Pipe Flow Rate Calculator

Find gallons per minute from pipe size and velocity, the velocity for a given flow, or the pipe size you need.

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GPM

Flow
Velocity
Inside diameter
Flow area
Other units
Velocity Check

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Name	Pipe	ID (In.)	GPM	Ft./Sec.	Remove

How to Calculate Pipe Flow Rate

Flow is the pipe's inside area times how fast the water moves. In US units, gallons per minute works out to a simple constant times velocity times the inside diameter squared.

Flow (gpm) = 2.448 × velocity (ft/s) × inside diameter (in)²

Velocity (ft/s) = 0.4085 × gpm ÷ inside diameter (in)²

Minimum inside diameter (in) = √(0.4085 × gpm ÷ velocity)

The constant comes from the units: one foot per second through one square inch is 0.00694 ft³/s, and a cubic foot is 7.48 gallons, so 60 × 7.48 × π ÷ 4 ÷ 144 = 2.448.

Worked Example

Flow at 5 Ft./Sec.

Size	Sch. 40 ID	Sch. 40 gpm	Copper L ID	Copper L gpm

Inside diameters from ASME B36.10 (Schedule 40) and ASTM B88 (copper tube).

Tips

Always use the inside diameter; nominal sizes are only names. The Copper Development Association recommends keeping velocity under about 8 ft/s for cold water and 5 ft/s for hot water to avoid erosion and noise. Faster flow also means more friction loss; see the friction loss calculator.