Flow rate, lift and head explained for small pumps

A pump label can look like a code. A flow figure, a height figure, sometimes a curve on a chart. Each one answers a plain question, and once you know the questions the code makes sense.

Flow rate: how much, how fast

Flow rate is the amount of water a pump moves in a set time. For small pumps it's usually litres per minute. A pump curve normally puts flow rate along the horizontal axis, the line across the bottom of the chart¹.

One trap: a single flow figure on a box can't tell you the whole story. The flow you get depends on what the pump is pushing against. That's what the next two ideas are about.

Head: how hard the pump has to work

Head is a height. It's the height of a column of water that would push back at the pump, written in metres or feet². Why use height? Because height turns straight into pressure. Each extra metre of fresh water adds about 9.8 kPa of pressure, so 10 metres of head is about 98 kPa².

It helps to think of head as "how much the water pushes back". There are three parts to it.

• Lift. The height from the water surface up to where the water comes out.
• Friction. Water rubbing against pipe walls.
• Fittings. Valves and bends add their own losses.

A pumping guide adds these up. It counts the static lift, the head lost to friction, and the losses from valves or pipe bends, all in metres of water³. Another source says losses happen as water flows through bends, valves and pipework¹. The total is called total dynamic head, or TDH.

Friction is the part beginners forget. A livestock sizing guide uses a rough rule of thumb in its worked example. It multiplies the pumping level and vertical rise by 1.1 to allow for friction⁴. That's a rough guess for one example, not a rule for your hose.

The same guide says it plainly: the greater the total dynamic head, the more energy it takes to pump the water⁴. More head means more power. The power draw of a 12V pump picks that up.

Suction lift: the part below the pump

Suction lift is the height from the water up to the pump's inlet. It's a different job from lifting water on the outlet side, because here the pump has to pull. Many centrifugal pumps sit below the water they draw from, which avoids the problem³.

If a pump struggles to pull, one maintenance guide says to reduce the suction lift or raise the liquid level⁵. In plain terms: move the pump closer to the water, or move the water closer to the pump. The checks in fixing a pump that won't draw water go through the rest.

Reading a pump curve

A pump curve is a chart. Flow rate usually runs along the horizontal axis. Pressure, shown as bar or as height in metres, runs up the vertical axis¹. The line on it shows the flow the pump gives at each level of pressure when it's working properly¹.

For a centrifugal pump the curve starts high on the left and slopes gently down to the right¹. So the higher the water has to go, the less you get.

Here's how to use it. Work out your total head. Find that figure on the vertical axis. Follow it across until you meet the curve. Where it meets is the duty point, and the flow right below it is what the pump will deliver¹.

The guide makes one more point that surprises people. A pump doesn't decide its own flow and pressure. The system it's installed in controls them¹. Change the hose or the height and you change the answer.

Common mistakes

• Reading only the top flow figure and ignoring the head. Your real flow sits on the curve, at your head.
• Forgetting friction and fittings, which add to the head³.
• Putting the pump high above the water and hoping it will pull. Many centrifugal pumps sit below the water for this reason³. Which kind of pump suits which job shows the designs that can pull.

When the numbers are close, check the maker's data for your exact model.