Choose a calculation mode, enter your known values, and get instant results
Enter flow, head & efficiency → get horsepower
Flow Rate
Total Head
Spec. Gravity
Enter HP, head & efficiency → get flow rate
Motor Power
Total Head
Spec. Gravity
Enter HP, flow rate & efficiency → get required head
Motor Power
Flow Rate
Spec. Gravity
Pump Efficiency
70%
10%30%55%75%99%
Use the pump's curve if you have it. Rough guide: small household pumps about 40–55%, larger centrifugal pumps near their best efficiency point 60–80%.
⚠ Please enter valid numbers (greater than zero) for all required fields.
Your Results
Brake Horsepower
—
HP
Power (kW)
— kW
Power (Watts)
— W
Water HP (WHP)
— HP
Flow Rate (GPM)
— GPM
Flow Rate (LPM)
— LPM
Flow Rate (m³/h)
— m³/h
Head (ft)
— ft
Head (m)
— m
Efficiency
— %
Pump Power Breakdown
📖 How To Use
How to Use This Pump Horsepower Calculator
This pump power calculator handles three common scenarios. Pick the one that matches your situation:
Choose a calculation mode
Use "Calculate HP" when you know your required flow and head and need to select a motor. Use "Calculate Flow Rate" when you have a fixed motor and want to know how much water it can move. Use "Calculate Head" when you need to find the maximum lift a given pump can achieve.
Enter flow rate and unit
Flow rate is how much water moves per unit of time — typically measured in GPM (gallons per minute) or LPM (litres per minute). Use the unit dropdown to match whatever your pump spec sheet or flow meter reports.
Enter total head and unit
Total head is everything the pump must overcome: the height from the water level it draws from to the outlet (static head), plus friction in the pipes and fittings, plus any pressure needed at the outlet. Enter it in feet, metres, psi or bar. If you only know the height, treat it as the minimum — the real total head is higher. The pipe size flow rate calculator shows how much head friction uses on a given pipe.
Set pump efficiency
Drag the slider to the pump's efficiency at your duty point — read it from the manufacturer's pump curve. Larger centrifugal pumps near their best efficiency point often reach 60–80%; small household pumps are usually lower, around 40–55%. If unsure, a lower value gives a safer (larger) motor size. A motor nameplate shows motor efficiency, not pump efficiency.
Hit Calculate — read all results
Results show brake horsepower (the shaft power the motor must deliver), water horsepower (the power that actually reaches the water), kilowatts, watts, and flow and head in several units. In HP mode the note under the results also suggests the next standard motor size and the electrical input. Copy or print with one click.
Always size up: Choose the next standard motor size above the calculated brake horsepower, leaving a margin — commonly 10–20%. A motor running at or beyond its rating runs hot and wears out sooner. For a 0.88 HP load, for example, a 1 HP motor (about 14% margin) is the usual choice; a 0.75 HP motor would be overloaded. Also check the motor covers the highest power on the pump curve, since a centrifugal pump can draw more when the head drops.
📐 The Formula
Pump Horsepower Formula Explained
Pump power comes from a simple energy balance: the power given to the water is its weight flow multiplied by the head it is raised through. In US units (specific gravity SG = 1.0 for water):
Brake HP (BHP) = (Flow GPM × Head ft × SG) ÷ (3960 × Efficiency)
Water HP (WHP) = (Flow GPM × Head ft × SG) ÷ 3960
BHP = WHP ÷ Efficiency
The constant 3,960 comes from the unit conversion: 1 HP = 550 ft·lbf per second = 33,000 ft·lbf per minute, and 1 US gallon of water weighs about 8.33 lb. So 33,000 ÷ 8.33 ≈ 3,960 GPM·ft per HP. If you know pressure instead of head, the same formula is HP = GPM × psi ÷ 1,714 — and for a fluid other than water, a given pressure equals a taller or shorter column (head = psi × 2.31 ÷ SG), which the calculator applies automatically.
