Grade & Slope Calculator
Convert between grade percent, slope angle, and rise-over-run ratio for grading, ramps, and drainage.
Reviewed by the ToolNestr Editorial Team — July 2026
How the grade & slope calculator works
Slope describes how steeply ground or a surface rises over a horizontal distance, and three trades state it three ways. Grade percent is the rise divided by the run, times 100. The slope angle is the actual angle in degrees, found with the arctangent of rise ÷ run. The ratio form, like 1:12, reads as one unit of rise for every twelve of run. All three describe the same steepness.
The most common mistake is treating grade percent and angle as the same number. They are not: a 100% grade is a 45° angle, because a rise equal to the run makes a half-square triangle. Grade percent climbs toward infinity as the surface nears vertical, while the angle simply approaches 90°. This calculator converts freely among all three, and also reports the true slope length — the along-the-slope distance — using the Pythagorean theorem on rise and run.
Knowing the grade matters for real limits. Wheelchair ramps under the ADA cannot exceed 8.3%, the 1:12 ratio. Driveways are usually kept under about 15% to avoid vehicles scraping, and roads below 6 to 8% for safe stopping. Site drainage often targets a gentle 1 to 2% so water runs off without pooling or eroding. Enter what you know — rise and run, an angle, or a percent — and read the rest.
Slope limits referenced here follow the ADA Standards for Accessible Design (ramp slope, §405) and typical civil engineering practice for roadway and site-drainage grading.
Pick what you know
Rise and run, a slope angle, or a grade percent.
Enter the values
Keep rise and run in the same units.
Read all forms
Percent, degrees, ratio, and slope length.
The formula explained
Grade percent
= rise ÷ run × 100. A 2 ft rise over 40 ft: 2 ÷ 40 × 100 = 5%.
Slope angle
= atan( rise ÷ run ) = atan(0.05) = 2.86°.
Slope length
= √(rise² + run²) = √(4 + 1600) = 40.05 ft along the slope.
Worked example
A driveway climbing 3 ft over a 25 ft run.
Tips for grading and slope work
Keep units consistent
Rise and run must be in the same unit before dividing — mixing inches of rise with feet of run is the single most common source of a wildly wrong grade percent. Convert everything to feet, or everything to inches, before entering the numbers.
Check local code before finalizing
ADA, IBC, and local municipal codes set hard maximums for ramps, stairs, and driveway aprons, and they sometimes differ from the general guidelines cited here. Always confirm the exact limit for your jurisdiction and application before pouring or grading to a calculated slope.
Real-world context
ADA ramp compliance is one of the most common reasons this calculator gets used. A ramp rising 2.5 ft (30 inches) over a 30 ft run computes to 2.5 ÷ 30 × 100 = 8.33% grade, right at the ADA's 1:12 maximum — and the slope length works out to 30.10 ft, about 4 inches longer than the flat run, which matters when a contractor is ordering ramp decking or handrail sections to exact length.
Road and driveway grading tells a similar story at a larger scale. A stretch of road climbing 18 ft over a 300 ft run comes out to 18 ÷ 300 × 100 = 6.00% grade, a 3.43° angle — right at the upper edge of what most road design guidelines consider safe for stopping distance in wet or icy conditions, which is why civil engineers rarely let sustained grades exceed 6 to 8%.
At the gentle end, a drainage swale dropping just 0.5 ft over a 50 ft run is exactly 0.5 ÷ 50 × 100 = 1.00% grade — barely over half a degree — which is the minimum most landscapers target so water actually moves toward a drain instead of pooling, but shallow enough that a small grading error can flatten it out entirely.
A steep residential driveway climbing 9 ft over a 45 ft run works out to 9 ÷ 45 × 100 = 20% grade, well past the roughly 15% ceiling most guidelines recommend — the kind of number that flags a project for a transition apron at the street or garage to keep the undercarriage from scraping.
Common misconceptions
"A 100% grade is a vertical, 90° wall." It is actually 45°. A 100% grade means the rise equals the run, forming a half-square right triangle — not a cliff face. Grade percent keeps climbing toward infinity as the surface approaches true vertical, while the angle only ever approaches 90°.
"A 1:12 ramp ratio and an 8% grade are basically the same thing, close enough." They round differently. A 1:12 ratio is 1 ÷ 12 × 100 = 8.33%, not 8%. On a long ramp run that quarter-percent difference compounds into inches of rise, which is exactly the kind of gap that fails an ADA inspection.
Related calculators
Frequently asked questions
How do I calculate grade percent from rise and run?
Grade percent equals the rise divided by the run, multiplied by 100. A 2-foot rise over a 40-foot run is 2 ÷ 40 × 100 = 5% grade. Rise and run must be in the same units.
What is the difference between grade percent and slope angle?
Grade percent is rise over run as a percentage, while the slope angle is the actual angle in degrees, found with arctangent of rise ÷ run. A 100% grade is a 45° angle — they are not the same number, which is a common mix-up.
What grade is too steep for a driveway?
Most codes and guidelines keep driveways under about 15%, with steeper short sections needing transition zones to avoid scraping. Wheelchair ramps under the ADA must not exceed 8.3% (a 1:12 ratio), and roads are typically held below 6 to 8%.
What does a slope ratio like 1:12 mean?
A ratio of 1:12 means one unit of rise for every twelve units of run — a rise-over-run of 1 ÷ 12, which is about 8.3% grade. Landscapers often state slope as a ratio, while engineers use percent and surveyors use degrees.
Sources & references
This tool uses standard formulas and reference values from:
- • American Concrete Institute — ACI 318, Building Code Requirements for Structural Concrete. concrete.org
- • ICC — International Residential Code (IRC), span, footing and framing tables. codes.iccsafe.org
- • APA – The Engineered Wood Association, allowable span and load guidance.
Estimates for planning only. Span, load and code values vary by jurisdiction — verify against your local adopted code and a licensed engineer before building.