Calculating water horsepower

How to the calculate the water horsepower coming out of the pumpĀ 16-12

Horsepower is measured using the units, foot pounds with one horsepower equal to 33,000 foot pounds. Since fluid has weight we can calculate how many pounds per minute we are pumping by finding out how much a gallon of our fluid weighs. After you have done that, multiply the gallons per minute you are pumping by 8.33 (the weight of a gallon of water) and then multiply that result by the specific gravity (the weight) of your fluid, and you will have the pounds per minute number you are looking for.

Once you have the pounds per minute you are pumping, you can multiply that number into the feet of head the pump is producing and you have foot pounds per minute that can be converted to horsepower.

Please take a look at the following pump curve.

Let’s use this chart for our example:

You are using the 13 inch impeller at 1750 rpm and pumping 300 gallons per minute of a fluid with a specific gravity of 1.0 to a head of 168 feet

300 gpm x 8.33 x 1.0 sg. = 2,499 pounds of fluid per minute.

We are pumping this fluid to a head of 168 feet so:

2,499 x 168 = 419,832 foot pounds per minute

Since 33,000 foot pound per minute equals one horsepower. We will divide and get:

419,832 / 33,000 = 12.73 horsepower.

This means that the pump is putting out 12.73 horsepower. Now, the next question is how much actual horsepower is required to do this?

Please take a look at the ascending lines on the bottom of the chart. Each line represents a different size impeller with the top line showing the horsepower required for a 13-inch impeller and the bottom line for a 9-inch impeller. The horsepower required is shown in the left column under bhp. (brake horsepower). Notice that it calls for 20 horsepower to move 300 gallons per minute with a 13-inch impeller.

If the pump were 100% efficient, all you would need would be 12.73 horsepower motor to drive the pump and it would do the job, but motors and pumps are not 100 % efficient because of friction losses and heat generation. This means that our actual efficiency is

12.73 hp out / 20 hp in = 0.64 or 64% efficient

Suppose the specific gravity of the fluid you are pumping is different than 1.0 (cold water). Just plug the new number into the formula and multiply the pump curve bhp by the same number. Using a specific gravity of 1.3, the change would look like this:

300 x 8.33 x 1.3 = 3248.70 pounds per minute

3248.70 x 168 = 545,781.60 foot-pounds per minute

545,781.6 / 33000 = 16.54 water horsepower out of the pump


20 x 1.3 = 26 horsepower is going into the pump

16.54 / 26 = 0.64 efficient


  • On February 18, 2018