Short answerTotal dynamic head is the vertical lift from the pumping water level to the outlet, plus friction in the pipe, plus any pressure wanted at the outlet at 2.31 ft per psi. It is the head to read off a pump curve, not the well depth.
Key takeaways
- TDH = lift from the pumping water level + rise to the outlet + pipe friction + outlet pressure.
- Setting the pump deeper below the water adds no lift.
- Each psi of outlet pressure adds 2.31 ft of head, which is why gravity tanks are popular.
The three parts
| Part | What it is | How to get it |
|---|---|---|
| Static lift | Pumping water level up to the ground | Driller's log, or measure with a well sounder while pumping |
| Elevation rise | Wellhead up to the highest outlet or tank inlet | GPS elevations, a site level, or a topo map; negative if the tank is downhill |
| Friction | Energy lost pushing water through pipe and fittings | Friction tables at your flow, times pipe length |
| Pressure (if any) | Pressure wanted at the outlet | psi x 2.31 = feet of head |
Static lift: the pumping level, not the static level
A well's static water level is where the water sits when nothing has been pumped for a while. Start pumping and the level drops until inflow matches the pump rate: that is the pumping level, and the difference is drawdown. On a low-yield domestic well drawdown can be tens of feet. The pump has to lift from the pumping level, so use it.
Small solar pumps help here. At 1 to 3 gpm many wells barely draw down at all, which is one reason solar pumping suits low-yield wells that would struggle to feed a big AC pump.
Elevation rise
Measure from the wellhead to the highest point the water reaches: usually the tank inlet, which on a tall tank is at the top, not the ground. A hilltop tank 60 ft above the well adds 60 ft. If the tank is downhill of the wellhead, the rise is negative and helps, as long as the pipe does not climb over a ridge on the way. If it does, the ridge is the high point that counts.
Friction
Friction rises with roughly the 1.85 power of flow and falls with nearly the fifth power of pipe diameter, so it is tiny at solar flow rates in properly sized pipe and punishing in pipe that is too small. At 2 gpm, 1 in polyethylene loses about 0.4 ft per 100 ft; 1/2 in loses about 12 ft. The friction loss tables give the numbers for poly and PVC.
Add about 10% to straight-pipe friction for elbows, tees and a check valve on a simple run. Add more for a run with many fittings, a filter or a meter.
| gpm | 1/2 in | 3/4 in | 1 in | 1-1/4 in | 1-1/2 in | 2 in |
|---|---|---|---|---|---|---|
| 0.5 | 0.90 | 0.13 | 0.03 | 0.01 | 0.00 | 0.00 |
| 1 | 3.26 | 0.45 | 0.11 | 0.04 | 0.02 | 0.00 |
| 1.5 | 6.90 | 0.96 | 0.24 | 0.08 | 0.03 | 0.01 |
| 2 | 11.8 | 1.63 | 0.40 | 0.14 | 0.06 | 0.01 |
| 3 | 24.9 | 3.46 | 0.85 | 0.29 | 0.12 | 0.03 |
| 4 | 42.4 | 5.90 | 1.46 | 0.49 | 0.20 | 0.05 |
| 5 | 64.2 | 8.92 | 2.20 | 0.74 | 0.31 | 0.08 |
| 6 | 89.9 | 12.5 | 3.08 | 1.04 | 0.43 | 0.11 |
| 8 | 100+ | 21.3 | 5.26 | 1.77 | 0.73 | 0.18 |
| 10 | 100+ | 32.2 | 7.94 | 2.68 | 1.10 | 0.27 |
| 15 | 100+ | 68.2 | 16.8 | 5.68 | 2.34 | 0.58 |
| 20 | 100+ | 100+ | 28.7 | 9.68 | 3.99 | 0.98 |
Pressure at the outlet
An open pipe into a stock tank needs no pressure. Anything that does: a pressure tank, drip emitters, a sprinkler, a house tap, adds its pressure as head. Each psi is 2.31 ft. A 40 psi pressure tank cut-out is 92 ft of extra head, which can double the duty of a shallow well pump. That is why most solar homesteads pump to a raised or hilltop tank and let gravity supply the pressure.
Worked example
Now go to a pump curve. You need a pump that delivers at least 2.5 gpm at about 148 ft, ideally with that point near the middle of its curve rather than at the end.
| Item | Value |
|---|---|
| Pumping level | 120 ft |
| Tank inlet above wellhead | 25 ft |
| Flow | 2.5 gpm |
| Pipe | 500 ft of 1 in poly at 0.61 ft per 100 ft, plus 10% |
| Friction | 3.4 ft |
| Outlet pressure | 0 (open into tank) |
| Total dynamic head | 148 ft |
Run your own numbers. The sizing calculator shows every step, for your state, lift and pipe.
Open the calculatorQuestions
- Does pipe length or depth matter more?
- Depth, almost always, at solar flow rates. Vertical lift is paid in full; friction in a correctly sized pipe adds only a few percent. Length starts to matter when the pipe is undersized or the run is thousands of feet.
- How many feet of head is 1 psi?
- 2.31 ft of water. So 10 psi is 23.1 ft and 40 psi is 92.4 ft.
Put your own gallons, lift and pipe in and see every step of the working.