Rise over run
Percent grade and slope calculator.
Enter a rise and a run in real units — feet and inches, metres, or plain numbers — and get the percent grade, the angle, the ratio and the fall per foot.
Units are welcome: 2 ft, 24 in, 0.6 m, or 2' 6"
Percent Grade
8%
Angle
4.57°
Ratio
1 : 12.5
Slope (m)
0.08
Fall per Foot
0.96″ per ft
Permille
80‰
80 mm per m
Sloped Length
25′ 15/16″
Moderate — Within the 1:12 limit the ADA sets for a ramp on an accessible route.
How That Compares
Where your grade sits against slopes you already have a feel for.
- 1%Minimum drainage fall on pavingEnough for water to run off a flat surface.
- 2.08%Drain pipe at ¼″ per footThe plumbing code minimum for small drain lines.
- 6%Steepest US interstate gradeInterstate design practice caps sustained grades near 6%.
- 8.33%ADA ramp maximum1:12 — the steepest ramp allowed on an accessible route.
- 20%Typical steep city streetThe famously steep streets in San Francisco reach roughly this.
- 35%Steepest paved public roadBaldwin Street in Dunedin, New Zealand, is about 1 in 2.86.
- 100%45 degreesRise equals run. The point most people badly misjudge.
The Formula
Percent slope and percent grade are two names for the same measurement, and it is the plainest measure of steepness there is:
grade = (rise ÷ run) × 100
The rise is the vertical change, the run is the horizontal distance covered, and both must be in the same unit before you divide. Mixing feet and inches is the usual source of a wrong answer by a factor of twelve — this calculator handles the conversion for you, so 2 ft over 300 in works exactly as you would hope.
Run Means Horizontal, Not Along the Slope
This is the definition people most often get wrong on site. The run is the horizontal distance between the two ends — the distance you would measure with a level held flat, not a tape laid on the ground. Measuring along the sloped surface gives a run that is too long, and therefore a grade that is too gentle.
How much does it matter? At 5% the sloped length is only 0.1% longer than the run, so nobody notices. At 20% it is 2% longer, and at 50% it is 12% longer. For gentle drainage work you can measure either way; for anything steep, measure the horizontal properly.
Getting a Feel for a Percent Slope
A percent grade is easy to compute and surprisingly hard to picture. These are worth committing to memory as anchors:
- 1–2% — reads as flat, but drains. The minimum fall on paving and pipework.
- 8.33% — the 1:12 ADA ramp limit. Noticeable, but manageable in a wheelchair.
- 6% — about as steep as a sustained interstate grade ever gets.
- 15% — where driveways start becoming a problem in ice.
- 100% — 45°. Rise equals run.
For the full picture, see the slope conversion chart, or convert a single value with the slope to degrees converter.
Common Questions
How do you calculate percent grade?
Divide the rise by the run and multiply by 100: grade = (rise ÷ run) × 100. A rise of 2 feet over a run of 25 feet is (2 ÷ 25) × 100 = 8%. Both measurements must be in the same unit, and the run is the horizontal distance, not the sloped distance.
Is the run the horizontal distance or the length along the slope?
The horizontal distance. Percent grade is defined as rise over horizontal run, so measuring along the sloped surface gives a smaller answer than the true grade. On gentle slopes the difference is negligible, but at 20% the sloped length is about 2% longer than the run.
What is a 1% grade?
A 1% grade rises one unit for every hundred units of horizontal distance — one foot in a hundred feet, or one centimetre in one metre. It is roughly the minimum fall used to drain a paved surface, and it reads as flat underfoot.
Can a percent grade be more than 100%?
Yes. A 100% grade is 45°, where the rise equals the run, and anything steeper than that exceeds 100%. A 200% grade rises two units for every one across, which is about 63.4°. The percentage only becomes undefined at true vertical, where the run is zero.
What is the maximum grade for a driveway?
Most local codes cap driveways somewhere between 12% and 20%, with 15% a common limit, and gentler transitions required at the top and bottom so vehicles do not bottom out. Anything above about 15% becomes difficult in ice or snow. Always check your local requirements.