Every pump is quoted with a headline maximum flow rate, often hundreds or in some cases, thousands of litres a minute. In practice, once it is installed, it will deliver less than this stated figure. The pump is not faulty and the figure is not incorrect, but it is widely misunderstood. This article explains what maximum flow actually represents, why the flow you achieve is always lower, and how to determine the figure you will obtain.
Maximum flow is measured at zero head
A pump's quoted maximum flow is measured with the pump discharging against no resistance: no vertical lift and no pipework. As soon as the pump has to raise water, or move it through a hose, the flow falls. The reverse applies to the quoted maximum head, the greatest height it can reach: that figure is measured at zero flow. A pump cannot deliver maximum flow and its maximum head at the same time. The two headline figures represent the opposite ends of its performance range, and your actual duty point lies somewhere between them.
What reduces flow in a real installation
Between test conditions and a working installation, two factors reduce the delivered flow.
Static head, the vertical lift. This is the vertical distance from the water surface to the discharge point. Every metre of lift reduces flow, because part of the pump's energy is used to raise the water against gravity rather than to move it.
Friction loss, the resistance of the pipework. Water moving through hose and fittings loses energy to friction, and the loss increases with the length of the run and as the bore narrows. A long, narrow or kinked hose can reduce flow considerably, so hose selection matters as much as the pump itself.
Adding the static lift and the friction loss together gives the total head. Locate that total head on the pump's performance curve, and the point at which it intersects the curve is the flow the pump will actually deliver. This is the true operating point, and it is almost always well below the headline maximum flow.
A worked illustration
Consider a site pump rated at 225 litres a minute maximum and 11 metres maximum head. These two figures describe its limits, not a guaranteed output. If it is required to lift water 5 metres and then move it through a length of hose, the total head might be 7 or 8 metres once friction is added, at which point the delivered flow could be well half the headline figure. Reduce the lift, or shorten and widen the hose, and the flow increases again. The same pump produces very different results depending on the system to which it is connected.
How to determine your actual flow
There is no need to estimate. Two tools convert the headline figure into a realistic one.
Read the pump curve at your total head. Every quality pump publishes a curve of flow against head. Determine your total head, locate it on the vertical axis, and read across to the curve and down to the corresponding flow. Our guide to how to read a pump curve sets out the method in full.
Estimate the friction loss with the calculator. Friction loss is the most difficult component to judge by inspection, so we provide a tool for it. Our Water Pump Performance Calculator estimates the friction loss for a given hose run, which can then be added to the static lift to establish the true total head.
Why correct sizing matters
Correct sizing determines whether a pump performs as intended, and errors are possible in both directions. If the pump is undersized, it will fall short of the required duty. If it is oversized, a different problem arises: the pump may empty a sump so rapidly that it short cycles, switching on and off repeatedly, which heats and wears the motor and is among the most common causes of early failure. The headline maximum flow figure frequently leads to oversizing for precisely this reason. We examine this and the other fundamentals of a durable installation in the engineering behind a reliable pump installation.
The correct approach in both cases is the same: design to the actual operating point rather than the headline figure. Read the performance curve at your total head, use the Water Pump Performance Calculator to establish the friction loss, and you can select with confidence. Alternatively, provide us with the lift, the hose run and the required flow, and we will match a pump to the actual duty.
Frequently asked questions
Why does my pump move less water than its max flow rating?
Because max flow is measured at zero head, with no lift and no pipe. On site you have both a vertical lift and friction in the hose, which together form the total head the pump works against. The higher that total head, the lower the flow, so your real flow is always below the headline figure.
Can a pump deliver its max flow and max head at the same time?
No. Maximum flow occurs at zero head and maximum head occurs at zero flow. They are the two opposite ends of the pump's performance curve, and your real duty sits somewhere between them.
How do I work out the flow I will actually get?
Add your vertical lift (static head) to the friction loss in your hose to get the total head, then read that total head against the pump's performance curve to find the flow. Our friction loss calculator estimates the hose losses, and our pump curve guide shows how to read the curve.
Does hose size affect pump flow?
Yes, significantly. A long, narrow or kinked hose adds friction loss and reduces flow, while a shorter, wider hose loses less. Matching the hose to the pump's outlet, and keeping runs short and straight, helps you get closer to the pump's potential.
