Gallons Per Minute Calculator
Calculate flow rate in gallons per minute from volume and time, or from pipe diameter and velocity.
Formula
GPM = volume (gal) ÷ time (min); or GPM = 0.4085 × D² × V (D in inches, V in ft/s)
- Pipe flow: derived from Q = A × V; A = π/4 × D²; converted to gallons/min.
- 0.4085 = π/4 × (1/144) × 7.48052 × 60 (converting ft²/s to gal/min).
- Typical residential water: 3–5 ft/s; max recommended: 8 ft/s for cold water.
Pipe: 2 in diameter, 5 ft/s velocity
Inputs
- Method: pipe
- Volume: 10 gallons
- Time: 2 minutes
- Pipe Inside Diameter: 2 in
- Flow Velocity: 5 ft/s
GPM = 0.4085 × 4 × 5 = 8.17 GPM.
Frequently asked questions
What is a typical house GPM demand?
A single-family home typically requires 5–10 GPM for peak demand. A garden hose delivers about 5 GPM at 40 PSI.