What Size Pump Do I Need for Drip Irrigation

What Size Pump for Drip Irrigation in One Sentence

The question what size pump for drip irrigation has one clear answer. Choose a pump that delivers the flow of your largest zone at the total head your system needs, with a margin, and check that result on the pump curve. Total head is lift plus pressure head plus friction.

Start With Your Largest Zone Flow

A drip system often runs in zones, and each zone runs on its own. The pump does not need to feed every zone at once. It needs to feed the largest zone, the one with the highest flow, at the head that zone needs. That is the first number to settle, and it sets the rest of the sizing work.

Start by finding the flow of the largest zone in litres per hour. The Drip Irrigation Pump Calculator asks for that flow first, because head and power both depend on it. Use the largest zone, not an average zone. An average hides the peak demand, and the pump must meet the peak.

Work Out Total Head in Three Parts

Total head is the total resistance the pump works against, in metres or feet of water. Lift is the height from the water surface to the highest emitter. Pressure head comes from the pressure the drip line needs. Friction is the head lost in the supply pipe. Add the three parts together.

Pressure head equals pressure times 10.2, in metres per bar. Friction equals pipe length times loss percent. Total head equals lift plus pressure head plus friction. Power equals 9.81 times flow in litres per second times head in metres divided by efficiency. Divide litres per hour by 3600 to get litres per second.

Measure lift from the water surface to the highest emitter, not from the ground under the tank. Pressure head is often the largest part of total head. At 1 bar, it is 10.2 metres. Friction is an allowance. The calculator gives no pipe diameter friction table, so friction here is pipe length times the loss percent you enter. Small pipes and many fittings add more friction.

The table below works through five metric examples. Each row uses 1 bar, so pressure head is 10.2 metres in every row.

Lift (m)	Pressure head (m)	Pipe length (m)	Friction at 5 percent (m)	Total head (m)
1	10.2	20	1.0	12.2
2	10.2	30	1.5	13.7
3	10.2	50	2.5	15.7
4	10.2	60	3.0	17.2
5	10.2	80	4.0	19.2

Lift adds head metre for metre. At 0.5 bar, pressure head is 5.1 metres. At 1.5 bar, it is 15.3 metres. Use the pressure your drip line needs.

Work Through the Page Example Step by Step

The page example uses 600 litres per hour at 1 bar, with 3 metres of lift and 50 metres of pipe at 5 percent loss. Pressure head is 1 times 10.2, or 10.2 metres. Friction is 50 times 5 percent, or 2.5 metres. Add 3 plus 10.2 plus 2.5 to get 15.7 metres of total head.

Now find power. First change flow to litres per second. Divide 600 by 3600 to get 0.1667 litres per second. Then use the power formula. Multiply 9.81 by 0.1667 by 15.7, then divide by efficiency. At 50 percent efficiency, the result is 51.3 watts, stated on the page as about 51 watts. Note that 50 percent is used only inside this worked example. The calculator gives no default efficiency figure, so do not treat 50 percent as a rule for every pump.

The page then gives the pump choice: at least 720 litres per hour at 19 metres. That is about a 20 percent margin on each number. Multiply 600 by 1.2 to get 720. Multiply 15.7 by 1.2 to get 18.84, rounded to 19. The page does not name the margin as 20 percent, so treat the 20 percent as arithmetic from those figures, not as a page rule.

Power: What the Watts Mean

Power in watts is the work the pump must do to move the flow against the head. It follows from flow, head and efficiency, not a size you pick on its own. A higher watt rating does not make a pump right. The right pump gives your flow at your head.

The table below uses the same total head values, with power at 50 percent efficiency, as in the page example. Each power value is rounded to the nearest whole watt.

Flow (L/h)	Flow (L/s)	Total head (m)	Power at 50 percent efficiency (W)
300	0.0833	12.2	20
600	0.1667	15.7	51
900	0.25	19.2	94
1200	0.3333	13.7	90

The 900 litre per hour row needs 94 watts, more than the 1200 litre per hour row at 90 watts, because its head is higher at 19.2 metres against 13.7 metres. Flow alone does not decide power. Head alone does not decide power. The two together, divided by efficiency, decide power.

Read a Pump Curve the Right Way

A pump curve shows what a pump can do across a range of heads and flows. The calculator tells you what flow and head to look for on a curve. It gives no pump curve data itself, so the curve you read must come from the pump you are considering. Read it at your head, not at the maximum head.

