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Sizing a well pump: total dynamic head
"What size pump do I need?" can't be answered by horsepower alone. Pumps are selected from a curve that pairs flow rate (gallons per minute) with total dynamic head (TDH) — the total resistance, expressed in feet, the pump must push against. Get TDH wrong and you either starve your fixtures or burn out an oversized pump.
The four components of TDH
| Component | What it is |
|---|---|
| Pumping water level (lift) | Depth to the water while pumping — the static level plus drawdown as the pump runs |
| Elevation lift | Vertical rise from the wellhead up to the storage or pressure tank |
| Pressure head | The system pressure you want at the tank, converted to feet (psi × 2.31) |
| Friction loss | Resistance from pipe length, diameter, fittings, and flow rate |
Add them up and you have TDH:
TDH = pumping level + elevation lift + pressure head + friction loss
Why drawdown matters
The water level in a well drops while you pump — that's drawdown. You must size to the pumping water level, not the resting level, or the pump will be undersized once it's actually running. A well that sits at 80 ft static but draws down to 140 ft while pumping has a 140 ft lift component, not 80.
Pressure converts at 2.31 ft per psi. Want 50 psi at the tank? That's about 115 ft of head on its own. Pressure is often the most overlooked chunk of TDH for people sizing by depth alone.
Push more GPM through undersized pipe and friction loss can dominate TDH on a long run. The fix is usually larger-diameter drop pipe — not a bigger pump fighting the restriction.
Well computes TDH from your system
Enter your pumping level, lift, pressure, pipe, and target flow and Well returns the TDH and the operating point to match against a pump curve — with sizing math drawn from extension and standards sources. Free to download.
Sources
- Penn State Extension (B. Swistock) — private water wells and pump systems
- USDA NRCS Conservation Practice Standard 642 (water well) and standard hydraulics references
Sizing math is a planning aid; have a licensed well/pump professional review your design.