Slope / grade calculator (ramp, roof, grade)
Calculate slope as a percentage, the angle and the ratio from rise and run.
Slope % = rise ÷ run × 100. Use the same unit in both fields.
How the calculation works
Slope (or grade) measures how much a ramp, roof or road "rises" relative to how far it "runs" horizontally. It can be a percentage, an angle or a ratio (1:N).
slope (%) = rise ÷ run × 100
angle = arctangent(rise ÷ run)
Step-by-step example
A ramp rising 1 m over 12 m horizontally:
- Percentage: 1 ÷ 12 × 100 = 8.33%
- Ratio: 1:12
- Angle: arctan(1/12) ≈ 4.76°
Useful references
- Accessible ramp: at most 8.33% (1:12).
- Roof: minimum slope depends on the tile type.
Note: percentage ≠ degrees. A 100% slope = 45° (rises as much as it runs), not 90°.
Examples
- Rises 2 m over 10 m: 20% slope (≈ 11.3°).
- A 1:12 ramp = 8.33% — the accessibility limit.
Frequently asked questions
What is the max accessible ramp slope?
Generally 8.33% (a 1:12 ratio), per accessibility codes.
Is a 100% slope a vertical wall?
No — it is 45°. Percentage is rise ÷ run; when equal, that is 100%. A vertical wall (90°) would be infinite.
How to convert percentage to degrees?
The angle is the arctangent of the percentage divided by 100. E.g. 25% → arctan(0.25) ≈ 14°.
Is a 100% slope the same as 90°?
No, it is 45°. Percentage is rise over run, and it has no upper limit — a vertical wall would be infinite.
What is the maximum slope for an accessible ramp?
The common reference is 8.33% (a 1:12 ratio), with steeper slopes allowed only in specific cases.
What slope do I need for drainage?
About 1–2% indoors toward the drain and around 2% outdoors, away from the building.
Related calculators
More Math calculators
See all →Scientific calculator
Free online scientific calculator: sine, cosine, tangent, root, power, log, ln, π and parentheses.
Percentage
Calculate percentages every way: X% of Y, increase, discount and percentage change.
Rule of three
Solve a simple rule of three (direct or inverse): enter three values and find the fourth.
Compound rule of three
Solve problems where two quantities vary at once (machines, days, output...), directly or inversely.
Area of shapes
Calculate the area of a square, rectangle, triangle, circle and trapezoid in seconds.
Logarithm
Calculate the logarithm of a number in any base, plus natural log (ln), log10 and log2.
Scientific notation
Convert a number to scientific notation (a × 10ⁿ) and back to decimal form.
Fraction to decimal
Convert a fraction into a decimal number and a percentage.
Updated on June 18, 2026 · by Rafael Rossi · Methodology & sources