Pipe Velocity Calculator | Flow Rate & Discharge

Pipe flow rate calculator

This tool converts a pipe's internal diameter and the fluid's velocity into a flow rate. Enter the diameter in millimeters, centimeters, or inches and the velocity in meters or feet per second, and it returns the result in cubic meters per hour, liters per second, liters per minute, and US gallons per minute. It also flags velocities above 3 m/s (9.8 ft/s), the point where pipe noise, vibration, and erosion typically start to appear.

The flow rate formula

Flow rate depends on the pipe's cross-sectional area and how fast the fluid moves through it. The area comes from the radius, half the diameter, using A = π × r². Multiplying that area by the velocity gives the volumetric flow rate, Q = A × v. Any diameter entered in millimeters, centimeters, or inches is converted to meters first (1 inch = 0.0254 m), and velocity in feet per second is converted to meters per second (1 ft/s = 0.3048 m/s), so the calculation always runs in SI units before converting back to the units you asked for.

Worked example

Take a 4-inch pipe carrying water at 6 ft/s, a velocity common on a building's main supply line. Converting the diameter gives 0.1016 m, so the radius is 0.0508 m and the cross-sectional area works out to π × 0.0508² ≈ 0.008107 m². The velocity converts to 1.8288 m/s. Multiplying area by velocity gives a flow rate of about 0.01483 m³/s, which the calculator reports as 53.38 m³/h, 14.83 L/s, or 235.01 GPM (US).

Common pipe sizes at 6 ft/s

The formula behind every result:

Q = A × v
A = π × (D / 2)²

The table below holds the same velocity constant at 6 ft/s, a typical design speed for domestic water lines, and varies only the diameter, so you can see how much the pipe size itself changes the discharge.