Slope and Grade Calculator
Convert slope between percent, ratio, degrees and fall over a run, and see the fall for any run length.
Slope & Grade Calculator
Percent, ratio, degrees and fall over a run are four ways of writing one slope. Drainage is specified in all four depending on who drew it, so this converts between them and checks the fall.
Quick answer: grade in percent, ratio and degrees
| Percent | Rise : run | Angle | Drop per 10 ft |
|---|---|---|---|
| 0.5% | 1 in 200 | 0.29° | 0.6 in |
| 1% | 1 in 100 | 0.57° | 1.2 in |
| 2% | 1 in 50 | 1.15° | 2.4 in |
| 5% | 1 in 20 | 2.86° | 6 in |
| 8.33% | 1 in 12 | 4.76° | 10 in |
| 10% | 1 in 10 | 5.71° | 12 in |
| 25% | 1 in 4 | 14.04° | 30 in |
| 50% | 1 in 2 | 26.57° | 60 in |
| 100% | 1 in 1 | 45° | 120 in |
Percent = rise ÷ horizontal run × 100; angle = arctan(percent ÷ 100). A 1:12 ramp is 8.33 %, 4.76°. 100 % is 45°, not vertical. Earthwork batters are written the other way round (horizontal : vertical), so check which number comes first.
| Percent | — |
|---|---|
| Ratio | — |
| Angle | — |
| Fall over the run | — |
| Per 10 ft / per metre | — |
| Slope length (hypotenuse) | — |
The colon reverses meaning between trades. A roof or a ramp is written rise over run, so 1:12 is one up in twelve along. That is 8.33%, or 4.76°, not 12%. Earthwork is written the other way round. OSHA defines maximum allowable slope as the ratio of horizontal distance to vertical rise (H:V) (29 CFR 1926 Subpart P, Appendix B), so the Type C limit of 1½:1 is 34° from horizontal. Read those same digits as rise over run and you get 56°, steeper than the 53° OSHA allows even in Type A, its most stable soil class. Find out which way the drawing writes it before you dig.
Ratio and percent are the same arithmetic. Degrees is not. A ratio is rise over run; percent is that times a hundred, so both scale. Degrees is the arctangent, which does not: 100% is 45°, not vertical. Round a 1:12 ramp to a tidy 5° and you have specified 8.75%, which is over the ADA limit. Convert it and carry the decimal.
A quarter inch per foot is one number doing two opposite jobs. IPC Table 704.1 makes it a minimum. It is the least fall permitted on horizontal drainage piping 2½ inches and smaller, where 3 to 6 inch pipe needs only 1/8 inch per foot. The 2010 ADA Standards make that same quarter inch a maximum. §403.3 and §405.3 cap cross slope at 1:48, and 1:48 is exactly 0.25 inch in 12, while §405.10 still requires landings subject to wet conditions to shed water. Detail that landing at a round 2% and the entire margin is a tenth of an inch over ten feet.
Check the adopted code, not this page. These are model-code and federal baselines, amended locally, and OSHA's ratio is the output of a competent person's soil classification, not a substitute for one. This page converts notation. It cannot tell you which notation the reviewer is holding.
What I check on a job
Where the run starts and ends. Before I trust the fall, I check where the run was measured. The page assumes horizontal distance between two spot elevations. A dimension taken along the sloped surface is a different number, though at paving grades the difference is a fraction of an inch and only matters on steep surfaces. Then I check both ends. The high end usually sits against something that cannot move, a door threshold or a slab edge, and the low end has to land on a drain rim or a grade that is already fixed. A correct percent between two wrong elevations is still a puddle. In practice the number goes wrong at the hand-off. The drawing says a percent, but the crew sets the surface with a string line and a level, so I give them the fall in inches over a stated run, as the tape reads it, and I put a level on the finished surface before the next layer covers it.
Percent is rise ÷ run × 100. Ratio “1 in 20” is the same as 5%.
Degrees are arctan(rise ÷ run). Degrees and percent agree only at zero and diverge
as the slope steepens, which is why mixing the two on one drawing causes trouble.
Common planning references: paved surfaces are usually drained at about 1–2%, and finished grade
is typically pitched away from a building for the first few feet. Accessible routes and ramps are
governed by accessibility regulations with their own limits and landing rules. Every one of those is
set by the applicable code and by the design, not by this page. Confirm the required figure for
your project rather than taking a number from here.
Worked example
A 2% slope over a 20 ft run: how much does it fall, and what angle is that?
- 2% is 2 in 100, i.e. a ratio of 1 in 50.
- Fall = 0.02 × 20 ft = 0.4 ft = 4.8 in, which the tape reads as 4-13/16".
- Angle = atan(0.02) = 1.15°.
It falls 4-13/16" (0.4 ft) over 20 ft, a 1-in-50 grade at about 1.1° — the slope a patio or a drain apron is usually laid at.
8 values on this page were driven in a browser and compared with independently derived figures on 2026-10-29 (2808 values across 228 calculators site-wide).
Questions
How do you convert a percent slope to a ratio?
Divide 100 by the percent: 2% is 1 in 50, 5% is 1 in 20. The calculator prints percent, ratio, degrees and the fall over your run together.
How much fall is a 2% slope over 20 feet?
0.4 ft, which is 4-13/16 inches on the tape.
Can I call a 1:12 ramp 5 degrees?
No. 1:12 is 8.33%, 4.76°. Round it to a tidy 5° and you have specified 8.75%, which is over the ADA limit. Degrees are an arctangent and do not scale with percent; convert and carry the decimal.
Why does a 2% landing leave almost no margin?
Because the 2010 ADA Standards cap cross slope at 1:48, which is 2.08%, 0.25 in. in 12. Detail a landing that must shed water (§405.10) at a round 2% and the whole margin is a tenth of an inch over ten feet.