Understanding Flow Rate, Head and Pump Curves

Every pump is sold with two headline numbers: a maximum flow rate and a maximum head. Both are true. Neither is achievable at the same time as the other. Understanding why is the difference between a system that works and one that disappoints.

The two numbers

Flow rate is volume per unit time — how much water moves. Australian pump specifications use litres per minute (L/min), though cubic metres per hour (m³/h) appears on submersible and bore pumps.

To convert: 1 m³/h = 16.67 L/min.

Head is the vertical height a pump can lift water, in metres. It is used instead of pressure because it is independent of the fluid and easier to work with in the field — you can measure head with a tape measure.

Head and pressure are directly related:

Head	Pressure (bar)	Pressure (kPa)	Pressure (psi)
10 m	1.0	100	14.5
20 m	2.0	200	29
30 m	2.9	290	43.5
43 m	4.2	420	61
50 m	4.9	490	71
132 m	12.9	1290	188

The working rule: 1 bar ≈ 10.2 metres of head ≈ 14.5 psi.

Why you never get both at once

A pump has a fixed amount of energy available. It can spend that energy pushing water high, or pushing a lot of water, but not both.

At zero head — the outlet is at pump level, wide open — all the energy goes into moving volume. This is maximum flow, sometimes called free flow or open discharge.
At maximum head — the outlet is as high as the pump can possibly push — all the energy goes into lifting. Flow drops to nothing. This is the shut-off head.
Everywhere in between, you get a trade: more head, less flow.

Plot that trade and you get the pump curve, which slopes down from left to right. Your system sits at exactly one point on that curve.

What this means in practice

Take the RM-JP100: 750 W, maximum flow 55 L/min, maximum head 43 m.

Fill a bucket at the pump — close to 55 L/min.
Feed a bathroom 25 m of head away — meaningfully less than 55 L/min, but plenty for the job.
Try to push water 43 m straight up — the water arrives, but barely trickles.
Ask for 45 m — nothing comes out at all.

The pump has not failed in that last case. It has been asked for more head than it can produce.

Calculating total dynamic head

Total dynamic head (TDH) is what your system demands of the pump. It has three parts.

1. Static head

The vertical distance from the water surface to the highest point of discharge. Measure it with a tape or from site levels — horizontal distance does not count here, only vertical.

If the pump is above the water (drawing from a tank outlet below it, or a dam), the suction lift counts as part of the static head too.

2. Pressure head

How much pressure you want left over at the outlet. If you are simply filling a tank, this is zero — you only need water to arrive. If you are supplying a house or sprinklers, you need residual pressure:

Use	Residual pressure wanted	Add to head
Filling a tank or trough	0 bar	0 m
Garden hose, drip irrigation	1.0–1.5 bar	10–15 m
Household taps and showers	2.0–3.0 bar	20–30 m
Sprinkler systems	1.5–2.5 bar	15–25 m

3. Friction head

Water rubbing against pipe walls costs energy. Friction loss rises steeply as pipe diameter falls and as flow rate rises — roughly with the square of velocity — which is why undersized pipe is such a common cause of poor performance.

A reasonable field estimate for correctly sized pipe is 5% of the total pipe run length. So 100 m of well-sized pipe costs about 5 m of head. Undersized pipe can cost several times that. Add extra for fittings: each elbow, valve or sharp bend is worth roughly 0.5–1 m.

Keeping water velocity below about 2 m/s on the discharge side keeps friction sensible:

Pipe / hose size	Comfortable flow at ~2 m/s
1” (25 mm)	up to ~60 L/min
1.5” (38 mm)	up to ~135 L/min
2” (50 mm)	up to ~235 L/min
3” (76 mm)	up to ~540 L/min
4” (100 mm)	up to ~940 L/min

On the suction side aim lower still, around 1–1.5 m/s, and never reduce the suction pipe below the pump’s inlet size.

Putting it together

TDH = static head + pressure head + friction head

Worked example

A farmhouse on tank water. The tank outlet is 2 m below the pump. The furthest bathroom is 6 m above the pump, 45 m of pipe away, through about six bends. Household pressure wanted.

Component	Calculation	Head
Suction lift	Tank outlet 2 m below pump	2 m
Static rise	Pump to bathroom	6 m
Pressure head	Household use, ~2.5 bar	25 m
Friction, pipe	5% of 45 m	2.3 m
Friction, fittings	6 bends × ~0.7 m	4.2 m
Total dynamic head		≈ 40 m

At 40 m of head, the RM-JP100 (43 m maximum) is right at the edge of its range — it would work, but with little flow to spare. The RM-SJP800 at 48 m maximum head, or the RM-JP200 at 50 m head and 130 L/min, gives comfortable margin.

That margin is the point of doing the calculation.

Reading a pump curve

Where a manufacturer publishes a curve, use it rather than the headline numbers.

Find your required flow on the horizontal axis.
Go up to the curve.
Read across to the head axis.

If that head is above what your system demands, the pump will do the job. If it is below, it will not — regardless of what the maximum head figure says.

The best efficiency point sits in the middle of the curve, not at either end. A pump running near shut-off head wastes energy and heats up; a pump running at free flow can overload its motor and cavitate. Choose a pump whose curve puts your operating point somewhere in the middle third.

Common mistakes

Counting horizontal distance as head. A 200 m run across flat ground adds friction, not static head. Roughly 10 m of head for 200 m of correctly sized pipe — not 200 m.

Forgetting residual pressure. Water reaching the shower head is not the same as a usable shower. Add the pressure head.

Sizing on maximum flow. The 55 L/min figure applies at zero head. At your actual head you will get less.

Undersizing pipe to save money. Halving the pipe diameter multiplies friction loss many times over. Pipe is cheaper than a bigger pump and cheaper than electricity over the pump’s life.

Ignoring suction lift. Every metre the pump sits above the water is a metre of head it must produce before any water leaves the outlet — and it eats into the 7–8 m practical suction limit discussed in How to Choose the Right Water Pump.