Pipe Slope Calculator
Find pipe or channel slope from rise (drop) and horizontal run. Results include percent grade, slope ratio, inches per foot, and feet of fall per 100 feet.
Enter your values, then press Calculate.
How to calculate
Enter the elevation difference (drop) and the horizontal distance between the two points. Field shots are often in inches of drop over a tape distance in feet. The calculator converts both values to feet, then reports percent, ratio, inches per foot, and fall per 100 feet.
Formula
Slope = rise ÷ run. Percent grade = slope × 100. Inches per foot = (rise in inches) ÷ (run in feet). Ratio 1:X uses X = run ÷ rise. Feet of fall per 100 feet equals the percent grade when rise and run use the same length unit.
Common use cases
- Sewer and storm lateral slope checks
- Trench grade from hub elevations
- Converting 1/8 in per foot to percent
- Checking minimum pipe slope from plans
Assumptions
- Drop and run are on the same pipe: invert to invert, not rim to invert.
- Run is horizontal station distance unless you deliberately enter slope length.
- Percent = (drop ÷ run) × 100. Inches per foot = drop inches ÷ run feet.
- A zero drop is a level line, not an error.
Limitations
- Does not look up a plumbing code minimum (¼ in/ft, 1%, 2%).
- Does not compute hydraulic capacity or Manning flow.
- Does not replace the percent-grade tool for roads and swales — those pages use rise/run without pipe units.
- Does not station the run. Pair with station difference if you only have STA values.
Worked examples
6 inches over 50 feet is 1 percent
Invert drop 6 in on a 50 ft lateral is 1% and 0.12 in/ft. That is flatter than the ¼ in/ft house-sewer rule of thumb, which is the point of checking.
1. Convert rise to feet
6 in ÷ 12 = 0.5 ft
2. Decimal slope
0.5 ft ÷ 50 ft = 0.01 ft/ft
3. Percent grade
0.01 × 100 = 1 %
4. Inches of fall per foot of run
(0.5 ft × 12) ÷ 50 ft = 0.12 in/ft
Quarter inch per foot
1. Convert rise to feet
0.25 in ÷ 12 = 0.0208 ft
2. Decimal slope
0.0208 ft ÷ 1 ft = 0.0208 ft/ft
3. Percent grade
0.020833 × 100 = 2.0833 %
4. Inches of fall per foot of run
(0.0208 ft × 12) ÷ 1 ft = 0.25 in/ft
Why pipe slope is not the same page as percent grade
Both tools divide rise by run. Pipe slope starts from a typical utility shot: drop in inches over a taped or stationed run in feet, then reports inches per foot next to percent and 1:X. Percent grade is the site/road page and does not lead with in/ft. Use this calculator for sewer, storm, and other laterals; use percent grade for drives, pads, and swales.
Inches per foot on the trench
Inches per foot = (rise in inches) ÷ (run in feet). A 6 in drop in 50 ft is 0.12 in/ft and 1%. 1/8 in/ft is 1.042%; 1/4 in/ft is 2.083%. Minimum slopes on the plans or in the governing standard are not computed here. Pair invert elevations with station difference to get rise and run, then enter those two numbers.
Quarter inch per foot is a unit, not a hidden default
Six inches of drop in 50 ft of run is 1.00% and 0.12 in/ft. A house sewer called at ¼ in/ft is 2.08%. The calculator will happily report a flatter slope if you type a smaller drop — it does not enforce the code. Use this page to check what the inverts you surveyed actually produce, then compare that number to the spec yourself.
Related calculators
- Percent Grade CalculatorConvert rise and run into percent grade, slope ratio, and feet per 100 feet.
- Bedding CalculatorEstimate pipe bedding cubic yards from length, width, and depth.
- Pipe Volume CalculatorCalculate internal volume of a circular pipe in cubic feet and gallons.
- Trench Excavation CalculatorConvert trench length, width, and depth into cubic yards of excavation.
FAQ
What is a 1% pipe slope?
A 1% slope drops 1 foot in 100 feet, or 0.12 inches per foot. A 6 inch drop in 50 feet is 1%.
How do I convert inches per foot to percent?
Divide inches per foot by 12, then multiply by 100. 1/8 in/ft is 1.042%, and 1/4 in/ft is 2.083%.
Does this replace the design slope on the plans?
No. Use it to check field measurements against the design. Minimum slopes for sewer and storm pipe come from the plans and the governing standard.
Should I use percent grade instead?
Use percent grade for roads, drives, and swales. Use this page when you care about inches per foot or fall per 100 ft on a pipe.
Disclaimer
These calculators are estimating tools for planning and field checks. They are not a substitute for approved plans, specifications, licensed professional engineering, or governing standards. Confirm densities, waste factors, and yields with your supplier and project documents.