Drain Pipe Size Calculator (DWV)

Drainage pipe carries loads, not just water. A toilet, a shower and a washer each contribute a drainage fixture unit value, and the pipe must be big enough - and sloped enough - to carry the combined load without backing up.

Drain Pipe Size Calculator picks a minimum DWV pipe size from your fixture load (DFU), the pipe context and slope, using typical IPC capacity tables.

• DFU to size: Minimum DWV pipe size from total load.
• Context aware: Horizontal branch, stack or building drain.
• Capacity shown: Max DFU of the chosen and next size.
• Slope check: Compares your slope with the code minimum.
• Quick fixture DFU: Add up fixtures without leaving the tool.
• Export: Copy or download the report.

1. Add up the fixture units - or enter a DFU total directly.
2. Pick the pipe context: horizontal branch, stack, or building drain.
3. Enter the run length and the slope you plan to use.
4. Read the minimum size and check the slope verdict.

Example - Bathroom branch. Toilet (3) + shower (2) + lav (1) = 6 DFU. A 2 in horizontal branch carries up to 6 DFU at 1/4 in/ft - just barely; a 2 in stack carries 10. Add a second bathroom and 3 in wins.

• Catch undersizing early: before pipe goes in the wall.
• Residential or commercial: same tables, any scale.
• Cross-check: pairs with the fixture-unit calculator.
• Free and private: no uploads, no logging.