Make an NPSH available calculator your operators can use on a tablet

The NPSH sheet sits on the engineer's laptop. The operator at the pump has a tablet, a tank level, a temperature, and a question: is there still enough suction head at this level and this temperature? The answer needs the site's air pressure, the water's vapor pressure at today's temperature, and the suction losses, which is why it lives in a spreadsheet in the first place.

This guide gives you the NPSH available formula for each kind of suction, the atmospheric pressure by altitude, the vapor pressure of water by temperature, specific gravity, and what the Hydraulic Institute publishes about NPSH margin. Then come worked examples checked against a U.S. Bureau of Reclamation example, a test table, the prompt that turns your sheet into one HTML file, and the steps to put it at one link your operators keep.

The sources were read on November 6, 2026: the Hydraulic Institute's Pump FAQs and its page for ANSI/HI 9.6.1, the Bureau of Reclamation's Engineering Monograph No. 40, the National Weather Service's pressure altitude formula, and the NIST Chemistry WebBook. Every number in the examples comes from a script.

The NPSH available formula

The Bureau of Reclamation defines NPSH as "the total suction head above vapor pressure at the highest point of the impeller inlet edge", and the NPSH available at a site as (Engineering Monograph No. 40, page 3):

NPSH = Ha + Hs − Hv − HL

The Hydraulic Institute's Pump FAQs define the same four terms:

• h_atm "is the absolute pressure (in feet of liquid being pumped) on the surface of the liquid supply level. In an open tank, it is the barometric pressure or the absolute pressure existing in a closed tank."
• h_vp "is the head in feet corresponding to the absolute vapor pressure of the liquid at the temperature being pumped."
• h_s "represents the static height in feet that the liquid supply level is above or below the pump centerline or impeller eye."
• h_f "is all suction losses (in feet), including entrance losses and friction losses through pipes, valves, and fittings."

Written with pressures, which is how a calculator takes them:

NPSHa = (P_surface,abs − P_v) / (ρ·g) + z_s − h_f

• P_surface,abs, the absolute pressure on the liquid surface, in Pa (or kPa × 1000).
• P_v, the liquid's vapor pressure at the pumping temperature, absolute, in Pa.
• ρ, the liquid's density at that temperature, in kg/m³; g = 9.80665 m/s².
• z_s, the liquid surface's height above the pump's datum, in m: positive for flooded suction, negative for suction lift.
• h_f, the suction-side losses at the flow you're checking, in m.

In US units: head in ft = psi × 144 ÷ density in lb/ft³, close to 2.31 × psi ÷ SG. Every term is head of the liquid being pumped, so the pressures divide by that liquid's density, not by water's.

The datum. HI measures h_s to "the pump centerline or impeller eye", and the Bureau of Reclamation to "the highest point of the impeller inlet edge". For a vertical or a large pump, ask the pump maker which datum its NPSH required is stated at; ANSI/HI 9.6.1 defines an NPSH datum plane (section 9.6.1.2.7 in its 2024 table of contents).

Open tank, closed tank, suction lift, flooded suction

Suction lift or flooded suction is only the sign of z_s. With a lift, z_s is negative and takes head away. The highest lift a pump can take follows from the formula: z_s ≥ NPSHr + margin + h_f + Hv − Ha.

From a suction gauge on a running pump, the Bureau of Reclamation's "absolute suction head less vapor pressure head" becomes:

NPSHa = (P_baro + P_gauge)/(ρ·g) + z_gauge + v²/(2g) − P_v/(ρ·g)

z_gauge is the gauge's height above the pump datum, and v the velocity in the pipe at the gauge. The gauge reading already includes the suction losses, so don't subtract h_f again. Keep the two methods apart in the page: one form for the tank method, one for the gauge method.

Atmospheric pressure by altitude

The National Weather Service's pressure altitude formula, from its wxcalc sheet, is h_alt = (1 − (P_sta / 1013.25)^0.190284) × 145,366.45, with the station pressure P_sta in millibars (hPa) and h_alt in feet. It is the standard atmosphere's lowest layer, which reaches 11 km (36,089 ft), far above any plant. Solved for the pressure:

P_atm (hPa) = 1013.25 × (1 − h_ft / 145,366.45)^(1 / 0.190284)
h_ft = site elevation in ft = elevation in m ÷ 0.3048

The formula's heads match the atmospheric pressure table in the Bureau of Reclamation's Engineering Monograph No. 40 (fig. 6, page 18) to within 1 mm at every elevation it lists. That table's sea-level 10.351 m is 101.325 kPa as head of water at 998.2 kg/m³, its density at about 20 °C.

