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Pipe Slope Calculator: Drain Fall per IPC Table 704.1

Find the minimum slope and total fall of a horizontal drain from IPC Table 704.1. This is drain piping only — not a wheelchair ramp, a roof pitch, or a gutter slope.

ft

IPC Section 704.1: drainage piping upstream of a grease interceptor needs at least ¼ in/ft (2%), regardless of diameter.

Your estimate

Minimum fall

5in

0.42 ft over 40 ft of run

Minimum slope

⅛ in/ft

1:96 (1.04%)

Fall in feet

0.42 ft

IPC Table 704.1: 2½ in and smaller → ¼ in/ft; 3 to 6 in → ⅛ in/ft; 8 in and larger → 1/16 in/ft. Piping upstream of a grease interceptor is ¼ in/ft minimum (IPC Section 704.1). The ratio is rise:run (⅛ in/ft = 1:96, about 1.04%). This is horizontal drain fall only — not an ADA ramp, a roof pitch, or a gutter slope.

Estimates only — confirm with your supplier and local code. First published · Last reviewed by CalcTheJob.

How to use the pipe slope calculator

  1. Enter the run length and the nominal pipe size.
  2. Turn on the grease-interceptor switch if the line drains to a grease interceptor (the piping upstream of it). IPC Section 704.1 sets a minimum of ¼ in/ft (2%) for that piping regardless of diameter.

A wheelchair ramp uses a different rule — see the ramp slope calculator. Gutter flow is on the gutter size calculator.

IPC Table 704.1

Horizontal drainage piping: 2½ inches and smaller → ¼ in/ft; 3 to 6 inches → ⅛ in/ft; 8 inches and larger → 1/16 in/ft. Fall = length × slope. This is the table of minimum slopes, not a vent or a pressure main.

A worked example: 40 ft of 3-inch pipe

A 40 ft run of 3 in drain at ⅛ in/ft falls 5 inches (0.42 ft). The same 40 ft of 2 in pipe at ¼ in/ft falls 10 inches. An 8 in line at 1/16 in/ft falls 2.5 inches. A 40 ft, 4 in line draining to a grease interceptor needs ¼ in/ft, or 10 inches of fall, instead of the 5 inches a 4 in line would otherwise need.

Sources & assumptions

How this calculator gets its numbers. Full method: our methodology.

  • Minimum slopes follow IPC Table 704.1 for horizontal drainage piping: 2½ in and smaller → ¼ in/ft; 3 in to 6 in → ⅛ in/ft; 8 in and larger → 1/16 in/ft. Fall = length × slope.
  • Drainage piping upstream of (draining to) a grease interceptor uses a minimum of ¼ in/ft (2%), regardless of diameter — IPC Section 704.1 exception, referenced by Table 704.1 footnote a.
  • The slope ratio is rise:run in the same units: ¼ in/ft = 1:48 (2.08%), ⅛ in/ft = 1:96 (1.04%), 1/16 in/ft = 1:192 (0.52%).
  • This is drain fall only, not an ADA ramp, a roof pitch, or a gutter slope.

Sources

  1. International Plumbing Code, via UpCodes, IPC Table 704.1 — Slope of Horizontal Drainage Piping. Minimum slopes: 2½ in or less, ¼ in/ft; 3 to 6 in, ⅛ in/ft; 8 in or larger, 1/16 in/ft. Section 704.1: where drainage piping is upstream of a grease interceptor, slope not less than ¼ in/ft (2%).

Last reviewed . The adopted plumbing code and a design professional win.

Frequently asked questions

How much fall does 40 feet of 3-inch drain need?

IPC Table 704.1 requires ⅛ in/ft for 3 to 6 in pipe, so 5 inches of fall.

What is the minimum slope for 2-inch drain pipe?

¼ in/ft for 2½ in and smaller.

Is this the same as a wheelchair ramp or a gutter slope?

No. Ramp slope is on the ramp slope calculator. Gutter flow is on the gutter size calculator. This page is horizontal drain fall only.

When is ¼ in/ft required on a larger pipe?

When the drainage piping is upstream of (drains to) a grease interceptor — IPC Section 704.1 sets ¼ in/ft (2%) for that piping regardless of size. The calculator has a switch for that case.

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