Why pumps talk in feet and gauges talk in psi
A pressure gauge measures force per unit area. A pump curve is plotted in feet. They describe the same thing, and the reason the two units coexist is that each one is natural to a different job.
A centrifugal pump imparts a fixed amount of energy per pound of fluid, regardless of what the fluid is. Spin a given impeller at a given speed and it will lift water 100 ft, brine 100 ft and gasoline 100 ft — the head is a property of the machine. But the pressure it develops is not: 100 ft of water is 43.3 psi, 100 ft of SG 1.2 brine is 52.0 psi, and 100 ft of SG 0.72 gasoline is only 31.2 psi. Publishing pump curves in feet means one curve serves every fluid; publishing them in psi would need a curve per fluid.
Static pressure in a building works the same way in reverse. Every foot you climb costs 0.433 psi, so a fixture 40 ft above the street loses 17.3 psi before any water moves. That is often the single largest term in a service-sizing pressure budget, and it is the reason a top-floor shower runs poorly while the basement hose bibb is fine.
Deriving 2.309 and 0.433
Both constants come from one fact: fresh water at 60 °F weighs 62.366 pounds per cubic foot.
Take a column of water one foot tall standing on a one-square-foot base. It contains one cubic foot, so it weighs 62.366 lb, and it presses on 144 square inches. The pressure at the base is therefore 62.366 ÷ 144 = 0.433097 psi per foot. Invert that and you get 144 ÷ 62.366 = 2.30894 feet per psi.
You will often see 2.31 and 0.433 quoted instead. Those come from rounding the density to 62.4 lb/ft³, which is water somewhere in the low 50s Fahrenheit. The difference between 2.31 and 2.309 is 0.05% — irrelevant against any real pressure gauge, but worth knowing about when a published table and your calculation disagree in the fourth digit.
Specific gravity handles every other fluid. A fluid with SG 1.2 is 20% denser, so each foot of it presses 20% harder: 0.433097 × 1.2 = 0.51972 psi per foot. Going the other way, a given pressure corresponds to a shorter column of a dense fluid, so head is divided by SG rather than multiplied. Keeping those two directions straight is the single most common error in this conversion, and the reason the calculator prints both constants for your fluid.
In SI the same physics reads p = ρgh. Water at 999.01 kg/m³ and g = 9.80665 m/s² gives 9,796 Pa per metre, so 1 bar is 10.21 m of water and 1 m of water is 9.80 kPa.
Worked example: a well pump for a two-storey house
A submersible pump sits 180 ft down a well. The static water level is 120 ft below grade, the pressure tank is at grade, and the tank cuts out at 60 psi. Friction in the drop pipe at design flow is 8 ft. What total head must the pump develop?
- Convert the cut-out pressure to head. 60 psi × 2.30894 ÷ 1.000 = 138.54 ft. This is the pressure the pump must still be delivering when it shuts off.
- Add the static lift. The pump must raise water from the standing level 120 ft below grade to the tank at grade: 120 ft.
- Add friction. 8 ft.
- Total dynamic head. 138.54 + 120 + 8 = 266.5 ft. That is the number you take to the pump curve.
- Sanity-check in psi. 266.5 × 0.433097 = 115.4 psi. A pump rated for 266 ft at your design flow is therefore developing about 115 psi across itself, which tells you what the drop-pipe and check-valve ratings must cover.
- Pressure at the pump inlet. The pump is 180 ft down and the water stands at 120 ft, so there are 60 ft of water above it: 60 × 0.433097 = 26.0 psi of submergence.
Now suppose the well is being used to move a 25% propylene glycol solution at SG 1.02 rather than water. The pump still develops 266.5 ft, because head is a property of the impeller. But the 60 psi cut-out now corresponds to 2.30894 × 60 ÷ 1.02 = 135.8 ft, so the delivered pressure at the tank rises slightly for the same machine. The head stayed put; the pressure moved.
Pressure and head conversion reference
| psi | kPa | bar | Feet of water | Metres of water |
|---|---|---|---|---|
| 5 | 34.47 | 0.345 | 11.54 | 3.52 |
| 10 | 68.95 | 0.689 | 23.09 | 7.04 |
| 15 | 103.42 | 1.034 | 34.63 | 10.56 |
| 20 | 137.90 | 1.379 | 46.18 | 14.08 |
| 25 | 172.37 | 1.724 | 57.72 | 17.59 |
| 30 | 206.84 | 2.068 | 69.27 | 21.11 |
| 40 | 275.79 | 2.758 | 92.36 | 28.15 |
| 50 | 344.74 | 3.447 | 115.45 | 35.19 |
| 60 | 413.69 | 4.137 | 138.54 | 42.23 |
| 80 | 551.58 | 5.516 | 184.72 | 56.30 |
| 100 | 689.48 | 6.895 | 230.89 | 70.38 |
| 125 | 861.84 | 8.618 | 288.62 | 87.97 |
| 150 | 1034.21 | 10.342 | 346.34 | 105.56 |
Divide the feet column by specific gravity for a fluid other than water — SG 1.2 brine reaches only 192.4 ft at 100 psi, because a denser fluid needs a shorter column to make the same pressure.
