Slope and Grade Calculator
Convert between rise and run, percentage grade, angle and ratio.
Last reviewed September 19, 2026
Result
8.333%
No allowance applies. This is a conversion between four ways of writing the same slope, not a material estimate.
- Angle
- 4.764 °
- Ratio, one in
- 1:12
- Length along the slope, per unit of run
- 1.0035
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Results are estimates for planning only. Actual needs vary with site conditions, products and workmanship. Always confirm quantities with your supplier and follow local building codes and manufacturer instructions. Read the full disclaimer
How to use it
- Choose whether you know a rise and run, a percentage grade, or an angle.
- Enter it. The run is the horizontal distance, measured level, not along the ground.
- Read the other three forms, and the slope length factor.
How it is calculated
A slope is one ratio written four ways:
grade % = rise ÷ run × 100
angle = arctan(rise ÷ run)
ratio = 1 : (run ÷ rise)
slope length = √(1 + (rise ÷ run)²) per unit of runAll four come from the same rise over run, so entering any one of them reconstructs the others. Grade and angle are scale free: a rise of 1 in 12 and a rise of 10 in 120 are the same slope.
The slope length is the multiplier between horizontal distance and distance measured up the surface. At 1 in 12 it is 1.0035, which is negligible. At 1 in 2 it is 1.118, which is not.
Practical tips
Measure the run level, not along the ground. This is the single most common error. Laying a tape down a hillside measures the slope length, not the run, and using it as the run understates the grade.
Watch the units in a ratio. A 1 in 12 ramp means one unit of rise for twelve of run in the same unit. Mixing inches of rise with feet of run gives an answer twelve times out.
Grade and angle are not interchangeable in speech. A contractor saying “a 30 per cent slope” and one saying “30 degrees” mean very different things: 30 per cent is about 16.7 degrees. When it matters, ask which.
Check drainage over the whole run, not the ends. A patio can have the right overall fall and still hold water in the middle. Straightedge and level across it in several places.
Long ramps need landings. A continuous run at the maximum grade is usually limited in length, with level landings between sections. The ratio alone does not make a ramp usable or compliant.
With your numbers
- Angle = arctan(rise ÷ run) = 4.764 °
- Ratio = one in 12
- Grade = 1 ft rise ÷ 12 ft run = 8.333%
Quick reference
No allowance applies. This is a conversion between four ways of writing the same slope, not a material estimate.
| Job | Grade |
|---|---|
| 1 in 12 ramp | 8.333% |
| 1 in 20 | 5% |
| 1 in 8 | 12.5% |
| 2% fall for drainage | 2% |
| 5% grade | 5% |
| 45 degrees | 100% |
| 1 m in 12 m | 8.333% |
| 1.5% fall | 1.5% |
Common questions
- What is a 1 in 12 slope as a percentage and an angle?
- 8.333 per cent and 4.764 degrees. It is the ratio most often specified for wheelchair ramps, and it means one unit of rise for every twelve of horizontal run.
- What is the difference between grade, slope and angle?
- They describe the same thing in different units. Grade is the rise divided by the run as a percentage. The angle is the arctangent of that same ratio. A one-in-x ratio is its reciprocal. A 100 per cent grade is 45 degrees, not vertical, which is the point people most often get wrong.
- Why is a 100 per cent grade only 45 degrees?
- Because grade is rise over run, not a fraction of a right angle. When the rise equals the run the grade is 100 per cent and the angle is 45 degrees. Beyond that, grade keeps climbing without limit as the angle approaches 90.
- What fall does a drain or a patio need?
- Commonly around 1 to 2 per cent for surface drainage, which is roughly a 10 to 20 mm fall per metre, or a quarter inch per foot. Pipework has its own requirements that depend on diameter. Check what applies rather than guessing, because too much fall is a problem for drains as well as too little.
- Is the run measured along the ground or level?
- Level. The run is the horizontal distance between the two points, not the distance you would walk. On a steep slope those differ noticeably, which is what the slope length figure is for.
- How do I measure an existing slope?
- Put a level on a straight board with one end on the high point, lift the low end until the bubble centres, and measure down from the raised end to the ground. That drop over the board length is your rise over run.