How to Calculate Pump Head, Flow and HP (With Worked Examples)

To size a pump you need two numbers: total head (how high the water must be lifted, plus losses, in metres) and flow (how much water per minute or per hour). From these, motor power can be estimated with a simple formula: HP ≈ (Flow in LPM × Total head in m) ÷ (4,500 × pump efficiency). This guide explains each step with two practical examples.

The three numbers you need

• Head (metres): the total height the pump has to push water, plus friction and any outlet pressure. 1 metre is about 3.28 feet.
• Flow (LPM or LPH): the volume of water delivered per minute or per hour.
• Power (HP or kW): the motor size needed to deliver that flow at that head. 1 HP is about 0.746 kW.

Head and flow are linked: as head rises, a pump delivers less water, as shown on its performance curve.

Step 1: Calculate total head

Total head = static head + friction losses + pressure required at the outlet

Static head

Static head is the vertical distance the water is lifted, from the lowest water level to the highest point of discharge.

• Surface pumps (monoblocks): add the suction lift (water level to pump) and the delivery height (pump to outlet). In practice, keep suction lift within about 6–7 metres; beyond that, pumps struggle to prime.
• Submersible pumps: measure from the pumping water level, the level the water drops to while the pump runs, up to the outlet. In a borewell, this is lower than the resting level.
• Buildings: allow about 3 metres per floor as a rule of thumb, plus the tank height above the roof.

Friction losses

Pipes, bends, valves and foot valves all cause friction, which rises sharply when pipes are too narrow.

• For short domestic runs with properly sized pipes, add roughly 15–25% of the static head.
• For long pipelines, many bends, or drip systems with filters, use the pipe manufacturer's friction charts.
• As a rough guide, choose pipe sizes that keep water velocity around 1–2 metres per second.

Pressure required at the outlet

Drip and sprinkler systems need pressure at the outlet, so add it to the head. 1 bar (about 1 kg/cm²) is roughly 10 metres of head. Drip systems often run at around 1–1.5 bar, while many sprinklers need 2 bar or more; use your irrigation designer's figure.

Step 2: Work out the flow

• Filling a tank: Flow (LPM) = tank capacity in litres ÷ desired fill time in minutes.
• Homes and buildings: Indian city water planning commonly uses around 135 litres per person per day. Multiply by occupants, then decide how quickly you want to pump it.
• Irrigation: usually specified by the drip or sprinkler system designer.
• Borewells: flow is capped by the bore's yield. A pump that delivers more than the bore can supply will run dry.

Useful conversions

Step 3: Calculate HP

The hydraulic power needed to lift the water is approximately:

• Hydraulic HP ≈ (Flow in LPM × Head in m) ÷ 4,500
• or, in kilowatts, Hydraulic kW ≈ (Flow in LPS × Head in m) ÷ 102

No pump is 100% efficient, so divide by the pump efficiency (the share of the motor's output that becomes water power):

• Motor HP ≈ (Flow in LPM × Head in m) ÷ (4,500 × efficiency)

As an approximate guide, small domestic pumps often have an efficiency of around 30–45%, while larger, well-matched pumps can reach 50–60%. If unsure, use 0.4–0.5 for a first estimate, then round up to the next standard motor size without big jumps. Dividing by the overall pump-set efficiency (pump and motor together) instead gives roughly the power drawn from the supply.

Worked example 1: Sump to overhead tank in a house

An independent G+2 house in Hyderabad has an underground sump and a 2,000 litre roof tank that should fill in about 30 minutes.

1. Suction lift: the lowest sump water level is 2.5 m below the pump.
2. Delivery height: three floors × 3 m = 9 m, plus 2 m to the tank inlet = 11 m.
3. Static head: 2.5 + 11 = 13.5 m.
4. Friction: about 25 m of reasonably sized pipe with a few bends; add 20% ≈ 2.7 m.
5. Total head: about 16 m.
6. Flow: 2,000 L ÷ 30 min ≈ 67 LPM.
7. Motor HP: (67 × 16) ÷ (4,500 × 0.4) ≈ 0.6 HP.

A 0.75–1 HP pump is a practical choice, and with modest suction lift and head, a single phase self-priming monoblock is the type usually considered. Confirm the exact model against its pump curve.

Worked example 2: Borewell for drip irrigation on a farm

A farmer in Telangana has a borewell feeding a drip system. In summer, the water level while pumping drops to about 60 m (roughly 200 feet) below ground. The drip designer specifies 100 LPM at 1.2 bar.

1. Pumping water level to ground: 60 m.
2. Delivery height above ground: 2 m.
3. Drip pressure: 1.2 bar ≈ 12 m.
4. Friction in column pipe, mains, filters and fittings: assume about 8 m.
5. Total head: 60 + 2 + 12 + 8 = 82 m.
6. Flow: 100 LPM (6,000 LPH).
7. Motor HP: (100 × 82) ÷ (4,500 × 0.45) ≈ 4 HP.

A 5 HP borewell submersible is a sensible starting point if the bore can yield 100 LPM through summer. Pump diameter follows the bore size, stage count follows the pump curve, and the pump must sit safely below the lowest pumping level. See our borewell pump size guide and agricultural irrigation pump guide.

Step 4: Check the pump curve, not just the HP

Two pumps of the same HP can deliver very different flow at the same head. After calculating head and flow:

• Find a pump whose curve passes through, or just above, your duty point (required head and flow).
• Prefer a pump that operates near the middle of its curve, where efficiency is usually best.
• Avoid a pump that only reaches your head at the edge of its curve, or one that delivers far more flow than needed.

Common sizing mistakes

1. Using the resting water level for borewells instead of the summer pumping level.
2. Ignoring friction, especially with long or narrow pipes.
3. Forgetting outlet pressure for drip and sprinklers.
4. Oversizing, which wastes electricity and can empty a sump or bore quickly. See our tips on energy-efficient water pumps.

Need help with sizing?

Every site is different, so Aqua Pump Engineer can check your numbers and suggest a suitable pump from the Arrot-z product range. Call +91-7799794047 or +91-9790854516 with your water source, depths, pipe length and water requirement.