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.
- •The calculator needs horizontal run and does not calculate the sloped walking-surface length.
- •A slope result alone does not establish accessibility, safe use, structural adequacy, or code compliance.
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.
Horizontal run is the level plan distance from the start to the end of a straight ramp segment. Sloped length follows the walking surface and is longer. If rise and run are known, ideal sloped length can be found with the Pythagorean formula, square root of rise squared plus run squared. This calculator does not return that length or add transitions and landings.
Total rise may need several ramp runs. Rules can limit rise or length between landings, require level spaces at doors and turns, or set maneuvering clearances. Dividing total rise by a target ratio only finds combined horizontal run. It does not lay out individual runs or prove that they fit the site.
Accessibility criteria depend on the facility, route, alteration status, jurisdiction, and adopted standards. Requirements can address maximum slope and cross slope, clear width, landings, door maneuvering space, handrails, edge protection, guards, curb ramps, detectable warnings, headroom, drainage, and changes in level. The 1:12 comparison in this tool is only an early reference.
A ramp that is technically flat enough can still be difficult or unsafe. Surface slip resistance, water, ice, leaves, lighting, exposure, abrupt transitions, excessive cross slope, narrow approaches, and inadequate maintenance affect use. Mobility devices and users also vary. Good design considers the whole route, not one calculated number.
Structural design is separate from slope. Materials, spans, supports, connections, foundations, guard loads, corrosion, soil, frost, drainage, and vehicle or occupant loads require suitable design. Temporary ramps and portable products also need capacity, anchorage, edge, and setup checks from their manufacturer.
The implementation rounds run-per-rise to two decimals before comparing it with 12, then displays percent and angle to one decimal. A measurement very close to 1:12 can be affected by rounding and field error. Use design dimensions and the applicable standard rather than a rounded status label at a compliance boundary.
Measure rise vertically and run horizontally with suitable tools. Do not follow sloped ground or a board when measuring run. Survey error, uneven landings, deflection, and finished-surface buildup can change the completed slope. Inspect the installed route rather than relying only on planned dimensions.
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
- Treating total run as one uninterrupted segment when intermediate landings may be required
- Ignoring cross slope, door clearance, drainage, surface condition, and approach space
- Using the rounded 1:12 status as a permit, inspection, or accessibility determination
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