Grade & Fall Calculator
Work out grade, fall or slope between two points as a percentage, ratio or angle with this free browser-based grade and fall calculator.
Reduced levels
Grade = (start RL − end RL) ÷ distance. Positive = fall, negative = rise.
Result
Before you rely on this: First-pass guide only. Verify safety-critical or regulated work against the relevant standards, your project requirements and a qualified professional.
How to use this calculator
- RL / Fall mode: enter the start RL, end RL and horizontal distance to get the fall, grade %, degrees, ratio and mm/m.
- Convert grade mode: choose a ratio, percent, degrees or mm/m, type the value, and read every other form at once.
- Optionally add a distance to see the fall over it, and copy the result string.
How it works
Grade is rise over run. From reduced levels, grade = (start RL − end RL) ÷ horizontal distance — a positive value is a fall, negative is a rise.
All grade forms are linked: percent = slope × 100, degrees = atan(slope), the 1:N ratio has N = 1 ÷ slope, and mm per metre = percent × 10. Fall over a distance = slope × distance.
Worked example
100.0 to 98.5 over 30 m. 1.5 m fall over 30 m = 5% = 1:20 = 2.862° = 50 mm/m. In Convert mode, 1:14 = 7.143% = 4.086° = 71.43 mm/m.
Common mistakes
- Mixing up rise and fall — a fall lowers the RL (start higher than end).
- Confusing a 1:N ratio with a percentage: 1:20 is 5%, not 20%.
- Reading degrees as percent — 5% is only about 2.86°.
Frequently asked questions
How do I convert 1:20 to percent?
Percent = 1 ÷ N × 100, so 1:20 = 5%. The Convert mode does this instantly along with degrees and mm/m.
Is 5% the same as 1:20?
Yes — a 5% grade falls 5 m over 100 m, which is 1 m over 20 m, i.e. 1:20.
How do degrees compare to percent?
Degrees = atan(slope). A 5% grade is about 2.86°, and 100% (1:1) is 45°. They only match near zero.
What grade do drains need?
It varies by pipe size and code, but common minimums are around 1% (1:100). Always check the relevant standard.
Related tools
- Ramp Length & Gradient Calculator
- Slope & Horizontal Distance Calculator
- Vertical Difference Calculator
- RL Difference Calculator
- Rise and Fall Levelling Worksheet
- Traverse Closure Ratio Calculator
Explore more in Surveying, Coordinates, GIS & CAD.
Cite this tool
Zerdly. (2026). Grade & Fall Calculator. Retrieved from https://www.zerdly.com/tools/grade-and-fall-calculator/
Tip: Enter any known values to calculate the remaining results.
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