Drain Pipe Slope

Slope (in per ft) × run length = total drop. Standard ¼" per ft.

Inputs

Result

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How to use this calculator

  • Enter pipe size + run length.
  • Pick slope (¼"/ft standard).
  • Read drop.

About this calculator

Drain pipes need slope so water carries solids. IPC standard: ¼" per foot (2.08% slope) for pipes ≤ 2.5". For 3" and larger, ⅛" per ft (1.04%) is acceptable and often required (steeper = water races ahead, leaves solids behind). Too flat: water doesn't flow, sediment builds up. Too steep: liquid outpaces solids → blockage. Building codes typically require ¼" per ft for residential drain lines.

Frequently asked

Why ⅛" for big pipe?+
Big pipes carry water in shallow stream — needs only gentle slope. Steep slope outruns solids.
Vent stack required?+
Yes — drains need vents to prevent siphoning trap seals. IPC requires vent within 6 ft of trap.
Maximum slope?+
No hard max but >½" / ft = solid fall-back. ¼" is sweet spot for residential.
Cleanout access?+
IPC requires cleanouts every 100 ft on horizontal runs, at base of stacks, and at every 90° turn.
Backfall?+
If pipe goes uphill anywhere — water stagnates. Catastrophic plumbing failure.

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