Total head is the single number that decides whether a pump reaches the tank or stalls halfway up the hill. It is also the number most often guessed. Here is how to work it out properly.
Head is measured in metres, and it describes resistance rather than distance. A pump rated at 60 m will lift water 60 m vertically only if there is no pipe friction and no pressure required at the outlet — which never happens. In a real system, the vertical lift is only one of three components.
The three components of total head
1. Static head
The vertical distance from the water surface being pumped from to the point being discharged to. For a borehole, measure from the pumping water level (the level while the pump is running, not the rest level) up to the discharge point. That difference matters: drawdown on a hard‑working hole can be many metres.
2. Friction loss
Energy lost to the pipe walls and fittings. It rises steeply with velocity — roughly with the square of flow rate — so it is dominated by pipe diameter. Bends, valves, non‑return valves and strainers all add to it.
3. Pressure head
Any pressure required at the outlet. Discharging into an open tank needs none. A sprinkler line needing 2.5 bar at the nozzle needs 25 m of pressure head on top of everything else. Convert bar to metres by multiplying by 10.2.
The formula
Total head = static head + friction loss + pressure head
That is the whole calculation. The work is in getting each term right, particularly the friction loss.
Estimating friction loss
For a first pass, use a friction loss table and multiply by the pipe length. The figures below are indicative, for smooth plastic pipe (HDPE or uPVC) carrying clean water. Steel and older pipe run higher; always work from the internal diameter, not the nominal outside diameter.
| Internal diameter | Flow | Velocity | Friction loss per 100 m |
|---|---|---|---|
| 50 mm | 5 m³/h | 0.7 m/s | ~1.6 m |
| 50 mm | 10 m³/h | 1.4 m/s | ~5.6 m |
| 50 mm | 20 m³/h | 2.8 m/s | ~20 m |
| 80 mm | 20 m³/h | 1.1 m/s | ~2.4 m |
| 80 mm | 40 m³/h | 2.2 m/s | ~8.8 m |
| 100 mm | 40 m³/h | 1.4 m/s | ~3.0 m |
| 100 mm | 80 m³/h | 2.8 m/s | ~11 m |
| 150 mm | 100 m³/h | 1.6 m/s | ~2.2 m |
A useful sanity check: keep velocity below about 1.5 m/s on the suction side and between 1.5 and 2.5 m/s on the discharge side. Above 3 m/s you are paying for friction and inviting water hammer and erosion.
For fittings, adding 10 to 15 per cent to the straight‑pipe friction is adequate for a typical run. Where the pipework is short and fitting‑heavy — a plant room, a booster skid — calculate the fittings properly using equivalent lengths, because they can exceed the pipe itself.
A worked example
A farm needs 8 m³/h from a borehole to a storage tank. The pumping water level is 42 m below the collar, the tank sits 18 m above the collar on a rise, and the pipe run is 320 m. A float valve at the tank needs a little pressure to seat, so allow 5 m.
Option A: 63 mm OD HDPE (about 55 mm internal)
- Velocity at 8 m³/h: about 0.9 m/s — comfortable.
- Friction: roughly 2.8 m per 100 m, so about 9 m over 320 m.
- Add 10 per cent for fittings: about 10 m.
- Total head = 42 + 18 + 10 + 5 = 75 m
Option B: 50 mm OD HDPE (about 40 mm internal)
- Velocity at the same flow: about 1.8 m/s.
- Friction: roughly 13 m per 100 m, so about 42 m over 320 m.
- Add 10 per cent for fittings: about 46 m.
- Total head = 42 + 18 + 46 + 5 = 111 m
The pipe decides the pump. The same borehole, the same tank and the same flow. The narrower pipe adds 36 m of head — close to a 50 per cent larger duty, a bigger motor, and higher running cost for as long as the system exists. The wider pipe usually pays for itself within a season.