For metric inputs, the equivalent formula is:
Power (kW) = (Flow m³/h × Head m × SG × 9.81) ÷ (3600 × Efficiency)
Power (HP) = Power (kW) ÷ 0.7457
Brake horsepower is the power at the pump shaft. The electricity the motor draws is higher again: input kW = shaft kW ÷ motor efficiency. Small single-phase motors are often around 65–80% efficient; larger premium-efficiency motors reach 90% and above.
Key Variables
Variable
Description
Typical Range
Flow Rate (Q)
Volume of fluid moved per unit time
1–10,000+ GPM
Total Head (H)
Static lift + friction losses + pressure needed at the outlet
5–500+ ft
Specific Gravity (SG)
Fluid density relative to water (water = 1.0)
0.8–1.8 typical
Pump Efficiency (η)
Ratio of water power out to shaft power in
about 0.40–0.85
Water HP (WHP)
Power delivered to the water (the minimum, with no pump losses)
Calculated output
Brake HP (BHP)
Actual shaft power required from motor
WHP ÷ efficiency
Unit Conversions
Convert From
Convert To
Factor
1 HP
kW
× 0.7457
1 kW
HP
× 1.3410
1 GPM
LPM
× 3.7854
1 LPM
GPM
× 0.2642
1 ft of head
m of head
× 0.3048
1 psi
ft of head (water)
× 2.3067
1 bar
ft of head (water)
× 33.456
📊 Reference Data
Typical Pump HP by Application
Rough motor sizes for common water pump jobs, for orientation only. The ranges are wide because head, pump type and efficiency vary — always size from your own flow and total head using the calculator above.
Application
Flow Rate
Head
Typical HP
Notes
Rooftop tank fill (residential)
10–25 LPM
8–15 m
0.25–0.5 HP
Single household
Rooftop tank fill (apartment)
30–80 LPM
12–20 m
0.5–1.5 HP
4–10 units
Borehole/deep well
10–50 LPM
30–120 m
0.5–3 HP
Submersible pump
Swimming pool circulation
100–300 LPM
10–25 m
1–3 HP
Runs for hours — efficiency matters
Agricultural irrigation
200–2000 LPM
15–50 m
2–30 HP
Field supply
Fire suppression system
500–2000 LPM
30–80 m
10–75 HP
Designed by specialists (e.g. to NFPA 20)
Municipal water supply
5000+ LPM
20–100 m
50–500+ HP
Distribution pump
⚡
Efficiency Sweet Spot
Centrifugal pumps are most efficient at their Best Efficiency Point (BEP). Operating far from BEP increases power consumption and wear.
📐
Always Use TDH
Total Dynamic Head (TDH) = static lift + friction losses + any pressure needed at the outlet. Friction depends on pipe diameter, length, fittings and flow. Never size a pump on height alone.
🔒
Safety Margin
Select the next standard motor size above the calculated BHP, commonly with a 10–20% margin. Check it also covers the highest power on the pump curve, not just your duty point.
🧮 Worked Example
Worked example: sizing a motor for 50 GPM at 100 ft
A pump must move 50 GPM of water (SG = 1.0) against a total head of 100 ft — the lift plus friction, worked out beforehand. The pump curve shows 70% efficiency at that point.
How do I calculate pump horsepower from flow rate and head?
Use the standard formula: BHP = (GPM × Head ft × SG) ÷ (3960 × Efficiency). For example, a pump moving 50 GPM against 100 ft of head at 70% efficiency handling water (SG = 1.0) requires (50 × 100 × 1.0) ÷ (3960 × 0.70) = 1.80 HP. This calculator does all the conversions automatically, so you can input any unit combination.
What is the difference between water horsepower and brake horsepower?
