Head pressure calculator

How do I convert feet of head to psi?

Multiply by 0.4335: each foot of water column adds about 0.4335 psi. 100 feet of water gives 43.35 psi.

How do I convert psi to feet of head?

Divide by 0.4335, or multiply by about 2.307: 50 psi pushes water up about 115 feet.

Where does 0.4335 come from?

NIST lists one foot of water (at 39.2 °F) as 2,988.98 pascals, and 1 psi as 6,894.757 pascals; the ratio is 0.43351 psi per foot.

Does warmer water change it?

Slightly — warm water is a little less dense. NIST’s inch of water at 60 °F works out to 0.4331 psi per foot, a 0.1% difference.

Is this the pressure at my tap?

It is the static pressure from height alone, such as from a water tower or tank above you. Flowing water loses some pressure to pipe friction — see the friction loss calculator.

How much pressure does a water tower or tank give?

Only the vertical height of the water surface above the point counts, not the size of the tank or the length of the pipe between them. Each foot adds 0.4335 psi, so water standing 100 feet above a tap gives 43.35 psi when nothing is flowing.

What water pressure does the plumbing code allow?

Section P2903.3.2 of the 2021 International Residential Code limits static water pressure to 80 psi and requires an approved pressure-reducing valve where the main pressure is higher. 80 psi is 184.5 feet of water. Local codes can differ, so check local code.

How the conversion works

The pressure at the bottom of a column of liquid is density × gravity × height, so for a given liquid it depends only on the height of the column, not on the width or shape of the container. NIST's conversion table defines the foot of water at 39.2 °F, near the temperature at which water is densest, as 2,988.98 pascals; a psi is 6,894.757 pascals, which gives 0.4335 psi per foot. The answer is gauge pressure, measured above the surrounding air; absolute pressure adds one standard atmosphere, 14.70 psi. The factor is for water. A liquid that is denser or lighter than water gives proportionally more or less pressure for the same height.

Worked example

A house has 55 psi at the water meter, and a shower is 25 feet higher. The height costs 25 × 0.4335 = 10.84 psi, leaving 44.2 psi when the water is still. Table P2903.1 of the 2021 IRC lists 20 psi of flow pressure for a shower with a thermostatic or balanced-pressure mixing valve at 2.5 gpm, so about 24.2 psi is left to cover the friction in the pipes and fittings. 20 psi is itself 46.1 feet of water.

Common mistakes

Using the length of the pipe instead of the vertical height is the usual error; a long, flat run adds friction but no head. Another is reading head as a flowing pressure: it is the static value, and moving water loses pressure to friction, which the friction loss calculator estimates. Confusing gauge and absolute pressure also happens, as does applying the water factor to another liquid. The 80 psi code limit is a static pressure, so it is judged with no water flowing.

Sources: NIST Special Publication 811, “Guide for the Use of the International System of Units,” Appendix B.8, conversion factors for units listed alphabetically (foot of water at 39.2 °F, inch of water at 60 °F, psi, standard atmosphere); USGS Water Science School, “Water Density”; International Residential Code 2021, section P2903.3.2 and Table P2903.1 (ICC, codes.iccsafe.org, read October 27, 2026). Related: friction loss calculator, pressure calculator, pipe size calculator, expansion tank calculator. How we check our calculators.