About the Drain Pipe Slope Calculator
This drain slope calculator works out how much a horizontal drain line has to fall over its length. Enter the run in feet, the pipe diameter and the slope you plan to use, and it returns the total drop in inches and millimeters, the slope as a percentage and an angle, and whether the slope meets the minimum in the International Plumbing Code (IPC).
It is meant for DIYers, plumbers and remodelers laying out a bathroom or kitchen drain, a basement bathroom, a building sewer to the street or a mortar bed for a tiled shower. Choose “Code minimum” to apply the IPC Table 704.1 rule automatically: 1/4 in per foot for pipe 2½ in and smaller, 1/8 in per foot for 3–6 in pipe and 1/16 in per foot for 8 in and larger.
In shower mode the length is the distance from the drain to the farthest edge of the floor, and the slope defaults to the 1/4 in per foot (2%) that the IPC requires for shower liners. The figures are from the 2009 IPC, the edition published free online, and later IPC editions use the same slope table. The International Residential Code (IRC Section P3005.3) uses the same 1/4 and 1/8 in per foot minimums but has no separate 1/16 in tier for 8 in and larger pipe. Local codes (including the Uniform Plumbing Code used in some western states) can differ, so confirm with your inspector before you set pipe.
With the default inputs, the total drop is 2.5 in. Change any value above to recalculate instantly.
How to use the drain pipe slope calculator
- 1Choose whether you are sloping a drain pipe or a shower floor.
- 2Measure the horizontal run in feet (for a shower, from the drain to the farthest edge).
- 3Pick the pipe diameter so the code minimum slope can be applied.
- 4Leave the slope on “Code minimum” or choose a steeper slope.
- 5Read the total drop and mark it on your framing, trench or mortar screed.
Formula and method
Drain slope in the US is stated in inches of fall per foot of horizontal run, so the total drop is simply the run in feet multiplied by the slope. Dividing the slope by 12 converts it to a rise-over-run ratio, which gives the grade as a percentage (1/4 in per ft ≈ 2.08%, 1/8 in per ft ≈ 1.04%) and the angle via the arctangent.
The code minimum follows IPC Table 704.1: 1/4 in per foot for horizontal drainage pipe 2½ in or smaller, 1/8 in per foot for 3 to 6 in pipe and 1/16 in per foot for 8 in and larger. IRC Section P3005.3 sets the same 1/4 and 1/8 in per foot minimums for houses, without the 1/16 in tier. For shower floors the IPC requires the liner to be pitched 1/4 unit vertical in 12 units horizontal (2%) toward the drain.
- Run
- Horizontal length of the pipe or floor in feet
- Slope
- Fall in inches per foot of run
- Drop
- Total fall from the high end to the low end, in inches
Worked examples
20 ft run of 3 in drain at code minimum
A 3 in pipe falls in the 3–6 in group, so the code minimum is 1/8 in per foot. Over 20 ft that is 20 × 0.125 = 2.5 in of drop, a grade of 0.125 ÷ 12 = 1.04%, or about 0.60°.
12 ft of 2 in drain at 1/4 in per foot
A 2 in drain must slope at least 1/4 in per foot. Twelve feet × 0.25 in = 3 in of fall (76 mm), a 2.08% grade.
1¼ in trap arm laid at 1/8 in per foot (too flat)
Pipe 2½ in and smaller needs 1/4 in per foot, so 1/8 in per foot fails. The 4 ft run should drop 1 in, not 0.5 in.
Shower floor 3 ft from drain to wall
Shower liners must be pitched 1/4 in per foot, so a point 3 ft from the drain must sit 3 × 0.25 = 0.75 in higher than the drain.
Frequently asked questions
What is the minimum slope for a drain pipe?+
Under IPC Table 704.1, horizontal drain pipe 2½ in and smaller needs at least 1/4 in of fall per foot, 3 to 6 in pipe at least 1/8 in per foot, and 8 in and larger at least 1/16 in per foot. The IRC uses the same 1/4 in figure for small pipe but requires only 1/8 in per foot for any pipe 3 in or larger.
Can a drain pipe have too much slope?+
Plumbing codes set minimums, not a maximum for horizontal drains, but very steep runs can let water outrun solids and are hard to support. When you need to lose a lot of height, plumbers usually add a vertical section with 45° or long-sweep fittings.
How much slope does a 4 inch sewer line need?+
A 4 in building drain or sewer must slope at least 1/8 in per foot under the IPC, so 50 ft of pipe needs 6.25 in of fall. Many plumbers use 1/4 in per foot when the depth allows.
What is 1/4 inch per foot as a percentage?+
One-quarter inch per foot is 0.25 ÷ 12 = 2.08% grade, about 1.19°. One-eighth inch per foot is 1.04%, and 1/16 in per foot is 0.52%.
How much should a shower floor slope?+
The IPC requires shower pan liners to slope 1/4 in per foot (2%) toward the drain, and the finished mortar bed is normally built to the same pitch so water reaches the drain without ponding.
Does the Uniform Plumbing Code use the same slopes?+
No. The UPC requires 1/4 in per foot (2%) as the norm and allows 1/8 in per foot only for pipe 4 in or larger where a steeper slope is impractical and the authority having jurisdiction approves it. A 3 in pipe at 1/8 in per foot passes the IPC but not the UPC.
Sources
- International Plumbing Code 2009 — Table 704.1 Slope of Horizontal Drainage Pipe and Section 417.5.2 Shower Lining (Public.Resource.Org edition)
- International Residential Code 2012 — Section P3005.3 Horizontal Drainage Piping Slope (Public.Resource.Org edition)
- Uniform Plumbing Code 2009 — Section 708.0 Grade of Horizontal Drainage Piping (Public.Resource.Org edition)