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Slope Percent to Degrees Calculator

Convert a percentage gradient into degrees for setting out drainage, ramps and driveways.

Computed in your browser — nothing you enter is uploaded. Figures are presented for United States against IRC 2024, and every formula is cited under regulatory standards below.

Last verified 2026-08-26 · v1.0.0

Market
Imperial · sales tax

Slope angle

1.15 °

High confidence

Angle = arctan(percent ÷ 100). A larger ratio denominator means a gentler slope, while a larger percentage means a steeper one — the two run in opposite directions.

Drainage falls, ramp gradients and driveway slopes get expressed as percentages, ratios and degrees depending on which trade or standard is speaking, and this converts the first into the last. For the shallow angles that most site work involves, the percentage and the angle track each other closely — two percent is 1.15 degrees — but they diverge sharply as things steepen, so the approximation should not be carried into roof work. The genuinely confusing part is that ratios run the other way: a 1:12 ramp is steeper than a 1:20 one, while 8% is steeper than 5%. Keeping straight which convention a drawing uses prevents a fall being set out backwards.

Preliminary estimate, not certified engineering. This tool produces an indicative quantity calculation for planning purposes only — it is not a certified structural analysis, a guaranteed material takeoff, or a substitute for building department approval. Always verify measurements on-site and have a licensed contractor or structural engineer review any load-bearing, code-sensitive, or safety-critical work before purchasing materials or starting construction. Spotted an arithmetic or standards error? Report it to contact@craftquantities.com with your inputs — confirmed fixes become pinned regression tests.

[Schema Verified] Computed in alignment with American Concrete Institute (ACI 318-19) formulas and International Residential Code (IRC 2024) spatial boundaries.

Regulatory standards & verification citations

  • Slope percentage = (rise / run) x 100; angle = arctan(rise / run)
  • ADA maximum running slope for a ramp is 1:12, which is 8.33% or about 4.76 degrees

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Frequently asked questions

Is percent the same as a 1-in-X ratio?
Related but inverted. A 1:12 ratio is one unit of rise over twelve of run, which is 8.33%. Divide 100 by the percentage to get the ratio denominator, and remember that a larger ratio number means a gentler slope while a larger percentage means a steeper one.
What gradient does surface drainage need?
Commonly a 2% minimum fall away from a building for paving and hard landscaping, though local requirements vary and specific finishes have their own minima. Too flat and water ponds; the tolerance on a long run is what usually causes the problem.
Why do ramps use ratios instead?
Because accessibility standards were written that way and the ratio is easier to check on site with a level and a measure. A 1:12 maximum running slope is the familiar figure, equal to 8.33% or 4.76 degrees.