The formula gives a standard day. A barometer on site reads the pressure at your elevation on today's weather, and the page can take that reading instead. Use the pressure at the site, not a weather report's figure corrected to sea level.

Vapor pressure of water by temperature

From the NIST Chemistry WebBook (saturated water), with the vapor pressure as head at the water's own density:

The Hydraulic Institute's own water-properties table in its HI Data Tool cites the same NIST database, and states the uncertainty of the underlying equation "0.025% in vapor pressure".

Between rows, interpolate on the logarithm of the pressure. Vapor pressure curves upward, so a straight line between two rows overshoots. Checked against NIST's 1 °C table from 1 to 100 °C, interpolating these 5 °C rows linearly on P is off by up to 1.38%, and linearly on ln(P) by up to 0.17%. At 37 °C, NIST gives 6.282 kPa; ln(P) interpolation gives 6.275 kPa, a straight line 6.331 kPa, and taking the 35 °C row, as a VLOOKUP with an approximate match does, 5.629 kPa.

Or use the Antoine equation with NIST's constants for water, log₁₀(P) = A − B/(T + C), with P in bar and T in kelvin:

Each set is valid only in its range, and the ranges leave 1 K gaps at 303 to 304 K and 333 to 334 K. Against NIST's 1 °C table, these rows were within 0.11%. A table is easier to check against the sheet than a piecewise equation, so most sheets are better copied as a table. Above 100 °C, or for a liquid other than water, put that liquid's vapor pressure table in the file, or take P_v as an input from its data sheet.

Specific gravity and liquids other than water

For any liquid, ρ = SG × the density of the reference water, and every pressure term divides by that ρ. The HI Data Tool's water table gives specific gravity on three reference temperatures, 39.2 °F, 60 °F, and 68 °F, so write down which one your liquid's SG is stated on. Between 39.2 °F and 60 °F, water's density changes by 0.1%.

A liquid with SG 0.80 and a vapor pressure of 20 kPa absolute, at sea level, with 2.0 m of flooded suction and 0.5 m of losses: Ha = 101.325 kPa ÷ (800 × 9.80665) = 12.92 m, Hv = 2.55 m, and NPSHa = 11.87 m. The same pressure is more head of a lighter liquid. The vapor pressure has to be that liquid's, not water's.

NPSH margin and what ANSI/HI 9.6.1 publishes

The pump's NPSH required comes from its maker, at the flow you run. HI's Pump FAQs: "NPSHR is a minimum NPSHA given by the manufacturer which is required for a pump to achieve a specified performance at a specified rate of flow, speed, and pumped liquid." And: "By Hydraulic Institute definition, the required NPSH of a pump is the NPSH available that will cause the total head (first stage head of multistage pumps) to be reduced by 3%." HI calls that value NPSH3. "The full published pump head will not, however, be achieved (by definition) when the NPSHA equals the NPSH3 of the pump."

So NPSHa equal to NPSHr is not enough. HI's FAQ on pump curves says "NPSH margin above this value is required for the pump to operate at the published head (refer to ANSI/HI 9.6.1)."

What HI's page for ANSI/HI 9.6.1, Rotodynamic Pumps Guideline for NPSH Margin, says in public:

• "To ensure optimal performance, the system supplied NPSHA must always exceed the pump's NPSHR."
• "Recommended minimum NPSH margins are provided for various applications within the following market segments": petroleum process, chemical process, electric power (non-nuclear), water and wastewater, pulp and paper stock, slurry, oil and gas, irrigation, general service, and building services pumps.
• The current edition supersedes ANSI/HI 9.6.1-2017. Among its changes: "Transition from the use of NPSH3 to NPSHR for NPSH margin determination", and "The pump manufacturer's supplied NPSHR value will be greater than or equal to the tested NPSH3 value."

