In short: a system with a 20 ft lift, a 100 ft run of 1-1/4″ PVC pipe carrying 12 GPM, and a 50 psi pressure target needs about 138 ft of total dynamic head. Pressure head alone accounts for over 80% of that — friction and velocity losses are almost an afterthought at this pipe size.
This is a sizing aid for pump curve selection, not a full engineering hydraulic design. Verify against your pump's actual published curve.
How to calculate total dynamic head
TDH adds up every form of resistance a pump has to push against, all converted into the same unit — feet (or meters) of water — so they can simply be summed:
Worked example. Take the default case: 20 ft of lift, 12 GPM through 100 ft of 1-1/4″ PVC pipe (C = 150), a 20% fittings allowance, and a 50 psi target. Friction loss from Hazen-Williams comes to 2.17 ft, plus 20% for fittings = 2.61 ft. Velocity works out to 2.65 ft/s, giving a velocity head of 0.11 ft. The 50 psi target converts to 115.34 ft. Sum them: 20 + 2.61 + 0.11 + 115.34 = 138.05 ft of TDH — matching the calculator's default result.
Pressure head usually dominates. In most home water systems, the pressure target contributes far more to TDH than friction or velocity — which is exactly what happened above. Before assuming you need a bigger pump, check whether your pressure switch setting or target pressure could reasonably come down instead.
TDH by pipe length
At the default 20 ft static head, 12 GPM, 1-1/4″ PVC and 50 psi target — only pipe length changes.
| Pipe length | Friction loss | TDH |
|---|---|---|
| 50 ft | 1.30 ft | 136.75 ft |
| 100 ft (default) | 2.61 ft | 138.05 ft |
| 200 ft | 5.21 ft | 140.66 ft |
| 400 ft | 10.43 ft | 145.87 ft |
| 800 ft | 20.85 ft | 156.30 ft |
TDH by pipe size
At 100 ft, 12 GPM, PVC (C=150), 50 psi target — only pipe diameter changes. Notice how quickly friction loss falls as pipe size increases.
| Pipe size | Velocity | Friction loss | TDH |
|---|---|---|---|
| 3/4″ | 7.58 ft/s | 33.72 ft | 169.95 ft |
| 1″ | 4.63 ft/s | 10.14 ft | 145.81 ft |
| 1-1/4″ (default) | 2.65 ft/s | 2.61 ft | 138.05 ft |
| 1-1/2″ | 1.94 ft/s | 1.22 ft | 136.61 ft |
| 2″ | 1.17 ft/s | 0.36 ft | 135.71 ft |
| 3″ | 0.53 ft/s | 0.05 ft | 135.39 ft |
Worked examples
Shallow booster, short run
106.1 ft
A 10 ft lift, 10 GPM through 50 ft of 1″ PVC, and a 40 psi target come to 106.1 ft TDH — modest friction (3.6 ft) against a dominant 92.3 ft pressure term.
Deep well, long run
243.3 ft
A 120 ft lift, 12 GPM through 300 ft of 1-1/4″ PVC, and a 50 psi target reach 243.3 ft TDH — here static head is the largest single contributor.
Old galvanized pipe, high demand
158.8 ft
A 30 ft lift, 20 GPM through 200 ft of old, corroded 1-1/2″ pipe (C=100), and a 50 psi target come to 158.8 ft — friction loss climbs fast in older, rougher pipe.
What else affects the result
Pipe age and condition. Hazen-Williams' C factor drops as pipe roughens with age and mineral buildup — an old galvanized line can lose more than half its effective C compared to new pipe, sharply raising friction loss over time.
Suction-side considerations. TDH covers the full system, but if your pump also has a meaningful suction lift or long suction run, check that separately against cavitation risk with the NPSH calculator — TDH alone doesn't flag that problem.
Multiple fixtures or zones. If flow rate varies by which fixtures or zones are active, TDH should be checked at your peak simultaneous demand, not just an average flow rate.
Frequently asked questions
What is total dynamic head (TDH)?
Total dynamic head is the total resistance a pump must overcome, expressed in feet (or meters) of water: the sum of static head (elevation change), friction head (pipe and fitting losses), velocity head (energy needed to accelerate the water), and pressure head (any target pressure required at the outlet). It's the number you plot against a pump curve to select a pump.
What is the formula for total dynamic head?
TDH = static head + friction head + velocity head + pressure head, all expressed in the same head units (feet or meters of water). Static head comes from elevation change, friction head from the Hazen-Williams or Darcy-Weisbach equation, velocity head from V²/2g, and pressure head from the target discharge pressure converted to head.
Why does pressure head dominate TDH in most home water systems?
A typical 40 to 60 psi household pressure target converts to roughly 92 to 138 feet of head on its own — usually far more than the friction and velocity losses in a reasonably sized pipe run. That's why increasing the pressure setting on a system raises pump requirements more than almost any other single change.
How much should I add for fittings and valves?
A common shortcut is to add 15 to 30% to the straight-pipe friction loss to cover elbows, tees, valves and other fittings, rather than calculating each fitting's equivalent length individually. Systems with many fittings in a short run should use a figure toward the higher end.
Why does pipe diameter matter so much for TDH?
Hazen-Williams friction loss scales with roughly the inverse fifth power of diameter, so even a modest increase in pipe size cuts friction loss dramatically. Undersized pipe is one of the most common causes of a system needing far more pump than its flow rate alone would suggest.
How do I use TDH to select a pump?
Plot your required flow rate and calculated TDH together as one point — your system's operating point — against a pump's published performance curve. A suitable pump's curve should pass through or above that point, ideally close to its best-efficiency point rather than at the extreme edge of its curve.
Sources & references
- American Water Works Association (AWWA). Hazen-Williams friction loss references.
- Hydraulic Institute. Pump standards and system head curve guidance (ANSI/HI).
John Mac, PE
Water Systems Specialist · Austin, Texas
Texas-licensed Professional Engineer with 13+ years in residential water systems and 180+ completed projects. Writes and reviews the calculators on watercalculator.us. Read full bio
Texas PE (2016) · BS Civil Engineering, Hydraulics (2013)