Suction lift and NPSH
For surface pumps drawing water upward, there is a second limit that head alone does not capture. Net positive suction head available (NPSHa) must exceed what the pump requires (NPSHr), with a margin. At sea level the theoretical ceiling on suction lift is about 10.3 m; in practice, friction, temperature and altitude bring it down to roughly 6 or 7 m, and less on the Highveld.
If NPSHa falls below NPSHr the pump cavitates: vapour bubbles form and collapse against the impeller, which sounds like gravel in the casing and destroys the impeller surface. The fix is almost always to move the pump closer to the water, enlarge the suction pipe, or switch to a submersible.
Errors that show up most often
- Using the rest water level. Always use the pumping level. Drawdown is part of the duty.
- Measuring pipe by its outside diameter. A 50 mm class 10 HDPE pipe has roughly 40 mm of bore. Friction follows the bore.
- Forgetting the pressure requirement. Irrigation and firefighting duties are dominated by it.
- Adding a safety margin to the head. Padding the head pushes the pump right along its curve and away from best efficiency. Size accurately instead.
Check it against the curve
Once you have flow and total head, find them on the pump curve. The intersection should sit near the best efficiency point, the motor should not be overloaded at the far right of the curve, and the NPSHr line at your flow should be comfortably below the NPSHa you calculated.
If you would rather have it checked, send us the measurements. Read How to Choose the Right Water Pump for the rest of the selection process, or browse borehole and submersible pumps.
Pressure, head and the conversions you will need
Pump curves, pressure gauges, irrigation specifications and municipal standards all use different units for the same thing. Getting these conversions wrong is a common source of badly sized pumps, so it is worth keeping the table below to hand.
| Unit | Equals in metres of head | Common use |
|---|---|---|
| 1 bar | 10.2 m | Pump gauges, irrigation emitters |
| 1 kPa | 0.102 m | Municipal and engineering specifications |
| 100 kPa | 10.2 m | Same as 1 bar |
| 1 psi | 0.70 m | Imported equipment, older equipment |
| 1 metre of head | 9.81 kPa | The unit pump curves use |
Pipe material matters as much as diameter
Friction loss depends on the roughness of the pipe wall as well as its bore. The Hazen-Williams coefficient C expresses that roughness – a higher number means a smoother pipe and less loss. Two pipes of the same nominal size in different materials can differ by 30 per cent in friction.
| Material | Hazen-Williams C (new) | C after 20 years |
|---|---|---|
| HDPE / PVC | 150 | 140 |
| Fibre cement | 140 | 120 |
| Galvanised steel | 120 | 90 |
| Cast iron, unlined | 130 | 80 |
| Concrete | 130 | 110 |
Two consequences follow. First, always check the internal diameter rather than the nominal size: a 50 mm class 10 HDPE pipe has an internal bore closer to 40 mm, and friction responds to the bore, not the label on the coil. Second, on a system that will be rehabilitated rather than replaced, size the pump for the pipe it will actually be pushing through in fifteen years.
The system curve: why one number is never the whole story
Total head is not a single fixed value. It is a curve. Static head stays constant whatever the flow, but friction loss rises with roughly the square of flow, so the total head the system demands climbs steeply as you push more water through it.
Plot that system curve on the same axes as the pump curve and they cross at exactly one point. That intersection – not the figure in the catalogue – is where the pump will actually operate. It is why a pump rated for 10 cubic metres per hour delivers seven on a restrictive system, and why opening a second irrigation block changes the duty of a pump nobody touched.
Two practical rules come out of this. On a system dominated by static head, flow is relatively insensitive to pipe sizing. On a friction-dominated system – long runs, small bores – flow is very sensitive to it, and one pipe size up is usually cheaper than one pump size up. The same distinction decides whether a variable speed drive will save much, as explained in how VSDs improve pump efficiency.
Not sure of your numbers? Send the borehole depth, the tank height, the pipe run and the flow you need to the technical enquiry form and we will calculate the duty point with you.