Slope Percent to Ratio Converter
Convert a slope or grade percentage into a run-to-rise ratio (1 in N) and its angle in degrees — for ramps, roads and drainage.
How it works
Ratio 1 in N: N = 100 ÷ grade% · Angle = arctan(grade% ÷ 100)
A grade percentage is the rise as a percent of the horizontal run: a 25% grade climbs 25 units for every 100 forward. Expressed as a "1 in N" ratio, N is how far you travel horizontally for each unit of rise, so it is 100 divided by the percentage. The angle comes from the arctangent of rise over run — note that a 100% grade is not vertical, it is a 45° slope.
Worked example
A 25% grade rises 25 in 100, so N = 100 ÷ 25 = 4 — a 1 in 4 slope. Its angle is arctan(25 ÷ 100) = arctan(0.25) ≈ 14.04° from horizontal.
Frequently asked questions
Is a 100% grade a vertical cliff?
No. A 100% grade means the rise equals the run, which is a 45° angle. Grades can exceed 100% — a slope steeper than 45° — but they never reach "vertical" as a percentage.
What does "1 in 12" mean?
For every 12 units you travel horizontally you rise 1 unit — about an 8.33% grade. It is the maximum slope allowed for many accessible ramps.
How do I go from ratio back to percent?
Divide 100 by N. A 1 in 20 slope is 100 ÷ 20 = 5%.
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This calculator is for educational purposes. Double-check important results before acting on them.