Pipe slope (fall) calculator
Enter the horizontal length of the pipe and a slope to get the total fall, plus the fall at every 10 feet for setting stakes or string lines.
Fall is measured over the horizontal length, which is how drain pipes are laid out and staked; over any slope used for drainage the difference from the length along the pipe is negligible. The result is the vertical drop between the two ends of the run, not a pipe size, a code decision or a check of vent, trap or fixture limits. Take the minimum slope for your pipe size and application from the plumbing code your authority has adopted.
Drain pipe is laid to fall at a steady slope, usually written in inches per foot. A slope of 1/4 inch per foot drops 10 inches over 40 feet, which is a 2.08% grade, or 1 in 48. Enter a run and a slope above to get the total fall, the grade and the drop at every 10 feet for setting stakes or string lines.
Inches per foot, percent and ratio
Drain slope is written three ways. Plumbers usually say inches per foot, engineers say percent and drawings sometimes say 1 in N. They are the same slope, converted with one formula.
Fall = run × slope in inches per foot Percent grade = inches per foot ÷ 12 × 100 Ratio (1 in N) = 12 ÷ inches per foot
A slope of 1/4 in per foot is 2.083%, or 1 in 48, and it falls 2 1/2″ over 10 ft.
| Inches per foot | Percent grade | Ratio | Angle | Fall over 10 ft | Fall over 100 ft |
|---|---|---|---|---|---|
| 1/16 in per ft | 0.521% | 1 in 192 | 0.30° | 5/8″ | 6 1/4″ |
| 1/8 in per ft | 1.042% | 1 in 96 | 0.60° | 1 1/4″ | 12 1/2″ (1′ 0 1/2″) |
| 3/16 in per ft | 1.563% | 1 in 64 | 0.90° | 1 7/8″ | 18 3/4″ (1′ 6 3/4″) |
| 1/4 in per ft | 2.083% | 1 in 48 | 1.19° | 2 1/2″ | 25″ (2′ 1″) |
| 3/8 in per ft | 3.125% | 1 in 32 | 1.79° | 3 3/4″ | 37 1/2″ (3′ 1 1/2″) |
| 1/2 in per ft | 4.167% | 1 in 24 | 2.39° | 5″ | 50″ (4′ 2″) |
| 3/4 in per ft | 6.250% | 1 in 16 | 3.58° | 7 1/2″ | 75″ (6′ 3″) |
| 1 in per ft | 8.333% | 1 in 12 | 4.76° | 10″ | 100″ (8′ 4″) |
Total fall by run length
The drop between the two ends of a run at the three slopes used most for drain lines. Read across for the length you have.
| Run | 1/8 in per ft | 1/4 in per ft | 1/2 in per ft |
|---|---|---|---|
| 10 ft | 1 1/4″ | 2 1/2″ | 5″ |
| 20 ft | 2 1/2″ | 5″ | 10″ |
| 25 ft | 3 1/8″ | 6 1/4″ | 12 1/2″ (1′ 0 1/2″) |
| 30 ft | 3 3/4″ | 7 1/2″ | 15″ (1′ 3″) |
| 40 ft | 5″ | 10″ | 20″ (1′ 8″) |
| 50 ft | 6 1/4″ | 12 1/2″ (1′ 0 1/2″) | 25″ (2′ 1″) |
| 75 ft | 9 3/8″ | 18 3/4″ (1′ 6 3/4″) | 37 1/2″ (3′ 1 1/2″) |
| 100 ft | 12 1/2″ (1′ 0 1/2″) | 25″ (2′ 1″) | 50″ (4′ 2″) |
Minimum slope in the plumbing code
The International Plumbing Code (Table 704.1) sets minimum slopes for horizontal drainage piping by diameter, and asks for uniform alignment at a uniform slope. For the base table the minimum is 1/4 in per foot for pipe 2 1/2 in and smaller, 1/8 in per foot for 3 in to 6 in, and 1/16 in per foot for 8 in and larger.
| Pipe diameter | Minimum slope | Percent grade | Fall over 40 ft |
|---|---|---|---|
| 2 1/2 in or smaller | 1/4 in per ft | 2.08% | 10″ |
| 3 in to 6 in | 1/8 in per ft | 1.04% | 5″ |
| 8 in or larger | 1/16 in per ft | 0.52% | 2 1/2″ |
Treat this as a reference, not as the rule for your job. The residential code has its own table, the edition your jurisdiction adopted may differ, and local amendments can change the numbers. Some applications also set different slopes, for example piping upstream of a grease interceptor. Take the figure from the code your inspector enforces.