Water horsepower (WHP) is the theoretical minimum power needed to move the fluid — calculated assuming 100% efficiency. Brake horsepower (BHP) is the actual shaft power the motor must deliver, accounting for all efficiency losses in the pump. The relationship is: BHP = WHP ÷ Pump Efficiency. A pump with 70% efficiency needs about 43% more shaft power than the water horsepower (1 ÷ 0.70 = 1.43).
What is a good pump efficiency for water systems?
Most centrifugal pumps for water supply operate between 60% and 80% efficiency at their best efficiency point (BEP). Submersible pumps for deep wells typically run 55–75%. Small fractional-HP pumps (under 0.5 HP) often run only 40–55% due to losses at small scales. Values above 85% indicate premium high-efficiency industrial pumps. If you don't know your pump's efficiency, read it from the pump curve at your flow and head. Without a curve, about 40–50% is a safer guess for a small household pump and 60–65% for a larger centrifugal pump — a lower guess gives a bigger, safer motor size.
How do I convert pump horsepower to kilowatts?
1 mechanical horsepower = 0.7457 kilowatts. To convert HP to kW, multiply by 0.7457. To convert kW to HP, multiply by 1.3410. For example, a 2 HP pump = 2 × 0.7457 = 1.49 kW. This calculator automatically shows both HP and kW in the results — no manual conversion needed.
What flow rate do I need to fill a 1,000-litre tank in 30 minutes?
Required flow rate = tank volume ÷ fill time = 1,000 L ÷ 30 min = 33.3 LPM (2,000 L/h), or about 8.8 GPM — plus whatever water is being used while the tank fills. Lifting that flow 10 m takes 8.8 GPM × 32.8 ft ÷ 3,960 ≈ 0.073 HP of water horsepower (about 55 W). Pipe friction adds to the 10 m, so with, say, 3 m of friction the total head is 13 m and the water horsepower about 0.095 HP. A small pump at 45% efficiency then needs about 0.21 HP at the shaft, so the smallest common size, 0.25 HP, covers it with about 19% margin — as long as its curve shows it delivers 33 LPM at 13 m. The tank refill time calculator works the other way, from a known flow to a fill time.
Does pipe friction affect the pump horsepower I need?
Yes — significantly. The head input should include Total Dynamic Head (TDH), which is static head (the vertical lift) plus friction head losses in the pipe system. Friction losses increase with flow velocity, pipe length, smaller pipe diameter, and every elbow, valve, or fitting. As a very rough allowance on a short, simple residential run, some installers add 20–30% to the static head — but long or narrow pipes can lose far more. For proper sizing, work out the friction loss with the Hazen-Williams or Darcy-Weisbach equation, for example with the pipe size flow rate calculator, plus the loss through fittings and valves.
What size pump do I need for a rooftop water tank?
For a typical 3-storey building with a rooftop tank 10–12 m up and a 20 LPM fill rate: static head ≈ 12 m, plus friction ≈ 3 m, gives a total head of about 15 m. Water power = 1,000 kg/m³ × 9.81 × (20 ÷ 60,000 m³/s) × 15 m ≈ 49 W ≈ 0.066 HP. At 65% efficiency the shaft needs about 0.10 HP; at a more typical 45% for a small pump, about 0.15 HP. The smallest common size, 0.25 HP, covers either — but make sure the pump's curve actually delivers 20 LPM at 15 m, since that, not the horsepower alone, decides whether it works.
✅ Sources & Review
Sources and review
The formulas on this page are standard pump and unit-conversion relationships; the figures come from these references:
NIST SP 811 — unit conversions (1 HP = 745.7 W; 1 US gallon = 3.785 L; 1 ft = 0.3048 m; 1 psi = 2.31 ft of water).
Pump efficiency ranges are typical published values; your pump's own curve from the manufacturer is the authority for its efficiency and the flow it delivers at a given head.
Reviewed by John Mac, PE, Water Systems Specialist
Texas-licensed Professional Engineer with 13+ years in residential water storage and 180+ tank projects. John writes and reviews every calculator on this site.
