Gallons Per Minute (GPM) Calculator
Estimate flow rate based on pipe diameter, pressure, and length. This is a theoretical estimate using simplified hydraulic equations.
Note: This is a theoretical estimate using the Hazen-Williams equation. Real-world flow rates can vary significantly due to fittings, valves, viscosity changes, and other hydraulic losses.
How to Measure GPM
GPM (gallons per minute) measures volumetric flow — how many gallons of liquid pass a point in one minute. It's used to size pumps, select fixtures, and design irrigation or firefighting systems.
Bucket Method (Simplest Way)
Use a 1-gallon container, time how long it takes to fill in seconds. GPM = 60 ÷ seconds to fill 1 gallon. For other bucket sizes: GPM = (bucket gallons ÷ seconds) × 60.
Example Calculation
If a 5-gallon bucket fills in 15 seconds: GPM = (5 ÷ 15) × 60 = 20 GPM.
Convert GPM to LPM and GPH
Multiply GPM by 3.78541 to get LPM (liters per minute). Multiply GPM by 60 to get GPH (gallons per hour).
Example: 2 GPM × 3.78541 = 7.57082 LPM, and 2 GPM × 60 = 120 GPH.
Advanced: Estimate by Pipe Diameter & Pressure
The advanced calculator uses the Hazen-Williams equation to estimate flow based on pipe characteristics. This is useful for plumbing design but provides theoretical values — real systems have additional losses.
FAQ
GPM (gallons per minute) measures volumetric flow — how many gallons of liquid pass a point in one minute. It's used to size pumps, select fixtures, and design irrigation or firefighting systems.
Use a 1-gallon container, time how long it takes to fill in seconds. GPM = 60 ÷ seconds to fill 1 gallon. For other bucket sizes: GPM = (bucket gallons ÷ seconds) × 60.
Multiply by 3.78541. Example: 2 GPM × 3.78541 = 7.57082 LPM.
Many modern showerheads are limited to ~2.0–2.5 GPM in the US.
It's a theoretical estimate using simplified hydraulic equations; fittings, valves, and real hydraulic losses can change real flow. Use it as a starting point.
1 Imperial gallon ≈ 1.20095 US gallons. The tool supports both units.