Drainage Slope & Fall Calculator
Drainage geometry
Solve grade, fall, run or elevations
Result
Calculation summary
Enter values to see the result
Signed grade preview
Start, run and intended flow
How to use this calculator
- 1Choose whether to solve slope from measured fall, required fall from a target slope, available run from fall, or slope from start and end elevations.
- 2Enter horizontal run and vertical values in independent units; run must be horizontal rather than the sloped surface length.
- 3For elevation mode, enter values in the direction water is intended to travel so a lower end elevation produces positive fall.
- 4Compare percent, 1:n ratio, angle, mm/m and in/ft, then verify the required grade with the applicable project documents.
Formula
Slope % = vertical fall ÷ horizontal run × 100; angle = arctan(slope ÷ 100)
A positive result falls in the intended direction. A negative elevation result is an adverse rise and is flagged.
Calculation steps
- Convert horizontal and vertical inputs to meters.
- Rearrange fall ÷ run × 100 for the selected unknown.
- Preserve the sign when start and end elevations produce an adverse grade.
- Convert the decimal grade to a 1:n ratio and angle.
- Report equivalent millimeters per meter and inches per foot.
- Calculate end elevation when a starting elevation is supplied.
Worked example
A 30 ft horizontal run at 2% requires 0.6 ft of fall, equal to 7.2 in, 20 mm/m, 0.24 in/ft, a 1:50 ratio and an angle of about 1.146°.
Assumptions
- Run is the horizontal plan distance, not pipe length or sloped surface distance.
- Positive fall means the end is lower than the start in the intended flow direction; elevation mode preserves adverse negative values.
- The tool calculates geometry only. It does not choose a minimum drainage grade for a pipe, roof, pavement, swale or accessibility route.
- Required slope depends on system type, diameter, flow, roughness, settlement, tolerances, code and local design criteria.
- Survey accuracy, construction tolerance and localized low points can matter more than the average grade.
- Hydraulic capacity, velocity, self-cleansing, erosion, structural support and frost protection are outside scope.
Sources
Frequently asked questions
Is percent slope the same as degrees?
No. Percent is rise or fall divided by horizontal run times 100; degrees are the inverse tangent of that decimal ratio.
What does a 1:50 ratio mean?
It means one unit of vertical change for every 50 of horizontal run, equivalent to 2%.
Why is my slope negative?
In elevation mode the entered end is higher than the start, creating an adverse rise in the intended flow direction.
Does the calculator choose the required pipe slope?
No. Use the slope required by the applicable design, manufacturer and authority.
Can run and fall use different units?
Yes. They are converted internally before the grade is calculated.
Is horizontal run the same as installed pipe length?
No. Installed length follows the slope and is slightly longer; this calculator uses horizontal run.