Plumbing
Updated Apr 20, 2026

Pipe Size Calculator

Calculate the correct pipe diameter for water supply and drain lines based on flow rate, fixture units, and pressure.

How to Size Pipes

Proper pipe sizing ensures adequate water flow, prevents noise and erosion, and meets plumbing code requirements. Undersized pipes cause low pressure and poor performance, while oversized pipes waste money and can cause stagnation issues.

Fixture Units Method

Plumbing codes use Fixture Units (FU) to standardize demand. Each fixture is assigned a value based on its flow rate. Total fixture units determine the minimum pipe size:

Total FU=Σ(Fixture Units×Count)

Flow Velocity Limits

Water velocity in pipes must stay within limits to prevent noise (water hammer) and pipe erosion. The relationship between flow rate, velocity, and pipe diameter is:

V=GPM2.448×d2

Where V is velocity in ft/s, GPM is gallons per minute, and d is the internal diameter in inches. Target velocity is 5-8 ft/s for supply lines.

Rule of Thumb
For most residential projects, 3/4" supply lines serve up to 3 fixtures, and 1" lines serve 4-8 fixtures. Always verify with local code requirements, as minimum sizes vary by jurisdiction.

Pipe Sizing Reference

Common residential pipe sizes for supply and drain lines based on fixture unit loads:

Pipe SizeSupply (FU)Drain (DFU)Common Use
1/2"Up to 4-Individual fixture supply
3/4"Up to 8-Branch supply lines
1"Up to 18-Main supply, water meter
1-1/2"-Up to 3Sink, tub drains
2"-Up to 6Shower, laundry drains
3"-Up to 20Toilet drains, main stack
4"-Up to 160Building drain, sewer line
Code Requirements
Plumbing codes require minimum pipe sizes regardless of fixture unit calculations. Toilets always need a minimum 3" drain. Building drains connecting to the sewer must be at least 4". Always check your local plumbing code before finalizing pipe sizes.

Frequently Asked Questions