Enter horizontal run in feet and the chosen fall in inches per foot. The example 0.25 in/ft is an assumption, not a universal code minimum for every diameter. Select the actual required slope from the adopted code, run classification and design.
The calculator converts that entered geometry only.
The calculation assumes uniform fall and no sags. It does not impose a universal maximum slope or establish that water will outrun solids at a particular slope. Code rules and installation geometry must be checked for the actual fixture and pipe.
Use horizontal run, not the longer sloped pipe length, for this entered fall-per-foot convention.
Total drop (inches) from pipe run length
| Pipe run length | 20 |
|---|---|
| Slope | 0.25 |
| Total drop (inches) | 5 |
| Grade | 2.1% |
Common error: Adding extra slope to be safe. A steep drain can leave solids behind, and the fall must also fit within the depth of the building.
Drain Slope: formula
Drain Slope: changing Pipe run length
| Pipe run length | Total drop (inches) | Grade |
|---|---|---|
| 16 | 4 | 2.1% |
| 18 | 4.5 | 2.1% |
| 20 (example) | 5 | 2.1% |
| 22 | 5.5 | 2.1% |
| 24 | 6 | 2.1% |
Drain Slope: compare input ranges
| Input | Tested from | To | Total drop (inches): low input | Total drop (inches): high input | Swing |
|---|---|---|---|---|---|
| Pipe run length | 18 | 22 | 4.5 | 5.5 | 1 (20%) |
| Slope | 0.225 | 0.275 | 4.5 | 5.5 | 1 (20%) |
Drain Slope: individual input comparisons
| Input | Value | Total drop (inches) | Grade |
|---|---|---|---|
| Pipe run length | 16 | 4 | 2.1% |
| 18 | 4.5 | 2.1% | |
| 20 | 5 | 2.1% | |
| 22 | 5.5 | 2.1% | |
| 24 | 6 | 2.1% | |
| Slope | 0.2 | 4 | 1.7% |
| 0.225 | 4.5 | 1.9% | |
| 0.25 | 5 | 2.1% | |
| 0.275 | 5.5 | 2.3% | |
| 0.3 | 6 | 2.5% |
Total drop (inches): Pipe run length against Slope
| Pipe run length / Slope | 0.2 | 0.25 | 0.3 |
|---|---|---|---|
| 16 | 3.2 | 4 | 4.8 |
| 18 | 3.6 | 4.5 | 5.4 |
| 20 | 4 | 5 | 6 |
| 22 | 4.4 | 5.5 | 6.6 |
| 24 | 4.8 | 6 | 7.2 |
Drain Slope: values and definitions
| Quantity and definition | Value used |
|---|---|
| Pipe run length — Horizontal length of the drain run, in feet. | 20 |
| Slope — Entered fall in inches per horizontal foot. Select the slope from the applicable adopted code and design; this input does not determine a legal minimum. | 0.25 |
| Total drop (inches) | 5 |
| Grade | 2.1% |
Pipe run length: half and double
| Pipe run length | Total drop (inches) |
|---|---|
| 10 | 2.5 |
| 20 | 5 |
| 40 | 10 |
A drain must fall enough for water to carry solids along, and too little or too much slope both cause trouble. A quarter inch per foot is the slope commonly required for small drain lines, and over a 20 foot run it drops 5 inches, a grade of about 2.1 percent. Too little slope lets solids settle and clog.
Too much can let water outrun the solids and leave them behind. The pipe size and the local code set the allowed range.
Confirm the slope your local plumbing code requires for the pipe size, and check the total drop against the height available between the fixture and the main line.
Size the pipe with the drainage fixture unit calculator, calculate the volume on the pipe volume page and flow on the flow rate calculator. The distance and slope and right triangle calculators handle the geometry, and the stair and rafter pages use similar slope math.
Is 1/4 inch per foot mandatory for every drain?
No. Required fall depends on the applicable adopted code, diameter and run classification. This tool does not choose the required minimum.
Does the calculator test a maximum allowed slope?
No. It converts a chosen slope to total fall and percent grade. Applicable restrictions require the actual design and code.
Why does 0.25 in/ft become 2.083%?
Divide the quarter-inch fall by 12 inches of horizontal run, then multiply by 100.
What slope should a drain pipe have?
The required slope depends on the adopted plumbing code, pipe size and installation. Enter the actual design slope to calculate drop, then verify the requirement with the responsible authority or design professional. A general inch-per-foot rule does not establish every pipe’s permitted grade.
Can a drain slope too much?
Grade must be considered with pipe size, layout, venting, connections and applicable requirements. This geometric calculator only converts slope to drop. It does not simulate transport of solids or establish a universal maximum slope; use the installation design and adopted code.
What is grade as a percent?
The drop divided by the length times 100. A quarter inch per foot is about a 2.08% grade.