Pipe Slope Calculator
Ten feet of 2 inch drain at the code minimum drops 2.50 inches. That minimum is 1/4 inch per foot, which is 2.0833 percent.
Last updated . Two inch drain is selected. The run starts at ten feet.
A pipe slope calculator for a horizontal drain. The drop is the run times the inches per foot. The diameter picks the 2021 IRC minimum, and a slope you type is compared with that minimum.
Storage or tankless hot water: water heater size calculator. A trench for the same line: trench calculator.
Drain drop
The diameter menu loads the 2021 IRC floor for that size: one quarter inch of fall on each foot through 2.5 inches, then one eighth inch of fall on each foot from 3 inches upward. The section’s rounded “2-percent” and “1-percent” words are not the fractions this form divides by 12.
Results
The code fraction, then the fall
2021 IRC P3005.3 requires uniform slope of at least 1/4 unit vertical in 12 units horizontal for a drain of 2 1/2 inches and smaller, and at least 1/8 unit vertical in 12 for a drain of 3 inches or more. The section calls those ratios a 2-percent slope and a 1-percent slope. The fractions are the rule this form uses. A quarter inch per foot is 0.25 ÷ 12 = 2.0833 percent. An eighth inch per foot is 1.0417 percent. Typing a flat 2 percent produces 0.24 inch per foot, and the note flags it as under the minimum for a 2 inch or 2.5 inch drain.
Drop in inches is the run in feet times the inches per foot. Ten feet at 0.25 is 2.50 inches, which is 63.50 millimeters. Ten feet of 3 inch, 4 inch, 6 inch, or 8 inch or larger pipe at the code minimum is 1.25 inches, 31.75 millimeters. The diameter list includes 8 inch or larger on the 1/8 side because the sentence says “3 inches or more,” not a cutoff at 6 inches. The Colorado DFPC printing of that sentence is on UpCodes.
Worked runs
| Run | Diameter | Slope | Drop | Against P3005.3 |
|---|---|---|---|---|
| 10 ft | 2 in or smaller | 0.25 in/ft (2.0833%) | 2.50 in | Meets |
| 10 ft | 3 in, 6 in, or 8 in or larger | 0.125 in/ft (1.0417%) | 1.25 in | Meets |
| 10 ft | 2 in or smaller | 2.0000% (0.24 in/ft) | 2.40 in | Under the minimum |
Pipe slope questions
How much does a 10 foot run of 2 inch drain drop?
Leave the run at 10 ft, the diameter on 2 in or smaller, and the slope mode on the code minimum. Slope reads 0.25 in/ft · 2.0833%. Code minimum reads 0.25 in/ft · 2.0833%. Drop reads 2.50 in · 63.50 mm. The note says this slope meets the 2021 IRC P3005.3 minimum for the selected diameter.
What is the drop for a 3 inch drain over the same 10 feet?
Set the diameter to 3 in and leave the run at 10 ft on the code minimum. Slope reads 0.125 in/ft · 1.0417%. Code minimum matches that. Drop reads 1.25 in · 31.75 mm. The note still says the slope meets P3005.3.
Do 4 inch, 6 inch, and 8 inch drains use the same minimum?
Yes. P3005.3 puts every diameter of 3 inches or more on 1/8 inch per foot. Set the diameter to 6 in, or to 8 in or larger, with the run still at 10 ft. Drop stays 1.25 in · 31.75 mm. The section does not switch an 8 inch drain to a shallower fraction.
What if I type 2 percent instead of the code fraction?
Set the slope mode to Percent and type 2, with the diameter still on 2 in or smaller and the run at 10 ft. Slope reads 0.24 in/ft · 2.0000%. Drop reads 2.40 in · 60.96 mm. The note says this slope is under the 2021 IRC P3005.3 minimum for the selected diameter. The section’s parenthetical “2-percent” is a rounded name. A quarter inch in 12 inches is 2.0833 percent, and 2.00 percent is 0.24 inch per foot.
What does P3005.3 actually say?
The 2021 IRC sentence, as printed for the Colorado DFPC residential code on UpCodes, says horizontal drainage piping shall be installed in uniform alignment at uniform slopes not less than 1/4 unit vertical in 12 units horizontal (2-percent slope) for 2 1/2-inch diameter and less, and not less than 1/8 unit vertical in 12 units horizontal (1-percent slope) for diameters of 3 inches or more. The drop on this page uses 0.25 and 0.125 inch per foot. The link is up.codes/viewer/colorado/irc-2021/chapter/30/sanitary-drainage.
What if the slope box is empty?
Set the mode to Inches per foot and leave the slope box blank. The run can stay at 10 ft. Code minimum still reads 0.25 in/ft · 2.0833% for a 2 inch drain. Slope reads Enter a slope. Drop reads —.
Is this the grade calculator or the roof pitch calculator?
No. Those pages turn a rise and a run into percent grade or a roof multiplier. This page is a drain: the fall over a pipe run, and the P3005.3 floor for the diameter you picked. A hot-water tank is the water heater size calculator.