How to calculate total dynamic head (TDH)

1. What “head” means

Pumps are rated in head— a height of water — rather than pressure, because head does not depend on the liquid's density in the way a pressure figure does, and it lines up neatly with the physical layout of a job. A pump that can produce 100 feet of head can, in still conditions, push water up a 100-foot column. Head and pressure are two ways of saying the same thing: one foot of water column is about 0.433 psi, so 100 feet of head is about 43 psi. Working in head lets you add a vertical rise measured with a tape to a friction figure and an outlet pressure on one common scale, which is exactly what sizing a pump asks you to do.

The word dynamic is the important one. A pump sitting idle only has to hold up the static column of water. The moment it starts moving water, friction in the pipe fights back and the pump has to work harder — so the head it must develop while running (the dynamic head) is always more than the still-water lift. Sizing a pump on static head alone is the classic way to end up with a pump that runs out of puff at the far tap.

2. The three parts of TDH

Static lift is the vertical distance from the surface of the water source to the point of discharge. It is measured with no flow, straight up: the horizontal length of the run does not add to it. If the source is below the pump (a well or a sump), that drop is a suction lift and counts too; if the source is above the pump (a header tank feeding down), the static part can even be negative and helps the pump.

Friction loss is the head the moving water gives up dragging along the pipe wall and through every elbow, tee and valve. Unlike static lift, it depends on how fast the water is moving, how narrow the bore is and how rough the pipe is — and it grows steeply as the pipe gets smaller. This is the part people most often underestimate. Work it out properly with the pipe friction loss calculator, which uses the Hazen-Williams equation, then bring the answer back here.

Pressure head is any pressure that has to be left over at the outlet — the 20–50 psi a home pressure tank, sprinkler or appliance expects, for example. It is not always present: if you are simply lifting water into an open tank, the required outlet pressure is zero and this part drops out. When it is present it is often the biggest of the three, because 40 psi alone is about 92 feet of head.

3. Converting pressure to head (table)

Because the three parts must be added on one scale, you convert any required pressure into a head. For water: feet of head = psi × 2.31, and metres of head = bar × 10.2. Common values:

The psi and bar columns are separate scales (they are not row-by-row equivalents), each shown next to its head conversion. These are exact unit conversions, not code values or estimates. The calculator does the conversion for whatever pressure you type in, so you can mix a lift in feet with a pressure in psi without doing it by hand.

4. Worked examples

A domestic well pump. The water sits about 25 feet below the discharge, the run loses about 15 feet of head to friction at the design flow, and the pressure tank needs 20 psi (≈ 46 feet). Add them: 25 + 15 + 46 ≈ 86 feet of total dynamic head, or about 37 psi at the pump. You would then look for a pump whose curve delivers your required gallons per minute at 86 feet or a little more. Notice that the pressure requirement, not the lift, is the biggest single piece here.

A flat booster run. Suppose there is almost no lift — the pipe runs level — but it is long and small, losing 20 feet to friction, and it must deliver 40 psi (≈ 92 feet) at the end. Now TDH ≈ 0 + 20 + 92 = 112 feet, even though the water never really climbs. This is why “it's all on one floor, so the pump barely has to work” is a trap: friction and pressure can dominate the head completely.

The same in metric. Raising water 8 metres, losing 4 metres to friction, and holding 1.5 bar (≈ 15.3 metres) at the outlet gives 8 + 4 + 15.3 ≈ 27.3 metres of head, about 2.7 bar at the pump. The method is identical; only the units on each part change.

5. Reading a pump curve with your TDH

TDH on its own does not pick a pump — you need it at a flow rate. Every centrifugal pump has a performance curve: head on the vertical axis, flow on the horizontal, sloping down to the right (the more it flows, the less head it makes). Find your design flow along the bottom, go up to your TDH, and you want a pump whose curve passes through or just above that point. Land too far below the curve and the pump will push more flow than you planned at lower head; land above it and the pump cannot deliver your flow. If you do not know your flow yet, measure or estimate it first — the GPM / flow rate calculator measures an existing supply from a timed fill.

Two practical cautions. First, friction loss is tied to flow, so TDH and flow are not independent: if you end up choosing a pump that runs at a higher flow than planned, its friction loss — and therefore the real TDH — rises too, so it is worth re-checking. Second, a very fast flow is a noisy, erosive, lossy flow; a quick look at the pipe velocity calculator tells you whether your chosen pipe is comfortably sized or is itself driving the friction up.

6. Getting the inputs right

The answer is only as good as the three inputs. For static lift, measure the true vertical difference between the water surface and the discharge — for a well, remember to include the drawdown level while pumping, not just the resting level, because the water drops as you draw from it. For friction loss, use the real inside diameter of the pipe (never the nominal size) and add the equivalent length of every fitting before you run the numbers; a run with many elbows can carry far more friction than its straight length suggests. For pressure head, use the pressure the equipment actually needs at the outlet, not the pressure at the source. Get those three honest and the sum is trustworthy; guess at any one of them and no amount of decimal places will save the result. The rest of the flow and sizing tools on the plumbing hub follow the same measure-first, then-calculate approach.

Common questions

How do you calculate total dynamic head (TDH)?

Add the three heads the pump works against: TDH = static lift + friction loss + pressure head. Static lift is the vertical rise from the water source to the discharge point; friction loss is the head lost in the pipe and fittings at your design flow; pressure head is any outlet pressure converted to a height of water (1 psi ≈ 2.31 ft, 1 bar ≈ 10.2 m). The sum, in feet or metres of water, is the pump head.

What is total dynamic head on a pump?

Total dynamic head is the total equivalent height of water a pump must work against while it is running: the physical lift, plus the friction the flow creates in the pipework, plus any pressure held at the outlet, all expressed as one head figure. It is the value you take to a pump's performance curve, read at your design flow rate, to choose the pump.

What is the difference between static head and dynamic head?

Static head is only the vertical distance water is raised, measured with no flow. Dynamic head (TDH) is what the pump sees while moving water: static lift plus friction loss plus outlet pressure. Static head is always part of TDH, but on a long or small-bore run the friction and pressure parts can add up to more than the lift itself.

How do you convert psi to feet of head?

For water, feet of head = psi × 2.31, because one foot of water column exerts about 0.433 psi. So 20 psi ≈ 46 ft, 40 psi ≈ 92 ft, 60 psi ≈ 138 ft. In metric, 1 bar ≈ 10.2 m of head. This lets you add a lift measured in feet or metres to a required pressure measured in psi or bar on the same scale.

Does pipe size affect pump head?

Yes, through the friction-loss part. A smaller bore or a longer run creates more friction loss, which raises the TDH and forces a bigger pump; the static lift and outlet pressure do not change with pipe size. Work the friction figure out with a friction loss calculator (flow, bore, length and material) and add it to the lift and pressure.