15 percent: a steep driveway
15% is 8.53°, about 1 in 6.67, and half as steep again as the 10% at the top of the usual driveway range. It rises 1.8 inches per foot, 15 feet over 100 feet of run.
The distance you travel is barely longer than the map distance: 100 feet of run is 101.12 feet along the slope. The steepness is felt in traction and in how fast water runs off, not in extra length.
A yard that drops 15% falls 4.5 feet across a 30 foot lawn.
15% slope: rise over a distance
A 15% slope rises 15 feet in every 100 feet of level distance.
| Run (level distance) | Rise in inches | Rise in feet | Length along the slope |
|---|---|---|---|
| 1 ft | 1.8 in | 0.15 ft | 1.01 ft |
| 10 ft | 18 in | 1.5 ft | 10.11 ft |
| 20 ft | 36 in | 3 ft | 20.22 ft |
| 50 ft | 90 in | 7.5 ft | 50.56 ft |
| 100 ft | 180 in | 15 ft | 101.12 ft |
Worked example
15 ÷ 100 = 0.15. The angle is the arctangent of that: arctan(0.15) = 8.53°. As a ratio, 100 ÷ 15 = 6.67, so the slope is 1 in 6.67. Per foot, 0.15 × 12 = 1.8 inches.
Nearby grades in degrees
| Grade | Angle | Ratio | Rise per foot |
|---|---|---|---|
| 8.33% | 4.76° | 1 in 12 | 1 in |
| 10% | 5.71° | 1 in 10 | 1.2 in |
| 12% | 6.84° | 1 in 8.33 | 1.44 in |
| 15% | 8.53° | 1 in 6.67 | 1.8 in |
| 20% | 11.31° | 1 in 5 | 2.4 in |
| 25% | 14.04° | 1 in 4 | 3 in |
| 30% | 16.70° | 1 in 3.33 | 3.6 in |
For any other rise, run or grade use the slope and grade calculator. The guide slope and grade for drainage and ramps covers how much fall each job needs.
Want to change anything else? The calculator above is the full Slope and Grade Calculator: edit any number and the results update instantly.
Tools & materials for this project
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Frequently asked questions
What is a 15 percent slope in degrees?
8.53 degrees. Divide the percent by 100 and take the arctangent: arctan(0.15) = 8.53°.
How steep is a 15 percent slope?
8.53°, or 1.8 inches of rise for every foot along the ground. It climbs 15 feet in 100 feet.
How much does a 15% slope rise per foot?
1.8 inches for every foot of run, which is 18 inches over 10 feet and 15 feet over 100 feet.
Related sizes and guides
Estimates only. Results assume standard materials and simple shapes. Supplier products vary, and the final call on quantity, structure and safety is yours. Check your local code and your supplier's guidance before ordering.