How to Calculate Pipe Size for Home Plumbing
Correct pipe sizing ensures adequate water pressure throughout your home and proper drainage in drain-waste-vent systems. Undersized supply pipes cause pressure drops; undersized drain pipes cause clogs.
Water Supply Pipe Sizing
Supply pipe sizing uses flow rate (gallons per minute) and pressure drop over distance.
Velocity Rule of Thumb
Water velocity in supply pipes should stay between 2โ8 feet per second (fps).
Flow rate (GPM) = Pipe area (sq ft) ร Velocity (fps) ร 449
Common pipe sizes and max GPM (at 6 fps):
| Pipe Diameter | Max Flow Rate |
|---|---|
| ยฝ inch | 2โ4 GPM |
| ยพ inch | 4โ8 GPM |
| 1 inch | 8โ15 GPM |
Fixture Unit Method
Each plumbing fixture is assigned "fixture units" (FU). Size the pipe to handle the total FUs it serves.
| Fixture | Fixture Units |
|---|---|
| Bathroom faucet | 1โ2 |
| Toilet | 3โ5 |
| Shower | 2 |
| Washing machine | 4 |
| Kitchen sink | 2 |
Drain Pipe Sizing
Drain pipes follow a different sizing scheme based on drainage fixture units:
| Pipe Size | Maximum DFUs |
|---|---|
| 1.5 inch | 3 DFU |
| 2 inch | 6 DFU |
| 3 inch | 20 DFU |
| 4 inch | 160 DFU |
Toilets require a minimum 3-inch drain. The main house drain is typically 4 inches.
Long Run Pressure Drop
For runs over 40 feet, upsize the supply pipe by one size to compensate for friction pressure loss.
Use our pipe size calculator for any plumbing configuration.