The Complete Guide to Total Dynamic Head
Every pump selection begins with two numbers: the flow rate in gallons per minute and the total dynamic head in feet. The flow is dictated by the process — how much water the cooling tower needs, how fast the tank must fill — and the total dynamic head is dictated by the system the pump must push against. Get the head right and the pump lands on its best-efficiency point; get it wrong by a few feet and the pump runs off its curve, cavitates, or wastes power for the life of the installation. This guide explains the four components that sum to total dynamic head and how the Pump Selection & TDH Calculator assembles them.
The first component is static head, the physical elevation the pump must lift the liquid. It is the vertical distance from the surface of the liquid on the suction side to the discharge point, measured at the pump. In the calculator this is entered as a suction static head and a discharge static head: a flooded suction tank above the pump contributes a positive suction head that helps the pump, while a lift below the pump enters as a negative number that adds demand. Static head depends only on geometry, not on flow — it is the same whether the pump runs at ten gallons per minute or a thousand.
The second component is friction head, the pressure the liquid loses overcoming friction in the pipe walls and fittings. Friction head grows with flow — roughly as the 1.85th power in the Hazen–Williams relation — so doubling the flow more than triples the friction losses. The dominant factors are pipe length, pipe diameter, pipe roughness, and the number and type of fittings. The calculator offers the two standard methods: Hazen–Williams with a roughness coefficient C that suits the pipe material, and Darcy–Weisbach with a dimensionless friction factor f. The fitting losses are handled by converting each fitting's K-factor to an equivalent length of straight pipe — a 90° elbow counts roughly like 30 pipe diameters of extra length — and adding that to the straight run.
The third component is pressure head, the head equivalent of any pressure the system exerts on the liquid beyond the atmosphere. A discharge that empties into a pressurized tank, or a suction fed from a pressurized vessel, carries a pressure difference that must be converted from pounds per square inch to feet of liquid. The conversion is 2.31 feet of water per psi at specific gravity 1.0, adjusted by dividing by the specific gravity for any other fluid: h = psi × 2.31 ÷ SG. A 50 psi tank pressure adds roughly 115 feet of water head — easily the difference between a small pump and a large one — which is why forgetting the pressure term is one of the classic pump-sizing failures.
The fourth component is velocity head, the kinetic energy of the moving liquid. It is V² ÷ 2g, where V is the flow velocity in the pipe and g is the acceleration of gravity. In most practical systems the velocity head is small — at a typical 6 ft/s velocity it is under a foot — but in short, high-velocity suction lines it can matter, and the calculator includes it for completeness. The flow velocity also appears implicitly in the friction terms, and it is the quantity that the velocity-head warning monitors: above roughly 10 ft/s in the discharge, friction, erosion, and water hammer all climb steeply.
The four components sum to the total dynamic head: TDH = H_static + H_friction + H_pressure + H_velocity. The pump's duty point is then the intersection of this TDH curve with the pump's head-flow characteristic, and the pump should be selected so that its best-efficiency point sits at or near the duty flow. This is the sense in which TDH is the single most consequential number in pump selection: the entire pump curve — impeller diameter, casing size, speed — is chosen to deliver that head at that flow.
From the TDH the required power follows directly. The hydraulic horsepower, the power delivered to the liquid, is HP = Q × TDH × SG ÷ 3960, where Q is in gpm, TDH in feet, and the constant 3960 converts the mixed units. The shaft horsepower, what the motor must actually deliver, is the hydraulic horsepower divided by the pump efficiency — a typical centrifugal pump at its best-efficiency point runs 60–80% efficient, so the shaft power is routinely 25–60% above the hydraulic power. The calculator applies a 15% motor margin and rounds up to the nearest standard motor size, which is the professional habit: motors are purchased in standard ratings, and a pump running at its margin is a pump that lasts.
The power formula makes the design tension visible. Because hydraulic power scales with flow and head directly, every foot of TDH that can be removed by good pipe sizing is an operating-cost saving that compounds over the pump's life. A system that needs 90 feet of head instead of 100 saves about 10% of the pump's energy forever, which is why the friction and static terms deserve as much attention as the pump itself. The calculator's per-component breakdown exists precisely so that a designer can see where the head is coming from and attack the largest term.
Put the pieces together and the workflow is short. Enter the flow, the fluid's specific gravity, the suction and discharge elevations and pressures, the pipe diameter and length, and the fitting counts; the calculator returns the four head components, the TDH, the hydraulic and shaft power, and a recommended motor, with the full formula trace underneath. It is the same calculation a pump engineer would do on a spreadsheet — automated, transparent, and free to use for as many systems as you need to size.