Where does 0.433 come from?

It is the weight of a one-foot column of water one square inch in cross section, at the density used for fire protection calculations. The inverse, 2.31 ft per psi, is the same constant read the other way.

Does temperature change it?

Water density varies slightly with temperature, but fire protection calculations use the single value. Antifreeze solutions are denser and are handled separately, with the fluid properties entered rather than assumed.

What is the reference point?

Whatever you measure the supply from, normally the base of the riser or the elevation of the hydrant used for the flow test. What matters is that the supply and the network use the same datum.

Does the hydrant elevation matter?

Yes. If the test hydrant sits at a different elevation from the base of the riser, the available pressure has to be corrected by that difference before it is compared against the demand. The full app asks for both elevations for exactly this reason.

Elevation pressure calculator

Convert a height difference to static pressure, and pressure back to head.

Preliminary. The reference tables behind this calculation are pending a second check against their printed source. Verify against your adopted edition before using a result in a submitted calculation.

13.0psi

pressure lost climbing 30.0 ft

Head for 1 psi: 2.3ft
Conversion: 0.4330psi/ft
Height: 30.0ft

The formula

p = 0.433 h

p: static pressure differencepsi
h: height above the reference pointft
0.433: pressure per foot of water columnpsi/ft

Which way the sign goes

Water climbing loses pressure and water dropping gains it. Enter a positive height for a sprinkler above the reference point and a negative one for a sprinkler below it. Getting this backwards on a multi-storey riser is a common and expensive mistake: a 30 ft climb costs about 13 psi, which is often the whole margin.

The full app derives every elevation change from the node elevations you enter, so the sign is never guessed and never typed twice.

Run the whole system, not one pipe. Same solver, pass or fail against your flow test, NFPA 13 report as a PDF.

Try the full calc

Questions about this calculation

Where does 0.433 come from?: It is the weight of a one-foot column of water one square inch in cross section, at the density used for fire protection calculations. The inverse, 2.31 ft per psi, is the same constant read the other way.
Does temperature change it?: Water density varies slightly with temperature, but fire protection calculations use the single value. Antifreeze solutions are denser and are handled separately, with the fluid properties entered rather than assumed.
What is the reference point?: Whatever you measure the supply from, normally the base of the riser or the elevation of the hydrant used for the flow test. What matters is that the supply and the network use the same datum.
Does the hydrant elevation matter?: Yes. If the test hydrant sits at a different elevation from the base of the riser, the available pressure has to be corrected by that difference before it is compared against the demand. The full app asks for both elevations for exactly this reason.

The other calculators

How these results are checked against worked examples · Pricing for the full app

Updated 2026-09-07