Brake Horsepower Calculator

Brake Horsepower (BHP) is the mechanical power that must be delivered to a pump shaft so the pump can overcome fluid head and move the required volume.

Use this calculator when you need to size motors, estimate electrical power draw, or check whether a selected pump will meet a specified flow and pressure at a given efficiency. BHP is the industry value pump manufacturers use to match pumps and drive motors.

How to use this calculator

1. Enter the pressure the pump must produce (use the unit selector to match psi, kPa, feet or meters of water, etc.).
2. Enter the flow rate you need (choose gpm, lpm, cms, cfs, acre-in/day, etc.).
3. Enter the pump efficiency (from the pump curve or manufacturer expressed as %).
4. Enter the motor (drive) efficiency (the motor’s efficiency in %).
5. Click Calculate. Results show Brake Horsepower (HP and kW) and the total power input required from the electrical drive (HP and kW).

How the calculator works

1. Your inputs (pressure, flow, pump efficiency, motor efficiency) are first converted to common units (pressure → psi, flow → gallons per minute).
2. The core hydraulic relation used is the standard pump BHP expression that relates flow (Q), head (H) and efficiency (η):
   BHP = (Q × H) / (3960 × η) when Q is in gpm and H is in feet of water. That “3960” is the unit conversion constant that converts gpm × feet to horsepower in imperial units. If you supply pressure (psi) instead of head, the tool converts psi to feet of water before applying the formula.
3. After computing BHP, the calculator computes total power input required by dividing the BHP by motor efficiency (as a decimal). Finally the code converts horsepower to kilowatts for display (1 HP ≈ 0.7457 kW).

PracticalTtips

• Always use the pump’s efficiency at the actual operating point (not the peak efficiency) pump curves show efficiency vs flow. If you don’t have a curve, use a conservative estimate.
• If you pump a fluid denser than water, include specific gravity in the calculation (your calculator currently assumes water unless you add an SG parameter). Many hydraulic formulas include SG as a multiplier with head or in the numerator.
• Round up the motor size to the next standard nameplate and add a safety margin when the pump runs continuously or near maximum duty.
• For well pumps: add the depth to the pumped water surface (converted to pressure/head) when determining required head.

What each input means

• Pressure: the pressure the pump needs to produce at the outlet (or the pressure equivalent of the total head). Use the unit selector to match how you measured it.
• Pressure Unit: selects the unit and tells the calculator how to convert to psi or feet of water.
• Flow Rate: how much liquid you want moved per time (gpm, lpm, m³/h, etc.).
• Flow Rate Unit: pick the flow unit used during measurement or design. The tool converts it internally to gallons per minute for the BHP formula.
• Pump Efficiency (%): the pump’s mechanical/hydraulic efficiency (from pump curve), not motor efficiency. Enter as percent (e.g., 75).
• Drive Motor Efficiency (%): the electrical-mechanical efficiency of the motor driving the pump (enter percent; used to calculate electrical input power).

Sources used

• Washington State Univ. Required Water Pump Horsepower (equations, units, usage notes).
• Calculator Academy. pump BHP formula and guidance on Q, DH, SG and E.
• PowerZone / Power Zone Equipment. pump power and unit-conversion approach for BHP.
• EngineeringToolbox. pump/fan power relationships and unit explanations (HP ↔ kW, AHP/BHP concept).
• CED / pump engineering references (PDF). BHP formula including SG and SI equivalents.