Maximum head is the head a pump can reach when flow is near zero. It is not the head at which the pump delivers useful flow to a drip zone. A pump with a high maximum head may still deliver little flow at the head your system needs. Always check flow at your head, not the maximum head.

To read a curve, start with your total head on the head axis. Move across to the curve, then down to the flow axis to read the flow at that head. If that flow meets your largest zone flow, with a margin, the pump is a candidate. If it falls short, the pump is not a candidate, even if its maximum head looks impressive.

If you need the flow for a zone first, the Drip Flow Rate Calculator can help you work it out. The Drip Irrigation Planner can help you set out the zone. Use the flow and the layout together, then size the pump to match them.

Add a Margin Before You Choose

The page example chooses at least 720 litres per hour at 19 metres for a need of 600 litres per hour at 15.7 metres. That margin is room for the real system to differ a little from the plan. Find the flow and head you need, then look for a pump that gives at least that flow at at least that head, with a margin on both numbers.

The table below applies about a 20 percent margin to the earlier rows, as arithmetic from the page example. Flow is multiplied by 1.2. Head is multiplied by 1.2 and rounded up to the next whole metre.

Need: flow (L/h)	Need: total head (m)	Look for at least: flow (L/h)	Look for at least: head (m)
300	12.2	360	15
600	15.7	720	19
900	19.2	1080	24
1200	13.7	1440	17

Use the table as a worked pattern, not as a buying list. Work out need, add a margin, then check the pump curve at the margined head. If the pump gives the margined flow at the margined head, it meets the test. Do not use margin as a reason to choose a pump far larger than the need. A pump far larger than the need can push too much pressure into the drip line.

Imperial Examples in Feet and Psi

Metric and imperial examples are kept separate here, and the imperial figures below are rounded equivalents, not exact conversions. One bar equals about 10.2 metres, or 33.5 feet, of head. One psi is 2.31 feet of head. Gravity gives only about 0.1 bar, or 1.4 psi, per metre, or 3.3 feet, of height.

The table below turns pressure in psi into feet of head with the page factor of 2.31 feet per psi.

Pressure (psi)	Head (ft)
10	23.1
15	34.7
20	46.2
25	57.8

A pressure of 15 psi gives 34.7 feet of head, close to the head from 1 bar, which the page states as 33.5 feet. Do not mix a metric head with an imperial pressure in the same calculation. Pick one set of units and work through the whole sizing in those units. The page example in imperial terms, as a rounded equivalent, is a total head of about 52 feet, with a pump choice of at least about 63 feet. Flow stays in litres per hour, because the page gives no flow conversion to gallons.

Rain Barrels, Wells and Low Gravity Head

A rain barrel on its own gives very little head. Gravity gives only about 0.1 bar, or 1.4 psi, per metre, or 3.3 feet, of height. A barrel raised by one metre gives about 0.1 bar, far below the 1 bar in the worked example. Use low pressure drip kits or a small pump.

Measure from the water surface in the barrel to the highest emitter, then multiply the height in metres by 0.1 bar per metre to get the gravity pressure. If that pressure falls short, you need a low pressure kit or a small pump. The Rain Barrel Calculator can help you work out what a barrel can supply.

Wells need a different check. The page states that wells need a pump rated for the water depth below ground. Lift is measured from the water surface in the well to the highest emitter, and that lift can be large. Check the pump rating against the water depth, not only against the total head at the surface.

FAQs

Q: What size pump do I need for drip irrigation?
A: Choose a pump that delivers the flow of your largest zone at the total head your system needs, with a margin, and check that result on the pump curve. Total head is lift plus pressure head plus friction.

Q: What is total dynamic head?
A: Total dynamic head is the total resistance the pump works against, in metres or feet of water. It is height, pressure and friction combined.

Q: How do I read a pump curve for drip irrigation?
A: Find your total head on the head axis, move across to the curve, then read the flow the pump gives at that head. Choose the pump only if that flow meets your largest zone flow with a margin. Do not choose by maximum head alone.

Q: Can a rain barrel run drip irrigation without a pump?
A: Gravity gives only about 0.1 bar, or 1.4 psi, per metre, or 3.3 feet, of height, so a rain barrel on its own gives very little head. Use low pressure drip kits or a small pump.

Q: How do I convert psi to feet of head?
A: Multiply psi by 2.31 to get feet of water. A pressure of 15 psi gives 34.7 feet of head.