Its 2024 table of contents lists definitions of an NPSH margin and an NPSH margin ratio (sections 9.6.1.2.5 and 9.6.1.2.6), and sections of "NPSH margin recommendations" by market segment. The recommended values are in the standard, which HI sells, and this guide doesn't quote them. Have the page compute both numbers, and take the required margin as an input from the standard or from your pump maker:

NPSH margin       = NPSHa − NPSHr        (m or ft)
NPSH margin ratio = NPSHa / NPSHr

Suction losses grow roughly with the square of the flow, and NPSHr changes with flow too, so read both at the highest flow the pump runs, not only at the duty point.

Worked examples

The Bureau of Reclamation's example. Engineering Monograph No. 40 (pages 7 and 8) takes a plant at 1,000 m with water at 35 °C, 1.0 m of suction head, and 0.3 m of suction losses: NPSH = 9.18 + 1.0 − 0.57 − 0.3 = 9.31 m. The script reproduces it. With 998.2 kg/m³, the density EM 40's atmospheric-head column implies, Ha = 9.181 m; with the density of water at 35 °C (994.0 kg/m³), Hv = 0.577 m, the monograph's 0.57 m. Unrounded, NPSH = 9.30 m. With the density at 35 °C for Ha too, Ha becomes 9.220 m and NPSHa 9.342 m. The 0.04 m between the two is the density used for Ha: pick one, and use it in the sheet and the page.

Open tank, flooded suction, at sea level. Water at 20 °C, the surface 3.0 m above the pump centerline, 0.8 m of suction losses, and an NPSHr of 4.5 m: Ha = 10.351 m, Hv = 0.239 m, NPSHa = 10.351 + 3.0 − 0.239 − 0.8 = 12.31 m. The margin is 7.81 m and the ratio 2.74.

Suction lift from a sump at 1,500 m. Water at 40 °C, the surface 4.0 m below the centerline, 1.2 m of losses, and an NPSHr of 2.5 m: P_atm = 84.550 kPa, Ha = 8.690 m, Hv = 0.759 m, and NPSHa = 8.690 − 4.0 − 0.759 − 1.2 = 2.73 m. The margin is only 0.23 m, a ratio of 1.09. With a required margin of 1.0 m, the lift can be at most 3.23 m.

Closed tank under pressure. Water at 90 °C in a tank held at 50 kPa gauge, at a site 300 m up, with the surface 5.0 m above the centerline and 0.6 m of losses: P_surface,abs = 97.771 + 50 = 147.771 kPa, Ha = 15.61 m, Hv = 7.414 m, NPSHa = 12.60 m. With the 50 kPa typed in as if it were absolute, the same page would show 2.27 m.

Closed tank at the boiling point. The same tank with its water at saturation, so P_surface,abs = P_v = 70.18 kPa: NPSHa = 5.0 − 0.6 = 4.40 m. Only the height and the losses are left.

From a suction gauge. Water at 25 °C, a gauge reading −30 kPa gauge 0.4 m above the centerline, a barometer at 101.3 kPa, and 30 L/s in a 150 mm pipe: v = 1.698 m/s, v²/2g = 0.147 m, and NPSHa = 71.3 kPa ÷ (997.0 × 9.80665) + 0.4 + 0.147 − 0.324 = 7.52 m.

US units. An open sump at 1,000 ft, water at 100 °F, a lift of 12 ft, 2.5 ft of losses, and an NPSHr of 9 ft: P_atm = 14.172 psia, Ha = 14.172 × 144 ÷ 61.99 = 32.92 ft, Hv = 0.951 × 144 ÷ 61.99 = 2.21 ft, and NPSHa = 32.92 − 12 − 2.21 − 2.5 = 16.21 ft. The margin is 7.21 ft and the ratio 1.80.

Test cases to check the page against your sheet

Enter each row in your sheet and in the AI's page, with the altitude formula, the NIST table interpolated on ln(P), and the water's density at the stated temperature. Ask for agreement within 0.01 in every column.

The inputs:

The expected results:

N2 is the case to watch: a ratio of 1.09 that a page should show plainly, next to the margin your standard asks for. N3 catches gauge pressure typed as absolute. N4 checks that a boiling liquid leaves only height and losses. N5 checks that every head divides by the typed-in liquid's density, here 800 kg/m³, and uses its vapor pressure, not water's. N7 runs the gauge form: its Ha is the absolute pressure at the gauge as head (71.3 kPa), it adds v²/2g = 0.147 m, and it doesn't subtract losses. N8 catches a lookup that takes the 35 °C row instead of interpolating.

