What Is the GPM Flow Rate Calculator?
This calculator finds the volumetric flow rate through a round pipe in gallons per minute (GPM) using the flow velocity and the pipe's inside diameter. It applies the standard relationship that flow rate equals velocity multiplied by the pipe's cross-sectional area, then converts cubic feet per minute into US gallons per minute with the factor 448.831.
How to Use It
Enter the flow velocity in feet per second (ft/s) and the inside diameter of the pipe in inches. The calculator converts the diameter to feet, computes the circular cross-sectional area, and returns the flow rate in GPM along with the area used.
The Formula Explained
The core equation is $$\text{GPM} = V \times A \times 448.831$$ where V is velocity in ft/s and A is the pipe area in square feet. The area comes from \(A = \pi \times r^2\), with \(r\) being the inside radius in feet. Because diameter is entered in inches, it is divided by 12 to convert to feet before halving. The constant 448.831 converts cubic feet per minute (ft³/min) to US gallons per minute, since 1 ft³ = 7.48052 gallons and there are 60 seconds in a minute (\(7.48052 \times 60 = 448.831\)).
Worked Example
Suppose water moves at 5 ft/s through a 2-inch inside-diameter pipe. The radius is \((2 \div 12) \div 2 = 0.08333\) ft, so the area is \(\pi \times 0.08333^2 \approx 0.021817\) ft². Then $$\text{GPM} = 5 \times 0.021817 \times 448.831 \approx 48.96 \text{ gallons per minute}.$$
FAQ
Does this work for any liquid? Yes. The geometry-based calculation is fluid-independent; it depends only on velocity and pipe size, not on the fluid's density or viscosity.
Is the diameter inside or outside? Use the inside (internal) diameter of the pipe, since that is the actual flow path. Nominal pipe sizes can differ from the true internal diameter.
What units does GPM use here? The result is in US gallons per minute. For Imperial (UK) gallons, multiply the result by approximately 0.8327.
