What is total dynamic head?
Total dynamic head (TDH) is the resistance your pump works against at a given flow, written as the height of a column of water. Pipe friction, fittings, the filter, the heater and any true lift all count. It is always a pair: so many feet at so many gallons per minute. A pump curve tells you the flow a pump delivers against a given head; your plumbing's head at each flow is its system curve; the pump runs where the two cross, as the Hydraulic Institute explains. There are two ways to find your number, and the calculator above does both: measure it with gauges, or add it up from the plumbing.
How to measure TDH with a pressure gauge and a vacuum gauge
The field method in the ANSI/APSP/ICC-7 checklist needs two readings with the pump running: pressure at the pump discharge, in psi, and vacuum at the pump strainer, in inches of mercury. Multiply the psi by 2.31, the vacuum by 1.13, and add them. The standard's own example, 14 psi and 6 in Hg, comes to 39.12 ft.
Read both at the pump speed and valve positions you care about, with clean baskets. A vacuum gauge threads into the strainer's drain plug. If the pressure gauge sits higher than the vacuum gauge, as one on top of a filter tank does, add that height in feet. A velocity correction only applies when the pipes at the two gauges differ in size, and it is small: 2-inch suction with 1½-inch discharge at 50 GPM adds 0.6 ft.
Total dynamic head field worksheet
pooltoolbox.com · Gauge method, ANSI/APSP/ICC-7
| Reading, pump running | Multiply | Feet of head |
|---|---|---|
| Pressure gauge: ________ psi | × 2.31 | ________ ft |
| Vacuum gauge at the strainer: ________ in Hg | × 1.13 | + ________ ft |
| Pressure gauge height above vacuum gauge | feet | + ________ ft |
| Total dynamic head | sum | = ________ ft |
Pump speed ________ RPM · Filter clean / dirty · Valves as normally run · Date ________ . Find this head on the pump's curve at that speed to read the flow.
| Pressure | Feet of head |
|---|---|
| 5 psi | 11.6 ft |
| 8 psi | 18.5 ft |
| 10 psi | 23.1 ft |
| 12 psi | 27.7 ft |
| 15 psi | 34.7 ft |
| 18 psi | 41.6 ft |
| 20 psi | 46.2 ft |
| 25 psi | 57.8 ft |
| 30 psi | 69.3 ft |
| Vacuum | Feet of head |
|---|---|
| 2 in Hg | 2.3 ft |
| 4 in Hg | 4.5 ft |
| 6 in Hg | 6.8 ft |
| 8 in Hg | 9 ft |
| 10 in Hg | 11.3 ft |
| 12 in Hg | 13.6 ft |
| 15 in Hg | 17 ft |
| 20 in Hg | 22.6 ft |
1 psi = 2.31 ft of water; 1 in Hg = 1.13 ft of water, the ANSI/APSP/ICC-7 factors. Computed by our engine.
Can I work out TDH from the filter pressure gauge alone?
No. A filter gauge gives the pressure side from the gauge onward, which is a floor, not the total. It leaves out the skimmer, the suction pipe and the strainer basket, the short run between pump and filter, and the gauge's own height. A 15 psi filter reading is 34.7 ft. Add a 6 in Hg vacuum and a gauge 3 ft up the tank and the true figure is 44.4 ft: the gauge alone missed 9.8 ft, 22% of it. The calculator reports a pressure-only reading as "at least" for that reason. Without a vacuum gauge, the other route is to add up the suction side from its plumbing in the calculator's second mode.
How to calculate TDH from the plumbing
The structure is the one on Florida's permit simplified TDH worksheet: head lost in the piping, plus the filter, plus the heater, plus everything else, all at one design flow. Here is a complete circuit at 50 GPM.
| Component | Head at 50 GPM | Share |
|---|---|---|
| Suction pipe and fittings | 2.67 ft | 10% |
| Return pipe and fittings | 4.24 ft | 16% |
| Cartridge filter, 100 sq ft | 7.78 ft | 29% |
| Heater | 7 ft | 26% |
| Skimmer, 2" outlet | 3 ft | 11% |
| Return fittings (exit) | 2.28 ft | 8% |
| Total dynamic head | 27 ft | 11.7 psi |
Computed by our engine. 2-inch Schedule 40: 40 ft of suction with three elbows, a tee branch and a valve; 60 ft of return with eight elbows, a tee and a valve; three ¾-inch return eyeballs. Equipment figures are reference points from the Sta-Rite engineering manual: a 100 sq ft cartridge filter at 17.5 ft at 75 GPM, a 2-inch skimmer at 3 ft at 50 GPM, and a heater at 7 ft, which the chart quotes without a flow and we take at 50 GPM. Your manuals override all three.
The pipe is 26% of this system's head; the equipment is 66%. That is the opposite of how most guides spend their words. Get the per-section figures from the pipe size calculator if you want to see where a run's loss comes from, and get the equipment figures from each model's manual, quoted with their flow.
What is a typical TDH for a pool?