You stay responsible for checking the results against your own standard and your pump maker's data before operators rely on them.

The prompt for ChatGPT, Claude, or Gemini

Paste your sheet's inputs and formulas as text, then send:

Turn my NPSH available spreadsheet into one HTML file that pump
operators open on a tablet. Keep my method exactly.

Rules for the code:
- Plain JavaScript functions inside the file. No eval, no new Function,
  no formula-parsing library, no fetch, and no outside data.
- Write every table into the file: the water table below, and my
  liquids' density and vapor pressure tables.
- Pressures are absolute inside the code. Every pressure field says
  "gauge" or "absolute" next to it, and gauge readings are converted
  by adding the barometric pressure.
- Convert the inputs to SI once, compute in SI, and show results in
  the units the operator picked (m or ft).

Inputs, tank form:
- Site: elevation (m or ft), or a barometer reading (kPa or psia).
  With an elevation, P_atm (hPa) = 1013.25 * (1 - ft / 145366.45)
  ** (1 / 0.190284).
- Suction source: open or vented tank / closed tank with its gauge
  pressure / closed tank at the boiling point (surface pressure = P_v).
- Liquid level above the pump datum (+ flooded, − lift).
- The liquid: water from the table below at its temperature, one of
  my liquids, or a liquid typed in: its density at the pumping
  temperature (or its SG and the water temperature the SG is stated
  on) and its vapor pressure, absolute, at the pumping temperature.
- Suction losses at this flow (m or ft).
- The pump's NPSHr at this flow, and the required margin, with a note
  saying where that margin comes from.

Gauge form, a second form for a running pump:
- The suction gauge reading (gauge, kPa or psig, negative for vacuum),
  the gauge's height above the pump datum, the barometer reading, the
  liquid as above, and the flow and the pipe's inside diameter at the
  gauge.
- NPSHa = (P_baro + P_gauge)/(ρ·g) + z_gauge + v²/(2g) − P_v/(ρ·g).
  Don't subtract suction losses in this form: the gauge reading
  already includes them.
- In this form, Ha is (P_baro + P_gauge)/(ρ·g), the absolute pressure
  at the gauge as head.

Formulas from my sheet (cell: formula):
[each result cell's formula, as text]

Water: interpolate density linearly and vapor pressure linearly on
ln(P) in this table: [T, density, vapor pressure]. Outside the table,
show a message instead of a number.

Results card at the top: NPSHa, large, then NPSHa − NPSHr and
NPSHa / NPSHr beside the required margin. Under it: P_atm, the surface
pressure (absolute), Ha, Hv, the density used, and in the gauge form
v²/2g, with units.

Layout: two columns on a tablet in landscape, one on a phone. Labels
above the fields, number fields with inputmode="decimal".
Put "NPSH check v1.0, [date]" at the bottom.

Test mode: a "Run test cases" button that runs these rows, each in its
form (N7 in the gauge form, the rest in the tank form), and shows each
result beside the expected value, marked pass within 0.01:
[the inputs and expected results tables above]

From the sheet to the page: what goes wrong

• Gauge pressure typed as absolute. In N3, 50 kPa gauge entered as 50 kPa absolute turns 12.60 m into 2.27 m, and a reading of psig taken as psia does the same in US units. Label every pressure field, and convert gauge readings in one place.
• A barometer figure corrected to sea level. A weather report's pressure at a site 1,500 m up reads about 101 kPa; the air there is about 84.6 kPa. That is 1.7 m of head of water.
• The temperature lookup. A VLOOKUP with an approximate match takes the row at or below the temperature: at 37 °C it reads 35 °C's 5.629 kPa where NIST gives 6.282 kPa, 0.07 m of head too little. The page should interpolate on ln(P), or do exactly what the sheet does, so the test rows match.
• The density of the heads. EM 40's atmospheric-head column implies one water density, 998.2 kg/m³, at every elevation; NIST's density changes with the temperature. On EM 40's example, Ha at 998.2 kg/m³ gives 9.30 m, and Ha at the water's own density 9.34 m.
• The sign of z_s. A lift is negative. A page that asks for "suction lift" as a positive number has to flip it, and its label should say so.
• Losses at the wrong flow. h_f from the duty point understates the losses at the pump's highest flow. Take the suction side's losses from your pump head and friction loss calculator at the flow you're checking.
• Losses counted twice. The gauge method's reading already includes the suction losses.
• Units. kPa, bar, and psi; m and ft; °C and °F. The NIST table's temperatures are °C; convert °F before the lookup, not after.
• Antoine's units. NIST's constants take T in kelvin and give P in bar, with log base 10. JavaScript's Math.log is the natural log; use Math.log10.

