How to calculate water flow volume & pressure

Updated March 23, 2017

You can calculate the flow rate, volume and pressure in a water tank using formulas from physics. Volume is the amount of space that an object takes up, and may be measured in litres, gallons, or cubic meters. Flow is the rate at which a certain volume of liquid passes through an opening; it may be measured in litres per second or gallons per minute. Pressure is the amount of force per square unit of area, and is measured in pounds per square inch (psi) or Newtons per square meter (pascals).

Measure the width, length and height of the water in meters.

Multiply the width, length and height to compute the volume in cubic meters. The formula is V = WLH, where V is the volume, W is the width, L is the length and H is the height.

Convert cubic meters to litres by multiplying by 1,000. Convert cubic meters to gallons by multiplying by 264.17.

Place an empty container below the faucet or release valve.

Open the valve and use the stopwatch to time the water flow for 15 seconds.

Measure the number of litres or gallons in the container, and divide that number by 15. This gives the flow rate in litres per second or gallons per second. The formula is F = V/T, where F is the flow rate, V is the volume and T is the time.

To convert to litres per minute or gallons per minute, multiply the number you obtained above by 4.

Use the hydrostatic pressure formula P = pgh, where p is the density of water in kg per cubic meter, g is the gravitational acceleration constant, h is the height of the water above the valve in meters and P is the pressure in pascals.

For water near the surface of the earth, p = 1,000kg per cubic meter, and g = 9.81 meters per second squared.

Convert pascals to psi by dividing by 6,894.76.

Use this example as a guide to calculate water pressure in a tank.

The height of the water in a tank is 4 meters above the valve. Applying the hydrostatic pressure formula gives you P = (1,000)(9.81)(4) = 39,240 pascals. In pounds per square inch, the pressure is 39,240/6,894.76 = 5.69 psi.


For a cylindrical tank, V = (3.14159)HR^2, where H is the height of the cylinder and R is the radius.

Things You'll Need

  • Tape measure
  • Stopwatch
  • Bucket
  • Calculator
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About the Author

Nucreisha Langdon has written professionally since 1991. She has ghostwritten more than 20 romantic fantasy novels, while her nonfiction work has appeared in the "Gainesville Sun" and the "Austin Chronicle." Langdon holds a Bachelor of Science in mathematics and a Bachelor of Arts in English from the University of Florida.