Flow Rate Calculator
Find the flow rate in gallons per minute (and other units) from a pipe's diameter and velocity, or from a volume and time.
Change the values and press Calculate to work out your own figures.
From pipe size and velocity
Enter the inside diameter and the velocity.
Show the working
- flow = π × (diameter ÷ 2)² × velocity
From a volume and time
Enter a volume and how long it takes.
Show the working
- flow = volume ÷ time
How to use it
Enter the inside diameter and the velocity. Enter a volume and how long it takes.
Lengths can be decimals, fractions or tape entries such as 12' 6 1/2" (in feet or inches). Prices are optional.
Key facts
flow = π × (diameter ÷ 2)² × velocity, or volume ÷ time
Bucket test
Time how long a 5 gallon bucket takes to fill to find a tap's or pump's flow: 30 seconds is 10 gpm.
Questions
What's the flow of a 1 in pipe at 3 ft/s?
7.34 gallons per minute.
Formulas
flow = π × (diameter ÷ 2)² × velocity, or volume ÷ time
Sources
Limitations
- Material densities are typical values (the midpoint of the commonly published range); real loads vary with moisture, grading and compaction, so check with your supplier or type in their density.
- Tank results are the geometric volume; real tanks have wall thickness and fittings.