A tablet layout for operators

• The site, set once. Elevation or barometer, filled in by whoever sets the page up, with the site's name, so operators only change what changes on the shift.
• The suction source as three buttons: open tank, pressurized tank, and tank at the boiling point. The pressure field appears only for a pressurized tank, labeled "gauge".
• Level, temperature, flow. The three numbers an operator reads off the plant, with their units, large enough to tap with gloves on.
• The result. NPSHa in large figures, the margin and the ratio beside the required margin, and the words for each state, not a color alone.
• The working, under it. P_atm, the absolute surface pressure, Ha, Hv, the density, and the suction losses used, so an engineer can check a screenshot.
• A version line. For example "NPSH check v1.2, 2026-11-06, NIST water table, margin per [your standard]".

Publish the NPSH check for your operators in 5 steps

1. Upload the HTML file

Use the box below, or sign up and click Upload an HTML file on your dashboard. Drop the file on Upload a file, or open Paste code and paste what your AI wrote. Convika takes .html and .htm files up to 2 MB, so the sheet itself doesn't upload: the AI turns it into HTML first. Click Create my preview.

2. Run the test cases on a tablet

Open the preview on the tablet the operators use and run the test rows. The page's scripts run as written in the browser, so the math happens on the tablet. Convika blocks eval and new Function and requests to other hosts, which is why the prompt asks for plain functions and every table in the file. The page can't read a live tank level or a plant historian; operators type what they read.

3. Choose who can open it

Click Publish page. Under Who can open this page?, choose:

• Public: "Anyone with the link can open it. It may show up in search results and in Convika's gallery." Free, with the Convika bar at the foot of the page.
• Private link: "Anyone with the link can open it, with no password or account. It stays out of search results and out of the gallery." Pro.
• Only people with the password: Convika's server checks the password before it sends the page. Pro.

Click Next. The first time, for a Public page, click Publish free. A private link or a password opens an order for Pro, $12 a month, and a first subscription starts free for 1 day with a card. A page that names your site, its pumps, and their duty points is better behind the password; a private link only stays out of search, and anyone who's forwarded it can open it. Anyone who opens the page can read its source, tables included.

4. Put the link where operators look

Add it to the shift handover notes or the pump's log sheet, and to the tablet's home screen, where the icon opens the live address. Convika doesn't make the page work offline or install it as an app, so the tablet needs a connection to open it.

5. Publish each new version at the same link

When a pump, a margin, or a table changes, ask your AI for the whole corrected file. On the page's screen, under Update from an HTML file, click Upload a new version, add the file, and click Upload as a new version. Run the test rows in the preview, then click Publish v2 at the same link (with your version number). Every operator's link and home-screen icon opens the new version. Keep the version line inside the page, and see updating at the same URL for the details.

On Free, a new workspace keeps 1 page live. The NPSH check and the pump head calculator can share one file as two tabs. To keep them as two pages, Light, a monthly add-on to Free ($3 a month for up to 5 live pages, with the Convika bar), or Pro keeps both live.

Ways to get an NPSH check onto a tablet

Sources: the App Store page for NPSH Calculator, PE-Calc's NPSH calculator, and Microsoft's pages on sharing a canvas app and running apps in a web browser, read on November 6, 2026. Convika's row comes from its own code.

Choose a store app when its method fits and the tablet has no signal at the pump. Choose Power Apps when your company already licenses it and operators sign in with work accounts. Choose a page from your sheet when operators should use your site's figures, your liquids, and your margin rule, from a link, and every correction should reach them at the same address.

Frequently asked questions

NPSH available calculator for pump suction: how do I make one my operators can use on a tablet?

