GPM Calculator
Use this GPM calculator to find a flow rate in gallons per minute from volume and time, or to solve for volume or time when the flow is known. It also gives GPM from pipe inner diameter and water velocity, and converts between GPM, L/min, m³/h and ft³/s.
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Flow rate
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Volume
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Time
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Flow from pipe size and velocity
Flow rate
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Flow rate converter
Flow rate
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Flow (gpm) = volume (gal) ÷ time (min). From a pipe, gpm = velocity (ft/s) × area (ft²) × 448.831, where 448.831 = 7.48052 gal per ft³ × 60 s per min; with the inner diameter d in inches this is gpm = 2.448 × d² × v. 1 US gallon = 3.785 L, so L/min = gpm × 3.785. Gallons here are US gallons; an imperial gallon is about 20% larger.
More in this group
- Pipe Volume Calculator Gallons, liters and cubic feet a length of pipe holds.
- Pool Volume Calculator Gallons, liters and fill time for rectangle, round, oval and kidney pools.
- Wire Size Calculator Minimum AWG for amps, length and voltage drop.
- Ohms Law Calculator Find volts, amps, ohms and watts from any two values.
How it works
- What is the GPM formula?
- GPM = gallons ÷ minutes. 100 gallons in 20 minutes is 100 ÷ 20 = 5 GPM. To time a faucet, divide the gallons collected by the seconds it took and multiply by 60.
- How do I find GPM from pipe size and velocity?
- GPM = 2.448 × d² × v, with d the inner diameter in inches and v the water velocity in ft/s. A pipe with a 2 in inner diameter at 5 ft/s is 2.448 × 4 × 5 = 48.96 GPM. Use the actual inner diameter, which differs from the nominal pipe size.
- How do I convert GPM to liters per minute?
- Multiply GPM by 3.785, the liters in a US gallon. 5 GPM is 18.93 L/min. One GPM is also about 0.2271 m³/h.
- Where does the 448.831 constant come from?
- One cubic foot holds 7.48052 US gallons. A flow of 1 ft³/s is 7.48052 gallons per second, or 7.48052 × 60 = 448.831 gallons per minute. So GPM = v × A × 448.831, and with a circular area this reduces to 2.448 × d² × v.
- How long does it take to fill a tank at a given flow?
- Set Solve for to Time and enter the flow and the volume. For example, 300 gallons at 5 GPM takes 300 ÷ 5 = 60 minutes. Pick the units you prefer for each value.