How to Calculate Slope Percentage
Slope percentage (also called percent grade) compares vertical rise to horizontal run:
Grade (%) = (rise ÷ run) × 100
Rise and run must be in the same unit before dividing — the calculator converts mixed units (say, inches of rise over feet of run) for you. The same slope can be written three ways:
- Percent: 8.33% — rises 8.33 units per 100 units of run
- Angle: 4.76° — the incline measured from horizontal, angle = arctan(rise ÷ run)
- Ratio: 1:12 — one unit of rise per 12 units of run
Measuring Rise and Run in the Field
For short distances — a ramp, patio, or driveway apron — rest a level on the surface, raise one end until the bubble centers, and measure the gap underneath: the gap is the rise and the level's length is the run. For longer ground slopes, use a line level on a taut string between two stakes, or a phone clinometer app for a quick angle reading (this calculator converts the angle to percent for you).
Slope Standards Worth Knowing
- Drainage minimum: 1–2% (¼ in per ft) for patios, pipes, and grading away from foundations
- ADA cross slope max: 2% on accessible routes
- Walkway limit: 5% — steeper than this and it must be built as a ramp
- ADA ramp max: 8.33% (1:12), with handrails and landings
- Interstate highway max: 6% sustained; mountain roads reach 7–10%
- Driveway comfort limit: 10–12%; most codes cap at ~15%
Grade, Angle, and Ratio Conversion Table
| Grade (%) | Angle | Ratio (1:N) | Typical Context |
|---|---|---|---|
| 0.5% | 0.3° | 1:200 | Near-flat sewer pipe minimum (4″+ lines) |
| 1% | 0.6° | 1:100 | Minimum drainage fall for patios and pipes |
| 2% | 1.1° | 1:50 | Maximum cross slope for accessible paths |
| 3% | 1.7° | 1:33 | Gentle graded road |
| 4% | 2.3° | 1:25 | Easy walking ramp |
| 5% | 2.9° | 1:20 | Max walkway grade before a ramp is required |
| 6% | 3.4° | 1:16.7 | Interstate highway maximum sustained grade |
| 7% | 4.0° | 1:14.3 | Steep mountain road |
| 8.33% | 4.8° | 1:12 | Maximum ADA wheelchair ramp |
| 10% | 5.7° | 1:10 | Steep hill, low gear for trucks |
| 12% | 6.8° | 1:8.3 | Steep residential driveway |
| 15% | 8.5° | 1:6.7 | Very steep driveway; common code maximum |
| 20% | 11.3° | 1:5 | Steep ski run or hiking trail |
| 25% | 14.0° | 1:4 | Black-diamond ski terrain |
| 33% | 18.3° | 1:3 | Typical maximum for tracked construction equipment |
| 50% | 26.6° | 1:2 | Extremely steep hillside; terraced construction |
| 66.7% | 33.7° | 1:1.5 | Common riprap embankment slope (1.5H:1V) |
| 100% | 45° | 1:1 | Rise equals run |
| 173% | 60° | 1:0.58 | Steeper than most staircases |
Slope Percentage vs. Slope of a Line
Percent grade is the construction and surveying way of writing the same m = rise/run that algebra uses for lines on a graph. If you are working from two coordinate points, need the line equation y = mx + b, or want the angle and distance between points with the full worked solution, use our Slope Calculator — it plots the line and shows every step.
Worked Example
You are building a ramp to a porch that sits 18 inches above grade, and you have 20 feet of room for the run:
- Convert to one unit: 18 in = 1.5 ft
- Slope = rise ÷ run = 1.5 ÷ 20 = 0.075
- Grade = 0.075 × 100 = 7.5%
- Angle = arctan(0.075) = 4.3°; ratio = 1 : 13.3
At 7.5% the ramp is under the 8.33% ADA maximum, so 20 feet of run is enough — though a gentler 1:16 (6.25%) would need 24 feet.