Pump specifications quote a maximum flow and a maximum head, and the two never happen at the same time. A pump delivers its biggest flow when it has nothing to push against, and its biggest head when it is delivering nothing. Sizing a pump means finding the point in between where it meets your job, and that takes a little arithmetic.
Quick answer
Add up your total head: the vertical distance from the water surface up to the pump (suction lift), plus the vertical rise from the pump to the discharge point, plus friction loss in the hoses. Then read the pump's performance curve to find the flow it delivers at that head. If the flow is not enough, you need a bigger pump, a shorter suction lift or a larger hose. To convert pressure, 1 psi equals about 2.31 feet of head.
In this guide
The terms
- Flow is the volume moved per unit of time, usually gallons per minute (GPM).
- Head is pressure expressed as a height of water. A pump with 100 feet of head can push water 100 feet straight up (at zero flow).
- Static suction lift is the vertical distance from the water surface up to the pump inlet.
- Static discharge head is the vertical distance from the pump up to the point where the water leaves the hose.
- Friction loss is the head lost to the hose, fittings and strainer. It rises steeply with flow and with longer or narrower hoses.
- Total dynamic head (TDH) is all of the above added together: what the pump actually works against.
Why flow falls as head rises
A centrifugal pump produces a certain amount of energy per gallon. The more of that energy is spent lifting the water, the less is left to move volume. The relationship is shown as a curve on the specification sheet: flow across the bottom, head up the side, with the pump’s performance sloping down from top left (high head, no flow) to bottom right (high flow, no head). The usable region is the middle.
Suction lift has a hard limit
A surface pump does not pull water up. It creates a partial vacuum and atmospheric pressure pushes the water up the suction hose. At sea level, atmospheric pressure can support about 33.9 feet of water, and no pump can exceed that. In practice, friction and the vapor pressure of water bring the usable limit to about 25 feet, and most engine-driven pumps are rated for 20 to 26 feet. Every foot of lift costs performance, so place the pump as close to the water as you safely can and let the discharge side do the long run. Altitude reduces the limit by roughly one foot for every 1,000 feet of elevation.
Friction loss
Water rubbing along a hose wall loses energy. The loss roughly quadruples when flow doubles, and it is far higher in a narrow hose. Manufacturers and hose makers publish friction tables, but two rules cover most situations:
- Use a discharge hose at least as large as the pump outlet. Going one size larger on long runs pays for itself in flow.
- Never use a suction hose smaller than the inlet. Starving the inlet causes cavitation, which sounds like gravel in the pump and destroys the impeller.
Fittings, valves and the suction strainer each add the equivalent of several feet of hose.
A worked example
Suppose you want to drain a flooded excavation into a ditch. The water surface is 8 feet below where the pump will sit. The ditch is 12 feet above the pump, 150 feet away, and you have 2-inch lay-flat discharge hose.
- Static suction lift: 8 feet.
- Static discharge head: 12 feet.
- Friction loss: for 2-inch lay-flat hose at around 100 GPM, published hose tables give something like 15 to 18 feet of head lost per 100 feet, so roughly 25 feet for 150 feet, plus a few feet for the strainer and fittings. Call it 30 feet.
- Total dynamic head: 8 + 12 + 30 = about 50 feet.
Now read the curve of the pump you are considering at 50 feet of head. A typical 2-inch transfer pump rated for 150 GPM at zero head and 100 feet maximum head might deliver somewhere around 80 GPM at 50 feet. Notice that more than half of the head is hose friction. If you need the excavation empty faster, a 3-inch pump with 3-inch hose, or simply a larger discharge hose, would do far more than a higher-horsepower 2-inch pump of the same design, because the hose is the limit.
Converting between PSI and feet
Pressure gauges read in psi; pump curves read in feet. One psi equals 2.31 feet of water, so a pump rated at 100 feet of head produces about 43 psi at zero flow. Sprinklers and fire nozzles are rated in psi at a given flow, which is why high-pressure pumps are described by pressure rather than by head.
Sizing checklist
- Measure the suction lift and keep it as short as possible.
- Measure the discharge rise and the hose length.
- Decide the flow you need (how many gallons, in how many minutes).
- Estimate friction loss from a hose table at that flow.
- Add them for total dynamic head, then add a 10 to 20 percent margin.
- Pick a pump whose curve gives your flow at that head, with the solids rating your water needs.
Our water pump buying guide covers the pump classes, and if a pump will not pull water at all, see why a pump will not prime.
Frequently asked questions
What does maximum head mean on a pump?
The height to which the pump could lift water if it were delivering no flow at all. It is the top end of the performance curve. A pump never works usefully at its maximum head, so compare flow at your actual head instead.
Does a bigger engine mean more flow?
Only if the pump end is designed for it. Flow is set by the impeller and housing; the engine just has to have enough power to drive them. Two pumps with the same engine can have very different flow and head.
How deep can a surface pump pull water from?
About 25 feet in practice at sea level, less at altitude. For deeper water, lower the pump closer to the surface, or use a submersible pump that pushes from below.
