How to use the slope calculator
Slope turns up all over a project: ramps and accessible routes, site grading, driveways, flat roofs, gutters, drainage pipes, landscaping and section drawings. Each discipline writes it differently. Civil drawings use percent grade, roofers use inches per foot, accessibility codes use ratios like 1:12 and surveyors often work in degrees. This calculator converts between all of them in one step.
- Pick how your slope is written and enter the value. If you measured on site or scaled off a drawing, choose rise and run and enter both in the same unit.
- Enter a horizontal distance to see how much the slope rises or falls over that length, and how long the sloping surface is.
- Read the checks. The note under the results compares your slope with the accessibility, drainage and grading limits that come up most often.
Slope formulas
Slope = Rise ÷ Run
Percent grade = Rise ÷ Run × 100
Angle = atan(Rise ÷ Run)
Ratio = 1 : (Run ÷ Rise)
Inches per foot = Rise ÷ Run × 12
Sloped length = Run × √(1 + (Rise ÷ Run)²)
The key rule is that run is always measured horizontally. If you measure along the surface instead, you'll understate the slope slightly on gentle grades and noticeably on steep ones. Percent and degrees are not proportional to each other: 10% is 5.71°, but 100% is 45°, so never scale one from the other.
| Ratio | Percent | Degrees | In per ft | Typical use |
|---|---|---|---|---|
| 1:48 | 2.08% | 1.19° | 1/4" | Max accessible cross slope, min low-slope roof |
| 1:20 | 5% | 2.86° | 0.6" | Max walking surface before it counts as a ramp |
| 1:12 | 8.33% | 4.76° | 1" | Max accessible ramp |
| 1:8 | 12.5% | 7.13° | 1.5" | Steep driveway |
| 1:3 | 33.3% | 18.43° | 4" | Steepest lawn most mowers handle safely |
| 1:1 | 100% | 45° | 12" | Steep embankment |
Worked examples
Checking a garden path
A path climbs 0.45 m over 12 m. The slope is 0.45 ÷ 12 = 0.0375, so 3.75%, about 1:26.7 or 2.15°. That's under the 1:20 limit, so it can be an accessible walking surface without handrails or landings.
Fall on a flat roof
A membrane roof needs a minimum of 1/4 inch per foot. That's 2.08%, so over a 9 m drainage run the roof must fall at least 9 × 0.0208 = 0.19 m. The tapered insulation package has to be at least 190 mm thicker at the high end, before allowing for crickets around the drains.
A driveway in degrees
A topographic survey shows a 7° slope along a proposed driveway. tan 7° = 0.123, so the grade is 12.3%, or about 1:8.1. Over a 15 m driveway that's a 1.84 m change in level. Many local authorities cap driveways somewhere between 12% and 20%, so check the local standard and the transition at the street.
Who this calculator helps
- Architects checking accessible routes, setting finished floor levels against site levels and drawing sections.
- Landscape architects grading paths, lawns, swales and planting beds.
- Civil engineers and site designers converting between survey angles and design grades.
- Plumbers and roofers turning inches per foot into percentages and total fall.
- Contractors setting out ramps, driveways and slabs that must fall to drains.
- Students and homeowners making sense of slopes on drawings, permits and survey reports.
Practical tips
- Design a little under every maximum. Built work rarely lands exactly on the drawn level, so a ramp drawn at exactly 1:12 can end up too steep.
- Check both directions. An accessible route can meet its running slope but fail its 1:48 cross slope.
- For slopes above 5% on a walking route, design it as a ramp and size it with the ADA ramp slope calculator.
- Roof pitches are usually written as x/12. The roof pitch calculator adds rafter length and roof area.
- Turning a site section into a drawing? Use the architectural scale converter to scale levels and distances.
Want to see how a regraded garden or new terracing would look? Upload a photo of the site and try Landscape Design.
