Pipe Flow Calculator

Flow Rate Is Just Area Times Speed

Every pipe carrying water is governed by the continuity equation: the volume passing a cross-section per second equals that cross-section's area multiplied by how fast the water is moving through it. Given a pipe's inside diameter and the water's velocity, this calculator converts straight to gallons per minute — the unit most plumbing codes and fixture specifications actually use.

The Formula

Q = A × v
A = π × (d / 2)²
GPM = Q(ft³/s) × 448.831

Diameter is entered in inches and converted to feet before the area calculation; the 448.831 constant converts cubic feet per second to US gallons per minute.

Where This Calculation Matters

Supply line sizing — checking whether an existing pipe can deliver the flow a new fixture or appliance needs.
Velocity limit checks — residential supply piping is typically kept under about 8 ft/s to control noise, erosion, and water hammer risk; this calculation confirms a given diameter isn't pushed past that.
Irrigation design — matching zone pipe diameter to sprinkler head flow requirements.
Fire suppression and industrial systems — verifying a main can deliver required flow at design velocity.

Flow Capacity by Pipe Diameter

Flow rate at common design velocities
Diameter	At 5 ft/s	At 8 ft/s
0.5 in	3.1 GPM	4.9 GPM
0.75 in	6.9 GPM	11.0 GPM
1 in	12.2 GPM	19.6 GPM
1.5 in	27.5 GPM	44.1 GPM
2 in	49.0 GPM	78.3 GPM
3 in	110.2 GPM	176.3 GPM
4 in	195.8 GPM	313.3 GPM

Capacity scales with the square of diameter, so a 2 in pipe carries roughly four times the flow of a 1 in pipe at the same velocity, not just double.

How to Use This Calculator

Enter the pipe's inside diameter in inches.
Enter the flow velocity in feet per second.
Select Calculate to get flow rate in both GPM and cubic feet per second.

Related Calculations

To size a pipe for a target flow rate instead, use the Pipe Size Calculator. For the pressure lost when that flow travels through a real length of pipe, see the Pipe Pressure Drop Calculator.