You are standing at the wellhead with a pressure gauge reading 45 PSI, a pump catalog open on your tailgate, and a curve that stubbornly refuses to mention PSI anywhere. The X-axis says GPM. The Y-axis says FEET. And you need to know, right now, whether this pump will actually deliver water to the barn on the hill — or whether you are about to install the wrong unit and find out the expensive way.
That is the whole reason PSI to feet of head conversion exists as a daily headache. Gauges speak pressure. Pump manufacturers speak head. Somebody has to translate, and today that somebody is you.
The good news: the math is one multiplication. The bad news: getting it backwards is one of the most common and most costly mistakes in pump sizing, and it happens to experienced people. Let us make sure it does not happen to you.
The PSI to Feet of Head Formula (And Why 2.31)
Here are the two numbers that matter:
- Feet of head = PSI x 2.31
- PSI = Feet of head x 0.433
So 45 PSI x 2.31 = 103.95 feet of head. That is your answer. That is the number you take to the pump curve.
Where does 2.31 come from? It is not arbitrary. Fresh water weighs roughly 62.4 pounds per cubic foot. A cubic foot is 144 square inches on its base. So a one-foot column of water presses down with 62.4 / 144 = 0.433 pounds per square inch. Flip that around — 1 / 0.433 = 2.31 — and you get how many feet of water it takes to make one PSI.
That is it. No magic. Just a column of water and its own weight.
Understanding this derivation matters more than memorizing it, because the moment you stop believing 2.31 is a mystical constant, you stop reversing it. You know instinctively that head numbers should be bigger than PSI numbers, roughly double-and-a-bit. If your answer came out smaller, you multiplied by the wrong one.
If you want the arithmetic done and double-checked instantly, the free PSI to feet of head calculator handles it in one field, and the feet of head to PSI converter runs the inverse when you are going the other direction.
A Worked Example You Can Actually Use
Say you are designing an irrigation zone. Here is the real situation:
- Water source: well, static water level 80 feet below grade
- Destination: sprinkler head on a rise, 25 feet above grade
- Required sprinkler operating pressure: 40 PSI
- Estimated friction loss through pipe and fittings: 18 feet
Step one, convert the sprinkler requirement: 40 PSI x 2.31 = 92.4 feet.
Step two, add the elevation. Lift from water level to grade is 80 feet. Rise from grade to sprinkler is 25 feet. Total static head = 105 feet.
Step three, add friction: 18 feet.
Total dynamic head = 92.4 + 105 + 18 = 215.4 feet at your design flow rate.
Now you go to the pump curve and find a pump that delivers your required GPM at roughly 215 feet or better. Notice what would have happened if you had only converted the 40 PSI and ignored elevation: you would have shopped for a 92-foot pump and installed something that could not even reach the top of the well casing, let alone push water uphill.
PSI to Feet of Head for Fluids That Are Not Water
The 2.31 factor is fresh water at ordinary temperature. Change the fluid and you change the density, which changes everything.
The correction is straightforward: divide your head result by the specific gravity of the fluid.
- Fresh water: SG 1.0. No change.
- Glycol mix (common HVAC): SG around 1.05. A denser fluid means the same pressure represents about 5% less head.
- Seawater: SG 1.025, so about 2.5% less head.
Example: 30 PSI in a 30% glycol loop. Water math gives 30 x 2.31 = 69.3 feet. Correct for glycol: 69.3 / 1.05 = 66 feet.
Is 3 feet worth chasing? On a residential loop, probably not. On a tall building riser or a system where you are already tight against the curve, absolutely — that is exactly the margin that turns “works” into “runs but never quite satisfies the far zone.”
Why Pump Curves Are in Feet at All
This trips people up, so it is worth saying plainly. Manufacturers plot in feet because head is fluid-independent and pressure is not.
A centrifugal impeller spinning at a given RPM throws fluid to a certain height. That height is basically the same whether the fluid is water, glycol, or diesel. But the pressure at the discharge changes with density, because heavier fluid in the same column pushes harder.
