Pipe Flow Calculator - Volumetric Flow Rate (Q = A × v)
Calculate volumetric flow rate through a pipe from its diameter and fluid velocity, in GPM, L/min, or m³/s.
Flow Rate
48.96 GPM
0.003089 m³/s
48.96 GPM
How to Calculate Pipe Flow Rate
Volumetric flow rate through a full, circular pipe is the cross-sectional area of the pipe multiplied by the fluid's average velocity. Enter the pipe's diameter and the fluid velocity, and choose whether to work in US units (inches, feet per second) or metric units (millimeters, meters per second).
Example
A 2-inch diameter pipe carrying water at 5 ft/s has a cross-sectional area of about 0.0218 ft². Flow rate Q = 0.0218 × 5 = 0.109 ft³/s, which works out to roughly 49 gallons per minute.
Common Use Cases
- Sizing a water pump or irrigation line for a target flow rate.
- Checking whether existing plumbing can deliver the flow a fixture requires.
- Estimating flow through HVAC, industrial, or process piping.
FAQs
Does this assume the pipe is completely full?
Yes — this calculates flow for steady, incompressible flow through a pipe running completely full. Partially filled pipes (like open channels or gravity drains that aren't flowing full) need a different open-channel flow formula, since the wetted cross-sectional area is smaller than the full pipe area.
What if I know the flow rate and need the velocity instead?
Rearrange the same formula: velocity = flow rate ÷ cross-sectional area. Calculate the pipe's area from its diameter using the formula above, then divide your known flow rate by that area.
Why does pipe diameter matter so much to flow rate?
Cross-sectional area scales with the square of the diameter, so doubling a pipe's diameter quadruples its area — and therefore roughly quadruples the flow rate at the same fluid velocity.