WireWorks

Pipe Flow Calculator

Pipe cross-sectional area times flow velocity, converted to gallons per minute.

Pipe inner diameter (in)

Flow velocity (ft/s)

Flow rate

12.24 GPM

Typical residential water supply velocity is 4-8 ft/s to balance flow against pipe noise and erosion.

What you entered

Pipe inner diameter (in): 1Flow velocity (ft/s): 5

1
Cross-sectional area of the pipe
π × (1/24)²= 0.0055 ft²
2
Flow rate in cubic feet per second
0.0055 × 5= 0.0273 ft³/s
3
Convert to gallons per minute
0.0273 × 448.831= 12.24 GPM

Result

Flow rate: 12.24 GPM

A 1" pipe at 5 ft/s carries about 12.239966773466671 gallons per minute.

Pipe flow rate by diameter and velocity

Flow is cross-sectional area times velocity. Because area scales with the square of diameter, doubling the pipe roughly quadruples the flow at the same speed.

Pipe diameter	Velocity	Flow rate
0.5 in	5 ft/s	3.06 gpm
0.75 in	5 ft/s	6.88 gpm
1 in	5 ft/s	12.24 gpm
1.5 in	5 ft/s	27.54 gpm
2 in	5 ft/s	48.96 gpm
3 in	5 ft/s	110.16 gpm
1 in	8 ft/s	19.58 gpm
2 in	8 ft/s	78.34 gpm

Domestic supply is normally designed around 5 ft/s and kept under 8. Faster water is noisy and erodes copper over time, so the answer to a flow problem is usually a larger pipe rather than more pressure.

Velocity is a design choice, not a fixed number

Plumbing codes and best practice typically target 4-8 ft/s flow velocity for water supply lines — too slow wastes pipe capacity, too fast causes noise, erosion ("water hammer"), and premature pipe wear.

Larger diameter increases flow a lot

Flow scales with the square of the diameter, not linearly — doubling a pipe's diameter roughly quadruples its flow capacity at the same velocity, which is why even modest diameter upgrades meaningfully increase capacity.

Frequently asked questions

I have a 3/4-inch pipe with water flowing at 6 ft/s — what's the flow rate?

About 6.9 GPM. The inner diameter of 3/4-inch copper pipe is about 0.785 inches (0.0654 ft), giving a cross-sectional area of 0.00336 sq ft, times 6 ft/s = 0.02017 cu ft/s = 9.05 GPM. Enter your exact pipe dimensions above.

What flow velocity is recommended for water supply lines?

4-8 ft/s is the typical design range. Below 4 ft/s wastes pipe capacity and can allow sediment to settle. Above 8 ft/s causes noise, erosion (water hammer), and premature pipe wear. Aim for 5-6 ft/s as a sweet spot.

Why does doubling the pipe size quadruple the flow?

Flow depends on cross-sectional area, which scales with the square of the diameter (A = pi x r^2). Doubling the diameter quadruples the area, so at the same velocity, four times as much water passes through.