Flow Rate Calculator

Enter either the pipe diameter and velocity or the volume of fluid passing in a given time to calculate the volumetric flow rate.

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How to Calculate Flow Rate

Volumetric flow rate, also referred to as volume flow rate, flow rate, or volume velocity, is defined as the volume of fluid passing a point in a given unit of time.[1] The SI unit of flow rate is cubic meters per second (m³/s).

Flow rate can be found by simply measuring the amount of fluid that fills a bucket in a given amount of time or using a formula.

Flow Rate Formula

The flow rate of a fluid can be calculated using the volumetric flow rate equation:

Q = A × v

Thus, the volume flow rate Q is equal to the cross-sectional area of the pipe A times the velocity of the fluid v.[2]

diagram illustrating the flow rate formula Q = A x v

To use the flow rate formula, substitute the cross-sectional area of the pipe and the velocity of the liquid. For a round pipe, the cross-sectional area can be found using the formula for circle area A = πr².

For example, let’s find the flow rate for the fluid passing through a 1″ diameter pipe at a velocity of 3 feet per second.

Start by finding the cross-sectional area of the pipe.

area = π × r²
area = 3.14 × (1 ÷ 2)²
area = 3.14 × 0.25
area = 0.785398 in²

Next, it will be easier to solve later if the velocity and the area are in the same units. Convert the area in square inches to square feet by dividing by 144.

A = 0.785398 in² ÷ 144
A = 0.00545 ft²

Finally, substitute the area and velocity in the flow rate equation and solve.

Q = 0.00545 in² × 3 ft/sec
Q = 0.01635 cubic ft/sec

Thus, the flow rate is 0.01635 cubic feet per second.

Alternate Flow Rate Formula

An alternative formula to solve flow rate is:

Q = V / t

The volume flow rate Q is equal to the volume of liquid passed V divided by the time passed t.

To use this formula, you need to be able to measure the volume of liquid that passes in a given time.

For example, let’s find the flow rate for a garden hose that can fill a 5-gallon bucket in 3 minutes.

Q = 5 / 3
Q = 1.67 gal/min

Thus, the flow rate is 1.67 gallons per minute.

Try our unit rate calculator to find the rate of other units.


  1. Engineers Edge, Fluid Volumetric Flow Rate Equation, https://www.engineersedge.com/fluid_flow/volumeetric_flow_rate.htm
  2. Chris Deziel, How to Calculate Flow Rates, Sciencing, https://sciencing.com/calculate-flow-rates-6199676.html