Ramp Slope Calculator
A ramp that rises 30 inches over 360 inches of horizontal run is a 1:12 slope, an 8.3 percent grade, and about a 4.8 degree angle. Use this ramp slope calculator to convert rise and run into the three formats people use most when planning a ramp. Enter the vertical rise and the horizontal run in inches, and the calculator shows the slope ratio, percent grade, and angle.
Quick answer
A 1:12 ramp slope means 1 inch of rise for every 12 inches of horizontal run.
What this tells you
- •A 1:12 ramp slope means 1 inch of rise for every 12 inches of horizontal run.
- •Percent grade is rise divided by run, multiplied by 100.
- •A 1:12 slope works out to 8.3% grade and about 4.8 degrees.
How to Use
- 1Measure the total vertical rise in inches.
- 2Measure the horizontal run in inches. Do not use the sloped walking surface length.
- 3Enter both values and click Calculate.
- 4Read the result as a slope ratio, percent grade, and angle in degrees.
- 5Use the 1:12 note as a planning check only, then verify the real project requirements.
How It Works
Formula
Slope Ratio = 1 : (Run / Rise)
Percent Grade = (Rise / Run) x 100
Angle = atan(Rise / Run) x 180 / piThe ramp steepness comes from rise divided by run. To express that as a 1:X ratio, divide the horizontal run by the rise. The same rise-to-run fraction also gives percent grade when multiplied by 100, and angle when passed through the arctangent function.
Calculation note: values are processed in the order shown above, using the current input units.
Worked Examples
30-inch rise with a 30-foot run
Divide 360 by 30 to get a 1:12 slope ratio. The same ramp has a rise-to-run fraction of 30/360 = 0.0833, which is an 8.3% grade and an angle of about 4.8 degrees.
24-inch rise with a 16-foot run
This ramp is steeper because it only gives 8 inches of horizontal run for each inch of rise. That pushes the grade to 12.5% and the angle to about 7.1 degrees.
6-inch rise with an 8-foot run
Longer run for the same rise makes the ramp flatter. Here the ramp gives 16 inches of run for each inch of rise, so the percent grade and angle both drop.
Common Ramp Slope Reference
Quick rise-to-run conversions for straight ramp planning.
| Slope Ratio | Percent Grade | Angle |
|---|---|---|
| 1:20 | 5.0% | 2.9° |
| 1:16 | 6.3% | 3.6° |
| 1:12 | 8.3% | 4.8° |
| 1:10 | 10.0% | 5.7° |
| 1:8 | 12.5% | 7.1° |
The 1:12 row is a common ADA-style planning reference, not a final code determination.
What does a 1:12 ramp slope mean?
A 1:12 ramp slope means every 1 inch of rise needs 12 inches of horizontal run. If a porch is 30 inches above grade, that works out to 360 inches of run, or 30 feet, before you account for landings or turns.
People also describe the same slope as an 8.3 percent grade or an angle of about 4.8 degrees. The math is the same. Only the format changes.
This calculator is useful for first-pass ramp planning, bid conversations, and sketch checks. It does not tell you whether a project is fully compliant because real ramp layouts can also depend on landings, handrails, edge protection, surface conditions, and local enforcement.
Common mistakes
- Using the sloped ramp board length instead of the horizontal run
- Mixing inches and feet in the same calculation
- Assuming a 1:12 slope by itself guarantees full ADA or local code compliance
- Ignoring landings, turns, and handrail requirements during planning