Pump Head Calculator
A pump does not care how deep the well is. It cares about total dynamic head, which is the lift plus everything the water fights on the way: friction in the pipe, losses through fittings, and the pressure you want at the far end. On a long run the lift is often the smallest of the three. Size on lift alone and you get a pump that runs, draws current, and delivers a fraction of its rated flow.
- Total dynamic head
- 164.0 ft 71.0 psi equivalent at 10 GPM — this is the number to look up on a pump curve
- Static lift
- 60.0 ft 37% of the total. Sizing on lift alone would have undersized this pump badly.
- Friction loss
- 11.6 ft 188 ft equivalent length of 1" PVC — 150 ft of pipe plus 25% for fittings
- Discharge pressure head
- 92.4 ft 40 psi × 2.31. Pressure you need AT the outlet, on top of getting the water there.
- Velocity in the pipe
- 3.9 ft/s Within the normal range
- If you only counted lift
- 60.0 ft The common mistake. It ignores 104.0 ft — a pump chosen this way delivers well below its rated flow, or nothing at all.
Reading a pump curve
Total dynamic head is only useful because pump curves are published against it. Knowing how to read one turns the number into a pump selection.
A pump curve plots head on the vertical axis against flow on the horizontal, and it slopes downward: the more head a pump must overcome, the less flow it delivers. At zero head — pumping into an open pipe at the same level — the pump delivers its maximum flow. At its shut-off head it delivers nothing at all.
Your system has its own curve, rising as flow increases because friction grows with the square of velocity. Where the two curves cross is the operating point: the flow and head the pump will actually produce in your installation. It is not a number you choose, it is where physics puts you.
This is why a pump rated "20 GPM" may deliver 8 in your system. The rating is a point on its curve, usually near its best efficiency, and your system head determines where on that curve you land.
Pumps also have a best efficiency point, and running far from it wastes energy and shortens life. A pump operating well to the left of its BEP recirculates and heats; far to the right it can cavitate. Selecting so the operating point sits near the BEP is what separates a specification from a guess.
Net positive suction head, and cavitation
This calculator sizes the pump for what it must push against. There is a second question — whether it can draw water in at all — and it has ruined a great many pumps.
Water boils at a lower temperature as pressure drops. A pump inlet creates suction, and if the pressure there falls below the vapour pressure of the water, bubbles form. They travel into the impeller, reach a region of higher pressure and collapse violently, eroding the metal. That is cavitation, and it sounds like the pump is drawing gravel.
Net positive suction head available is what your installation offers at the inlet: atmospheric pressure, less the suction lift, less friction in the suction line, less the vapour pressure of the water at its temperature. NPSH required is what the pump needs, and it comes from the manufacturer.
Available must exceed required, with margin. The variables that hurt are suction lift, long or undersized suction piping, hot water and altitude — atmospheric pressure at 5,000 feet is meaningfully lower than at sea level.
This is why submersible well pumps sidestep the problem entirely by sitting below the water, and why above-ground jet pumps are limited to roughly 25 feet of suction lift no matter how powerful they are.
Bigger pipe usually beats a bigger pump
Where friction dominates total head, the cheapest fix is rarely more pump.
Friction loss falls very steeply with diameter — in the Hazen-Williams form used here, with diameter to roughly the 4.87 power. Going up one nominal size can halve the friction component, and going up two can remove most of it.
The pump alternative costs more twice. A larger pump has a higher purchase price, and it pays for that friction in electricity every hour it runs, for the life of the installation. On a well pump running several hours a day, the operating cost difference exceeds the pipe upgrade within a season or two.
The arithmetic is easy to check with this calculator: run it at your intended pipe size, then again one size up, and compare the total dynamic head. If the difference is large, the friction component is dominant and the pipe is where the money should go.
Where static lift dominates — a deep well, a tall building — pipe size matters much less and the pump genuinely has to do the work.
What this is based on
- Total dynamic head = static lift + friction loss + pressure head
- Hazen-Williams friction loss with material C factors
- Conversion of 1 psi to 2.31 feet of water column
An estimate for planning. It does not calculate net positive suction head, account for suction-side losses separately, or model variable-speed pump behaviour. Confirm pump selection against the manufacturer’s curve.
Frequently asked questions
What is total dynamic head?
The total resistance a pump works against, expressed in feet of water. It is static lift plus friction loss plus the pressure head you need at the outlet. Pump curves are published against TDH, so it is the number you use to select one.
Why convert psi to feet?
Because pump curves are in feet and pressure requirements are in psi. One psi lifts water 2.31 feet, so 40 psi at the tap is another 92 feet of head the pump must produce on top of the physical lift.
What is equivalent length?
Every elbow, tee and valve causes turbulence, and that loss is expressed as the length of straight pipe that would cause the same loss. Adding a percentage is the quick method; the precise way is to look up each fitting and add its equivalent feet.
Should I use bigger pipe or a bigger pump?
Usually bigger pipe. Friction loss rises with roughly the square of flow and falls sharply with diameter, so one size up often removes more head than it costs. A larger pump instead pays for that friction in electricity every hour it runs.
Does this include net positive suction head?
No. NPSH governs whether the pump can draw water in without cavitating, and it is a separate calculation involving suction lift, water temperature and the pump’s own NPSH requirement. It matters most on shallow well and above-ground pumps.