So the manufacturer publishes the thing that does not move (feet) and lets you convert to the thing your gauge shows (PSI). It is actually the more honest choice. It just means the translation lands on your side of the fence.
Common Mistakes
Reversing 2.31 and 0.433. The number one error, by a wide margin. Sanity check: head is always the bigger number. 50 PSI is about 115 feet, not 22 feet. If your gut says “that seems small,” it is.
Reading head without reading flow. A pump curve is a curve, not a number. A pump rated for “230 feet max” delivers 230 feet at essentially zero flow — shutoff head. At your actual 20 GPM, it might deliver 180. Head and flow are a pair. Never quote one without the other.
Forgetting elevation and risers. Every vertical foot between your water source and your discharge point is head you must pay for. Second-story bathrooms, hillside sprinklers, rooftop coils — all of it counts.
Assuming this conversion gives you total dynamic head. It does not. TDH = static head + friction loss + velocity head. The PSI conversion handles the static and pressure-requirement portions. Friction loss is a separate calculation involving pipe diameter, length, material, and flow rate. Skip it and you will undersize.
Ignoring suction-side conditions. Lift, NPSH available, and suction pipe friction all matter. A pump that meets your discharge head but cavitates on suction is still a failed installation.
Using gauge readings from the wrong point. A gauge six feet up a riser is not reading the same pressure as one at the pump discharge. Know where your measurement is taken.
Honest Limitations of This Conversion
The 2.31 factor assumes fresh water at roughly room temperature. Water density does shift with temperature — hot water at 200 degrees F is measurably lighter than water at 60 degrees F — but for the great majority of pump and plumbing work the difference falls inside your other rounding errors. In high-temperature boiler and process applications, use density tables rather than the shortcut.
This conversion also tells you nothing about pipe sizing, valve authority, NPSH, or whether your pump will run near its best efficiency point. A pump that meets head and flow but sits at the far right edge of its curve will be loud, inefficient, and short-lived. Converting correctly is step one, not the finish line.
And a broader point: unit conversion is the connective tissue of field work. Whether it is pressure to head, or checking electrical draw on that pump with an amps to megawatts converter, the pattern is the same — get the translation right early so nothing downstream is built on a bad number.
FAQ
Is 2.31 exact or rounded?
Slightly rounded. Water density varies with temperature, so the precise factor moves a little. 2.31 is the accepted engineering standard and is plenty accurate for pump selection, plumbing, and irrigation design.
How many feet of head is 60 PSI?
60 x 2.31 = 138.6 feet. A quick mental estimate: double the PSI and add about 15%. 60 doubled is 120, plus 15% is 138. Close enough to catch a serious error.
Does pipe diameter change the PSI to head conversion?
No. The conversion itself is purely about fluid density. Pipe diameter affects friction loss, which is a separate quantity you add to head — it does not alter the 2.31 factor.
Can I convert PSI to head for air or gas?
Not with this method. The 2.31 factor is built on liquid column weight. Compressible gases behave differently and need different tools entirely.
My pump is rated 150 feet. What PSI should the gauge show?
At shutoff, roughly 150 x 0.433 = 65 PSI. Under actual flow it will read lower, because you are operating further right on the curve. If your gauge reads far below expectation at low flow, suspect a worn impeller, a partly closed valve, or an air leak.
Why did my pump underperform even though I converted correctly?
Almost always friction loss or elevation left out of the total. The conversion was fine; the head budget was incomplete. Rebuild the calculation with every vertical foot and every fitting accounted for.
Get the Number Right, Then Move On
Two numbers. 2.31 up, 0.433 down. Add your elevation, add your friction, read head and flow together on the curve, and correct for specific gravity if you are not pumping plain water.
When you want it checked in three seconds instead of three minutes, the PSI to feet of head tool is free and needs no signup. Run your number, then get back to the work that actually pays.