Pump Horsepower Calculator

About the Pump Horsepower Calculator

Calculates the brake horsepower a pump needs from flow (GPM), total head (ft), fluid specific gravity and pump efficiency.

How to use this calculator

1. Enter the flow rate, usually in gallons per minute, as \(Q\).
2. Enter the total head in feet as \(H\) and the specific gravity of the fluid as \(SG\).
3. Enter the pump efficiency as a decimal in \(\\eta\), such as \(0.75\) for \(75\\%\).
4. Use the result to compare hydraulic power and brake horsepower, then choose a suitable motor size.

The formula explained

The formula computes brake horsepower, which is the input power the pump needs after accounting for fluid load and efficiency. It uses flow rate, head, specific gravity, and efficiency to estimate the power required.

• Q = flow rate, in gallons per minute
• H = total head, in feet
• SG = specific gravity of the fluid
• \(\eta\) = pump efficiency, written as a decimal
• BHP = brake horsepower, the power required at the pump shaft

Step by step method

1. Find the hydraulic load by multiplying \(Q\), \(H\), and \(SG\).
2. Multiply \(3960\) by the efficiency \(\\eta\).
3. Divide the numerator by the denominator to get \(BHP\).

Worked example

Problem. A pump moves \(250\) gallons per minute against a head of \(60\) feet. The fluid has a specific gravity of \(1.2\), and the pump efficiency is \(0.75\). Find the brake horsepower.

1. Substitute the values into \(BHP = \frac{Q \cdot H \cdot SG}{3960 \cdot \eta}\).
2. Compute the numerator, \(250 \cdot 60 \cdot 1.2 = 18000\). Compute the denominator, \(3960 \cdot 0.75 = 2970\).
3. Divide, \(BHP = \frac{18000}{2970} \approx 6.06\).

Tips and common mistakes

• Make sure efficiency is entered as a decimal, not as a percent, so \(75\\%\) becomes \(0.75\).
• Use the correct head in feet, because a unit mismatch will give a wrong horsepower result.

Frequently asked questions