Slope Calculator: Convert Percent, Degrees, and Ratio

Most people get slope wrong because they mix up percent and degrees. A 45-degree hill is not a 45% slope – it's 100%. That mistake leads to driveways too steep, gutters pitched incorrectly, or paths that fail inspection. Here's what experts check first: the ratio of rise to run, and which unit the local code expects.

The math is simple: slope percent = (rise / run) × 100. Degrees are the arctangent of that ratio. For a 2-foot rise over 20 feet of run, you get 10%. That equals about 5.7 degrees. Use the slope calculator below to switch between percent, degrees, and ratio without doing trigonometry by hand.

Slope Calculator: Percent, Degrees, Ratio

Enter the rise and run in any unit – feet, meters, inches – and the calculator gives you the slope in all three forms instantly. It also converts between percent, degrees, and ratio if you already have one value. For example, 15% is roughly 8.5 degrees, and 1:20 is exactly 5%.

If you're working from a site plan, the Land planner can read the real slope of a drawn plot directly. That gives you the actual ground shape, not just a single line.

What Is a Steep Slope? Site Planning Bands

Site planners use three informal bands to classify terrain:

• Gentle: under 5% – easy to build on, minimal grading.
• Moderate: 5% to 15% – typical residential lots, may need retaining walls on downhill sides.
• Steep: over 15% – requires special engineering, erosion control, and often a geotechnical report.

These bands help you quickly judge whether a lot is feasible for your project. A 10% slope slopes enough to affect drainage, but it's not a cliff. At 20% (11.3 degrees), you're in challenging territory for driveways and foundations.

Driveway Slope Limits: What Experts Check

For residential driveways, typical municipal guidance sets a maximum of 15% – about 8.5 degrees. Where snow or ice is common, experts recommend staying at 12% or lower. That's because icy slopes above 12% become difficult or dangerous for vehicles to climb, especially with front-wheel drive.

Run the numbers: a 12% slope means a 1.2-foot rise per 10 feet of run. At 15%, it's 1.5 feet per 10. That extra 0.3 feet per 10 seems small, but it makes a real difference in winter. Check your local code before pouring concrete.

ADA Compliance for Walks and Ramps

If you're designing an accessible route, the ADA standards are strict:

• Accessible route: maximum 5% slope (1:20). That's a 1-foot rise over 20 feet.
• Ramps: up to 8.33% (1:12). A 1-foot rise needs at least 12 feet of ramp length.

Use the calculator to verify your design. For a 6-inch step, a 1:12 ramp requires 6 feet of run. But that same rise as an accessible route would need 10 feet – so you often see a ramp rather than a path.

Percent to Degrees Quick Reference

Here are conversions to keep in your head:

• 10% = 5.7°
• 20% = 11.3°
• 45° = 100%

Most slope questions are answered by these three. If you need others, the calculator handles any value.

How to Use the Slope Calculator for Land Planning

When you have a property survey, pick two points on a contour or edge where you know the elevation difference (rise) and horizontal distance (run). Input those numbers. If you have a drawing with contour lines, plot the area in the Land planner and it calculates the actual slope of the terrain.

This is the same method surveyors use: measure rise over run, not the distance along the ground surface. The calculator does the rest.

Common Mistakes to Avoid

• Using degrees when the code says percent, or vice versa. Always check the unit.
• Measuring run as the sloped distance, not the horizontal. That understates the slope.
• Forgetting that slopes over 15% are steep – you may need retaining walls, drains, or permits.

Start With the Range, Then Get Precise

First, figure out what range your situation falls into: gentle, moderate, or steep. That tells you whether you need special design. Then use the slope calculator to get exact values for your plans, permits, or build details.

Whether you're grading a backyard, planning a driveway, or checking an ADA path, the tool above converts everything for you. For a full plot analysis, use the Land planner to see slopes across your entire property.