Drain Sizing Calculator | Free Construction Calculators
Calculate the required drain pipe size based on fixture units and slope.
Fixture Unit Values (DFU)
Common Fixtures
- Toilet: 4 DFU
- Shower: 2 DFU
- Bathtub: 2 DFU
- Kitchen sink: 2 DFU
- Bathroom sink: 1 DFU
- Washing machine: 2 DFU
- Dishwasher: 2 DFU
- Floor drain: 1 DFU
Example Calculation
- 2 toilets = 8 DFU
- 2 sinks = 2 DFU
- 1 shower = 2 DFU
- 1 bathtub = 2 DFU
- Total = 14 DFU
- Required: 3" pipe
Drain Pipe Capacity
| Pipe Size | 1/4" slope | 1/2" slope |
|---|---|---|
| 1-1/4" | 1 DFU | 1 DFU |
| 1-1/2" | 3 DFU | 3 DFU |
| 2" | 6 DFU | 8 DFU |
| 3" | 20 DFU | 27 DFU |
| 4" | 160 DFU | 216 DFU |
| 6" | 620 DFU | 840 DFU |
Drain Slope Requirements
- Minimum slope: 1/4 inch per foot (2% grade)
- Standard slope: 1/4 to 1/2 inch per foot
- Maximum slope: Usually 3 inches per foot
- Too flat: Causes slow drainage and clogs
- Too steep: Water outrun solids, leaving deposits
Code Requirements
- Main drain: Minimum 3 inches for houses
- Branch drains: Based on fixture unit load
- Vent sizing: Typically half the drain size
- Commercial: Often requires one size larger
Important Notes
- Always check local plumbing codes for specific requirements
- Commercial buildings typically need larger pipes
- Never reduce drain pipe size downstream
- Proper venting is as important as proper sizing
- Consider future expansion when sizing mains