Have an AI rewrite your NPSH sheet as one HTML file with plain JavaScript and the water table inside it, check it against the sheet with test cases, and publish it at a link. Operators open it in the tablet's browser with no app. Use the prompt and the test table above.

What is the formula for NPSH available?

NPSHa = (absolute pressure on the liquid surface − vapor pressure) ÷ (ρ·g) + the liquid level above the pump datum − the suction losses. The Bureau of Reclamation writes it NPSH = Ha + Hs − Hv − HL, every term in head of the pumped liquid.

How do I calculate NPSHa for a closed or pressurized tank?

Use the tank's absolute pressure on the liquid surface: the barometric pressure plus the tank's gauge pressure. For a tank holding liquid at its boiling point, the surface pressure equals the vapor pressure, so NPSHa is the liquid level above the datum minus the suction losses.

What's the difference between suction lift and flooded suction in the NPSH formula?

Only the sign of the static head. With flooded suction, the liquid surface is above the pump datum and z_s adds head. With a suction lift, it's below and z_s takes head away. The highest lift follows from z_s ≥ NPSHr + margin + h_f + Hv − Ha.

How much NPSH margin do I need?

As much as your standard or your pump maker asks for. ANSI/HI 9.6.1 gives recommended minimum margins by application, and its values are in the standard, which the Hydraulic Institute sells. HI's FAQ says margin above NPSH3 "is required for the pump to operate at the published head".

Why does NPSH available use absolute pressure?

Vapor pressure is absolute, and the liquid boils when the absolute pressure drops to it. A gauge reading leaves out the atmosphere: in the pressurized-tank example, 50 kPa gauge entered as absolute turns 12.60 m into 2.27 m.

How does altitude change NPSH available?

The air pressure on an open tank falls with height, and Ha with it. The National Weather Service's formula gives 101.325 kPa at sea level, 89.870 kPa at 1,000 m, and 79.487 kPa at 2,000 m, which is 10.35 m, 9.18 m, and 8.12 m of water at 20 °C.

Can I turn my NPSH calculator xls into something operators use without Excel?

Yes. Paste its inputs and formulas into ChatGPT, Claude, or Gemini with the prompt above, and ask for one HTML file. Check it with the test rows, then publish it at a link. The workbook stays as your reference.

Can operators use it offline?

Not on Convika. Convika doesn't make pages work offline or install them as apps, so the tablet needs a connection to open the link. Store apps such as NPSH Calculator say they work offline.

Sources

• Pump FAQs (Hydraulic Institute): "What is the difference between NPSHR, and NPSHA" (entry dated April 2018) and the pump curve entry (read November 6, 2026, 17:16 JST)
• ANSI/HI 9.6.1 Rotodynamic Pumps Guideline for NPSH Margin (Hydraulic Institute): product page (read November 6, 2026, 17:17 JST)
• ANSI/HI 9.6.1-2024 table of contents (Hydraulic Institute): sections 9.6.1.2.5 to 9.6.1.2.7 and 9.6.1.4 (read November 6, 2026, 17:17 JST)
• Engineering Monograph No. 40, Selecting Large Pumping Units (U.S. Bureau of Reclamation, December 1978): NPSH, page 3; example, pages 7 and 8; fig. 6, page 18 (read November 6, 2026, 17:24 JST)
• Pressure Altitude (National Weather Service, El Paso, wxcalc) (read November 6, 2026, 17:15 JST)
• NIST Chemistry WebBook, Thermophysical Properties of Fluid Systems: water: saturation tables, 0.01 to 100 °C and 33 to 212 °F (read November 6, 2026, 17:14 and 17:28 JST)
• NIST Chemistry WebBook: water, Antoine equation parameters (read November 6, 2026, 17:15 JST)
• HI Data Tool: Water Properties (Hydraulic Institute) (last updated July 19, 2024; read November 6, 2026, 17:25 JST)
• NPSH Calculator (App Store) (version 1.2.2; read November 6, 2026, 17:35 JST)
• NPSH Calculator, NPSH Available vs Required (PE-Calc) (no date on the page; read November 6, 2026, 17:26 JST)
• Share a canvas app in Power Apps (Microsoft Learn) (dated September 17, 2026; read November 6, 2026, 17:27 JST)
• Run apps in a web browser, Power Apps (Microsoft Learn) (dated August 25, 2026; read November 6, 2026, 17:33 JST)