You will read "50 to 60 ft for an inground pool, 30 for above ground", or "40 to 70". Those figures are not wrong so much as missing their flow. The same example system, moved along its own curve:
| Plumbing | TDH at 50 GPM | TDH at 75 GPM |
|---|---|---|
| 2" Schedule 40 | 27 ft | 59.8 ft |
| 1½" Schedule 40 | 41.2 ft | 89.9 ft |
The worked example above with only the pipe size and flow changed. Pipe friction recomputed by Hazen-Williams; equipment and return fittings scaled by the square law from their quoted flows.
"Typical 50 to 60 ft" describes a single-speed pump pushing 75 GPM through 2-inch plumbing. The same pool at 50 GPM needs 45% of that head, and the same pool in 1½-inch pipe needs 53% more at 50 GPM. A typical figure cannot size your pump. Yours, at your flow, can.
Why TDH changes with pump speed
| Flow | 20 GPM | 30 GPM | 40 GPM | 50 GPM | 60 GPM | 70 GPM |
|---|---|---|---|---|---|---|
| System head | 4.5 ft | 9.9 ft | 17.4 ft | 27 ft | 38.6 ft | 52.2 ft |
The example system's curve, from our engine. No static lift, so the curve passes through zero.
At 25 GPM the example needs 6.9 ft against 27 ft at 50 GPM: 26%. Friction losses vary with the square of velocity, and pipe friction with flow to the power 1.85. So a head measured at full speed says nothing about low speed until it is moved along the curve, which the calculator does once you give it the flow. Power is head times flow, which is where the savings in the variable-speed calculator come from.
Does the height of the equipment pad add head?
No. Static head is the difference in liquid level between where the water comes from and where it goes, and it does not depend on flow. A pool loop starts and ends at the same surface, so once primed, whatever the pump lifts on the way up is handed back on the way down. Two of the guides ranking for this topic add it anyway: "if the pump is 3 ft above the pool, add 3 ft". Pad height matters for priming and for the pressure at the pump inlet. It is not part of TDH.
Water that leaves the pipe above the pool surface: a raised waterfall, a spillover from a raised spa, deck jets. A waterfall lip 4 ft above the pool takes the example from 27 ft to 31 ft at 50 GPM. Unlike friction, it does not shrink at low speed: at 25 GPM that system still needs 10.9 ft, 4 of them lift.
How much head do the filter, heater and valves add?
| Component | Published | At 40 GPM | At 60 GPM |
|---|---|---|---|
| 1½" push-pull backwash valve | 6 ft at 50 GPM | 3.8 ft | 8.6 ft |
| 1½" multiport valve | 5 ft at 50 GPM | 3.2 ft | 7.2 ft |
| 2" push-pull backwash valve | 7 ft at 75 GPM | 2 ft | 4.5 ft |
| 2" multiport valve | 3.5 ft at 75 GPM | 1 ft | 2.2 ft |
| Skimmer, 2" outlet | 3 ft at 50 GPM | 1.9 ft | 4.3 ft |
| Main drain, 2" outlet | 1.5 ft at 50 GPM | 1 ft | 2.2 ft |
| Cartridge filter, 100 sq ft | 17.5 ft at 75 GPM | 5 ft | 11.2 ft |
| Sand filter, 24" | 25 ft at 63 GPM | 10.1 ft | 22.7 ft |
| D.E. filter, 60 sq ft | 7 ft at 90 GPM | 1.4 ft | 3.1 ft |
Published column: the head-loss chart in the Sta-Rite Engineering and Design Manual, p. 17. Other columns: moved to that flow by the square law in our engine, which reproduces the chart's own second points (6.0 ft at 50 GPM becomes 13.5 ft at 75 GPM). An older manual, shown for scale; use your model's current manual and always note the flow its figure is quoted at.
A fixed allowance such as "heater, 15 ft" or "filter, 12 ft" is only true at one flow, which is why the calculator asks for the flow beside every loss. And one item is not fixed even then: a filter cleaned at 8 to 10 psi over its clean pressure adds 18.5 to 23.1 ft of head before it is cleaned, taking the example from 27 ft to 45.4 ft. The cleaning threshold is your filter manual's to set; the maintenance schedule covers the habit of reading it weekly.
Is my filter pressure too high?
On its own, a filter pressure is neither good nor bad. 25 psi is 57.8 ft on the pressure side alone, which is a lot for low speed and unremarkable for a large single-speed pump. Compare it with the clean reading at the same pump speed.
- High from the day the filter was cleaned: the restriction is downstream. Look for small return eyeballs, a partly closed valve or undersized return pipe, which the pipe size calculator will price.
- Rising over weeks: that is the filter loading. Clean it at the manual's threshold.
- Low, with weak returns: the problem is on the suction side, such as a full basket or an air leak. A vacuum gauge shows it as a high reading.
Using TDH to read a pump curve
Take the flow you need from the pump sizing calculator and the head at that flow from this page. That pair is a duty point. On a manufacturer's curve, find the flow along the bottom and the head up the side: a pump, or a speed of a variable-speed pump, whose curve passes through or just above that point will deliver it. A curve that passes far above means more flow than you asked for, more head than you calculated, and a larger electricity bill.