Pipe Flow Rate Calculator Formula

Understand the math behind the pipe flow rate calculator. Each variable explained with a worked example.

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Formulas Used

Flow Rate

flow_cfs = area_sqft * velocity

Flow Rate

flow_gpm = area_sqft * velocity * 448.831

Variables

VariableDescriptionDefault
diameterInside Diameter(in)2
velocityFlow Velocity(ft/s)5
radius_ftDerived value= diameter / 2 / 12calculated
area_sqftDerived value= pi * pow(radius_ft, 2)calculated

How It Works

Pipe Flow Rate

Q = A x V

Where A is the cross-sectional area of the pipe (pi x r squared) and V is the average fluid velocity. Convert inches to feet before computing the area. Multiply cu ft/s by 448.831 to get gallons per minute.

Worked Example

A 2-inch inside-diameter pipe with water flowing at 5 ft/s.

diameter = 2velocity = 5
  1. 01Radius in feet = 2 / 2 / 12 = 0.0833 ft
  2. 02Area = pi x 0.0833 squared = 0.02182 sq ft
  3. 03Q = 0.02182 x 5 = 0.1091 cu ft/s
  4. 04GPM = 0.1091 x 448.831 = 48.95 GPM

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