Total Dynamic Head (TDH) Pump Calculator
Total dynamic head for pump selection: static lift and discharge elevation, outlet pressure (psi, bar or kPa) minus any source pressure, Hazen-Williams pipe friction for Schedule 40, Schedule 80 or a custom pipe, fittings as a percentage or counted one by one, velocity head, specific gravity, NPSH available, a system curve, water and brake horsepower, motor size and running cost.
How it works
TDH = static head (suction lift + discharge elevation) + pressure head ((outlet psi − source psi) × 2.31 ÷ SG) + friction head + velocity head · Hazen-Williams h_f (ft per 100 ft) = 0.2083 (100 ÷ C)^1.852 Q^1.852 ÷ d^4.8655 (Q gpm, d in) · equivalent length = pipe + fittings (% of pipe, or Σ count × L/D × d) · velocity = 0.4085 Q ÷ d², velocity head = v² ÷ 2g · WHP = Q × H × SG ÷ 3,960 · BHP = WHP ÷ efficiency · NPSHa = atmospheric head − lift − suction friction − vapour pressure head
A pump has to overcome everything between the source and the outlet. That is the height it lifts the water (static head), the pressure the outlet needs (each psi is 2.31 feet of water, or 10.2 m per bar), the friction of the pipe and fittings, and the small kinetic energy of the moving stream. Added up, they give the total dynamic head. Together with the flow, it is the point on a pump curve you need to meet. A pressurised source, such as a tank or a city main, reduces the head by its own pressure. Friction is the term most often underestimated. It grows with roughly the square of the flow and the fifth power of the inverse diameter, so a pipe one size too small can double the head and the power. Hazen-Williams is the trade standard for water at ordinary temperatures. Fittings can be added as a 20–30% length allowance or counted one by one using equivalent lengths in pipe diameters. A foot valve with a strainer or a globe valve can add as much as a long run of pipe. The system curve shows how the needed head changes with flow, and the pump runs where its curve crosses it. NPSH available checks the suction side against cavitation: atmospheric head, which falls with elevation, minus the lift, suction friction and the vapour pressure of the water. It should stay a few feet above what the pump needs. Keep velocity at about 2–7 ft/s (0.6–2.1 m/s) for a quiet, durable system.
Worked example
Sixty gpm lifted 12 ft from a well and delivered 45 ft up at 30 psi through 350 ft of 2″ Schedule 40 copper (C 140, ID 2.067″) with a 25% fittings allowance. Static head is 57 ft and pressure head 30 × 2.31 = 69.3 ft. Friction is 6.4 ft per 100 ft × 437.5 ft = 28.0 ft, and velocity head is 5.7 ft/s → 0.5 ft. TDH is about 154.9 ft. Water horsepower is 60 × 154.9 ÷ 3,960 = 2.35 hp, and brake horsepower is 3.6 hp at 65%, so pick a 5 hp motor. At sea level with 60 °F water, NPSH available is about 33.9 − 12 − 0.6 = 21.3 ft before suction friction.
Sources: Grundfos ECADEMY — calculating total dynamic head (elevation, pressure, friction, fittings) · Engineering ToolBox — Hazen-Williams friction loss for water · Pumps & Systems — total head for the pump and system
Frequently asked questions
What is total dynamic head?
It is the energy per pound of liquid that the pump must add, expressed as a height of water: static lift, plus the pressure needed at the outlet, plus friction and velocity losses. Together with the flow, it sets the duty point you look for on a pump curve.
Should I count fittings or use a percentage?
A 20–30% allowance is fine for simple systems. Count them when there are many fittings or any big ones. A foot valve with a strainer is worth about 420 pipe diameters, around 70 ft on a 2″ pipe, and a globe valve 340. Choose Count them to enter each type.
Can I use metric units?
Yes. Flow takes L/min, m³/h or L/s, pressure takes bar or kPa, and the Metric switch shows lengths in metres. Heads, velocity and power are then shown in m, m/s and kW.
What is NPSH and why does it matter?
Net positive suction head available is the pressure margin at the pump inlet above the water’s vapour pressure. If it falls below the pump’s NPSH required, the pump cavitates, loses flow and erodes the impeller. High lifts, long or small suction pipes, hot water and high elevation all reduce it.
Why does the pump deliver more flow than designed?
If the real head is lower than calculated, for example because the pipe is short or has little friction, the pump runs out along its curve to a higher flow. That can overload the motor. The system curve table shows the head at other flows. Trim the impeller, add a valve or use a variable-speed drive.
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Hydraulic estimate with standard formulas and typical fluid properties (water unless stated); verify against pump curves, pipe data and the applicable code before specifying equipment.