Water Horsepower Calculator

Understanding Water Horsepower Calculator

Water Horsepower is the minimum power theoretically required to move a given volume of water against a given head.

How to use this calculator

1. Enter the flow rate \(Q\) in gallons per minute.
2. Enter the total head \(H\) in feet.
3. Enter the specific gravity \(SG\), which is usually \(1\) for water.
4. Read the water horsepower result, which is the theoretical minimum power in horsepower.

The formula explained

The formula \(WHP = \frac{Q \times H \times SG}{3960}\) computes the theoretical horsepower needed to move a liquid. For water, \(SG\) is usually \(1\), so the result depends mainly on flow and head.

• Q = flow rate in gallons per minute
• H = total head in feet
• SG = specific gravity of the liquid, compared with water
• WHP = water horsepower, the theoretical minimum power in horsepower

Step by step method

1. Multiply the flow rate \(Q\) by the head \(H\).
2. Multiply that result by the specific gravity \(SG\).
3. Divide by \(3960\) to get water horsepower.

Worked example

Problem. Find the water horsepower for a pump moving \(250\) gallons per minute against a head of \(60\) feet with water \(SG = 1\).

1. Use \(WHP = \frac{Q \times H \times SG}{3960}\).
2. Substitute the values, \(WHP = \frac{250 \times 60 \times 1}{3960}\).
3. Compute the result, \(WHP \approx 3.79\).

Tips and common mistakes

• Make sure the flow rate is in gallons per minute and the head is in feet, because the constant \(3960\) depends on those units.
• This gives theoretical power only, so real pump power will be higher because of efficiency losses.

Frequently asked questions