Mistakes this conversion invites
- Multiplying by specific gravity when you should divide. Head is divided by SG; pressure is multiplied by it. A dense fluid makes more pressure per foot and therefore needs fewer feet for a given pressure.
- Confusing gauge and absolute pressure. This converter works in gauge pressure, which is what every ordinary pressure gauge reads. Suction-side calculations, NPSH in particular, are done in absolute pressure, and you must add roughly 14.7 psi at sea level to convert.
- Using 2.31 in a chain of calculations that also uses 0.433. They are reciprocals of slightly different densities, so mixing them introduces a small inconsistency. Pick one density and derive both constants from it, which is what this page does.
- Forgetting temperature. Water at 200 °F is 3.6% less dense than at 60 °F, so a hot-water column produces 3.6% less pressure per foot. It rarely matters in a building; it matters in a boiler plant.
- Confusing static head with total dynamic head. The conversion here gives you the elevation and pressure terms. Friction must be added separately with the Hazen-Williams calculator or the Darcy-Weisbach calculator.
- Reading a gauge at the wrong elevation. A gauge mounted 6 ft above the pump discharge reads 2.6 psi lower than the discharge itself. On low-pressure systems that is a real error, not a rounding one.
Where the conversion shows up in a sizing job
In water service design the whole pressure budget is assembled in these units. Start with static street pressure, subtract the residual pressure the highest fixture needs, subtract 0.433 psi for every foot of elevation between the two, subtract the meter loss and any treatment-device loss, and whatever remains is what you may spend on friction. The water service pipe size calculator runs that arithmetic and converts the residue into an allowable loss per 100 feet.
In pump work the same conversion turns a duty point into a purchase. Total dynamic head is static lift plus friction plus any pressure the discharge must be delivered at, all expressed in feet. The sump pump capacity calculator assembles exactly that sum for a basement application, and the same structure applies to a booster set or an irrigation pump.
For gravity drainage the conversion is not used at all, because there is no pressure — flow is driven purely by slope, computed with the Manning's equation calculator and set with the pipe slope calculator. The distinction is worth holding onto: pressurised piping spends head, gravity piping spends elevation, and only the first one converts to psi.
Why a closed hydronic loop does not need the elevation term
Everything above assumes an open system — a well, a service line, an irrigation main — where the fluid arrives once and elevation has to be paid for on the way. A closed hydronic loop, the kind used for a boiler, chiller or radiant floor circuit, behaves differently: the same water leaves the pump, rises through the supply piping, falls back through the return piping, and arrives at the same pump inlet it started from. Because the column pushing down on the return leg is made of the same fluid at the same specific gravity as the column the pump must lift on the supply leg, the two cancel foot for foot. The pump's net lifting duty in a closed loop is therefore friction alone — the elevation term this calculator converts never enters the sizing, whether the loop rises one floor or thirty.
That cancellation applies only to the pump's net duty, not to what a gauge reads. A pressure gauge tapped into the same loop three floors up still reads lower than one at the pump discharge, by exactly 0.433 psi for every foot of elevation between them, because a gauge measures the local column above its own tap regardless of whether the loop is open or closed. What vanishes is the lifting work the pump has to do; the static pressure difference between two points in the loop does not.
The practical trap runs both ways. Adding static lift to a closed-loop pump selection, on the reasoning that the pump has to push water up three floors, oversizes the pump for no benefit. Going the other way, treating an actually-open connection — a make-up water line, an atmospheric expansion tank, a domestic hot water coil fed from the street — as if it were part of the closed loop reintroduces the elevation term at exactly that point, and skipping it there undersizes the pump. Before applying either budget, confirm which regime the specific leg of piping is actually in.
Key terms
- Head
- Energy per unit weight of fluid, expressed as a height. Because it divides out the density, the same head figure applies to a pump handling any liquid.
- Static head
- The elevation difference alone, with no flow. It is present whether or not the pump is running.
- Total dynamic head
- Static head plus friction head plus any required discharge pressure expressed as head. This is the figure you read against a pump curve.
- Specific gravity
- The ratio of a fluid's density to that of water at a reference temperature. Dimensionless, so it works identically in imperial and metric conversions.
- Gauge pressure
- Pressure measured relative to local atmospheric pressure — what an ordinary gauge shows. Absolute pressure adds atmospheric, about 14.7 psi at sea level.