Worked example: a 40 foot, 4 inch drain
A 4 in drain runs 40 ft in a straight line from a building to a connection. At the base-code minimum of 1/8 in per foot the pipe falls 5″ over the run. At 1/4 in per foot it falls 10″.
40 ft × 1/8 in per ft = 5 in 40 ft × 1/4 in per ft = 10 in Difference = 5 in
That difference is real depth. If the pipe leaves the building 18 in below grade on level ground, the far end is 23 in down at the minimum slope and 28 in at 1/4 in per foot. On a long run the extra fall adds up quickly: 100 ft at 1/4 in per foot falls 25″ (2′ 1″). Check that the receiving connection is low enough before you commit to a slope.
For the stakes, read the station list in the calculator: 10 ft is 2 1/2″, 20 ft is 5″, 30 ft is 7 1/2″, 40 ft is 10″. Each stake is cut that far below the reference at the high end.
Checking slope with a level and a block
A level with a block under one end checks slope without any other equipment. Rest the level on the pipe, lift the downhill end by the block thickness for your slope and level length, and the bubble centres when the pipe is at that slope.
| Level length | 1/8 in per ft | 1/4 in per ft | 1/2 in per ft |
|---|---|---|---|
| 2 ft level | 1/4″ | 1/2″ | 1″ |
| 4 ft level | 1/2″ | 1″ | 2″ |
| 6 ft level | 3/4″ | 1 1/2″ | 3″ |
A 4 ft level with a 1 in block checks 1/4 in per foot. Use a straight, calibrated level and check across the whole run, because a small error on a short level repeats along the pipe.
Setting slope in a trench
Pick a reference, such as the invert at the downstream end, a string line or a laser. Work out the fall at each station with the calculator and cut each stake or grade mark to that depth. Bed the pipe so its invert, the inside bottom of the pipe, follows the line. The invert drops steadily from start to end, so the top of a pipe follows it only while the diameter is constant.
To measure an existing pipe, take the elevation difference between two points and divide by the distance. A pipe that drops 18 in over 60 ft falls 18 ÷ 60 = 0.3 in per foot, or 2.5%. A level reading at both ends of a long run is more reliable than a level on a short section.
Mistakes to avoid
- Mixing up percent and inches per foot. 1/4 in per foot is 2.08%, not 0.25%.
- Measuring fall from different points. Use the same part of the pipe, usually the invert, at both ends.
- A sag between supports. A low spot holds water; support the pipe along its length and keep the slope uniform.
- Ignoring the depth the fall consumes. The downstream end is lower by the full fall. Check the connection before you dig.
- Treating a slope as a pipe size. The slope the code requires depends on diameter and application.
What this calculator does not decide
It converts a slope and a length to a fall. It does not size a pipe, set vent or trap distances, check fixture units or design a sewer or stormwater system. Those depend on your code, the fixtures and the soil. The fall is measured over the horizontal run, which is how drain lines are staked; for the gentle slopes used on drains the difference from the length along the pipe is small. Even at 1/2 in per foot, 100 ft of horizontal run is about 100.1 ft of pipe.
Sources
Minimum slopes for horizontal drainage piping are in the International Plumbing Code, Section 704.1 and Table 704.1, published by the International Code Council. Your jurisdiction adopts a particular edition and may amend it.
Frequently asked questions
What is the standard slope for a drain pipe?
The base plumbing code table asks for at least 1/4 in per foot for pipe 2 1/2 in and smaller, and 1/8 in per foot for 3 in to 6 in pipe. Your jurisdiction may adopt a different edition or amend the table, so confirm the minimum with your inspector.
How do I convert inches per foot to percent?
Divide the inches per foot by 12 and multiply by 100. 1/4 in per foot is 0.25 ÷ 12 × 100 = 2.08%. To go back, multiply the percent by 12 and divide by 100.
How much fall is 1/4 inch per foot over 100 feet?
25″ (2′ 1″). Multiply the run in feet by the slope in inches per foot.
Is a steeper slope always better?
No. Codes set minimum slopes. Whether a maximum applies, and how steep a run can be before it counts as a vertical drop, depends on your code and the application, so ask your inspector.
Do I measure the run along the pipe or horizontally?
Horizontally is the normal way to stake and lay out a drain. For slopes as gentle as drain lines the difference from the length along the pipe is negligible.