How it works
A pump has to lift the water, push it against the system pressure, and overcome friction in the pipe. Add those up, in feet of water, and you have the total dynamic head. Pump curves are drawn against TDH, so you match the flow and TDH you need to a point on the curve.
Friction comes from the Hazen-Williams equation applied to the pipe length plus an equivalent length for fittings and valves. Hydraulic horsepower is the useful power going into the water; dividing by efficiency gives the shaft (brake) horsepower the motor must supply.
Worked example
A well pump delivering 15 gpm through 250 ft of 1-inch PVC (plus 20 ft of fitting allowance), lifting 150 ft to a 50 psi system:
- Pressure head = 50 psi × 2.31 = 115.5 ft
- Friction = Hazen-Williams over 270 ft (pipe plus fittings) = 31.4 ft
- Velocity head = 5.57² ÷ (2 × 32.174) = 0.48 ft
- TDH = 150 + 115.5 + 31.4 + 0.48 = 297.4 ft
- Brake hp = 15 gpm × 297.4 ft ÷ 3,960 ÷ 55% = 2.05 hp
| Input | Value |
|---|---|
| Flow rate | 15 gpm |
| Vertical lift (pumping water level to discharge point) | 150 ft |
| Required discharge pressure | 50 psi |
| Pipe inside diameter | 1.049 in |
| Pipe length | 250 ft |
| Extra length for fittings and valves (equivalent length) | 20 ft |
| Pipe material (C factor) | PVC / HDPE (C=150) |
| Pump efficiency | 55 % |
| Result | Value |
|---|---|
| Total dynamic head | 297.4 ft |
| Total dynamic head as pressure | 128.7 psi |
| Pipe friction head | 31.4 ft |
| Pressure head | 115.5 ft |
| Hydraulic horsepower (water power) | 1.13 hp |
| Brake horsepower required | 2.05 hp |
| Water velocity in the pipe | 5.57 ft/s |
Assumptions and limits
- Fresh water at ordinary temperatures (specific gravity 1). 1 psi = 2.31 ft of water.
- Fittings are added as one lump of equivalent pipe length that you estimate.
- Friction is computed on a single pipe diameter. Change of pipe size along the line is not modeled.
- Motor efficiency, service factor, and voltage drop are not included in brake horsepower.
Common questions
What is total dynamic head?
The total pressure a pump must generate, in feet of water: lift, plus discharge pressure, plus friction losses while water is flowing.
How do I convert psi to feet of head?
Multiply psi by 2.31. A 50 psi pressure switch setting is 115.5 ft of head.
Do I use the static or pumping water level for a well?
Use the pumping level. When the pump runs, the water level in the well drops (drawdown), and the pump has to lift from there.
Should I size the pump to the highest TDH?
Size it so the flow you need falls near the middle of the pump's curve at the TDH you calculated, including your worst case such as the pump cut-out pressure.
Sources
- Hazen-Williams equation, US customary form (see the Hazen-Williams calculator on this site).
- Hydraulic horsepower = gpm × head (ft) ÷ 3,960 (Hydraulic Institute standard relationship). 1 psi = 2.31 ft of water at ordinary temperatures.
Updated 2026-09-30