Slope & Grade
How to Calculate Percent Grade
Percent grade = rise / run x 100. How to measure rise and run in the field with a string line or level, worked examples, and target grades for common projects.
Percent grade is the language of drainage, driveways, ramps, and every conversation with an excavation contractor. The formula takes ten seconds; what takes practice is measuring rise and run correctly in a lumpy yard. This guide covers both.
The formula and its family
Rise = Grade ÷ 100 × Run · Run = Rise ÷ Grade × 100
All three arrangements matter in the field: computing an existing slope (first form), figuring how much fall a path needs (second), and finding how far you can run before hitting a grade limit (third).
Grade, angle, and ratio - one slope, three languages
| Grade | Angle | Ratio (rise:run) | Rule-of-thumb meaning |
|---|---|---|---|
| 1% | 0.57° | 1:100 | Minimum pipe slope; 1/8 in per ft |
| 2% | 1.15° | 1:50 | Patio drainage; 1/4 in per ft |
| 5% | 2.86° | 1:20 | Foundation grading minimum |
| 8.33% | 4.76° | 1:12 | ADA ramp maximum |
| 10% | 5.71° | 1:10 | Comfortable driveway max |
| 25% | 14.0° | 1:4 | Steep lawn; mowing limit approaching |
| 50% | 26.6° | 1:2 | Typical cut-slope limit in good soil |
| 100% | 45° | 1:1 | Reference point: not vertical! |
Conversion math lives in Percent Grade to Degrees, or let the calculator translate all three at once.
Field method 1: string line and line level ($10)
- Drive stakes at the top and bottom of the slope you're measuring.
- Tie a string to the uphill stake at a marked height; stretch tight to the downhill stake.
- Hang a line level mid-string; raise/lower the downhill end until level; tie off.
- Rise = (string height on downhill stake) − (string height on uphill stake). Run = taped distance between stakes.
Example: string sits 4 in up the uphill stake and 22 in up the downhill stake, stakes 30 ft apart: rise = 18 in = 1.5 ft; grade = 1.5 ÷ 30 × 100 = 5%.
Field method 2: the 4-ft level ($0 if you own one)
For short distances: set one end of the level on the high ground, lift the free end to bubble-center, measure the gap below it. Gap 1.25 in over a 48-in level = 1.25 ÷ 48 × 100 = 2.6%. Repeat and average a few placements - single readings on rough ground mislead.
Field method 3: laser level (rent ~$40/day)
For whole-yard work: one setup gives elevations at any number of points via a receiver rod. Note elevation and taped distance for each point; grade between any two = elevation difference ÷ distance × 100. This is the method for planning swales, drains, and pads where you need a profile, not just one number.
Measured rise and run? The calculator returns grade, angle, ratio, and fall-per-distance instantly.
Open the Slope CalculatorCommon mistakes
Taping along the ground. Run is horizontal. On a 20% slope, surface-taping inflates run ~2% (small); on steep banks it's no longer small - and mixing the two definitions across a project compounds.
Negative grades hiding in a positive average. A path can fall 2% overall yet rise through a 6-ft section - where every puddle will live. Check intermediate stations, not just endpoints.
Confusing per-foot rules with totals. "1/4 inch per foot" and "2%" are the same thing; "2 inches over the patio" is not a grade until you divide by its length.
When to call a professional
Legal grade matters - drainage crossing property lines, floodplain elevations, ADA compliance, driveway permits - are surveyed, not string-lined. And slopes showing tension cracks or slumping deserve a geotechnical eye regardless of what any level says.
Frequently Asked Questions
What is the formula for percent grade?
Grade (%) = rise / run x 100, with rise and run in the same units. A 6-inch drop over 25 feet: 0.5 / 25 x 100 = 2% grade.
What does a 2% grade mean in inches?
About 1/4 inch of fall per foot, or 2 feet of fall per 100 feet. It's the standard slope for patios and paved surfaces that must shed water.
Is percent grade the same as degrees?
No. Percent compares rise to run; degrees measure the angle. 100% grade = 45 degrees. For small slopes they look similar (2% is about 1.15 degrees) but they diverge steeply.
How do I measure grade with just a level?
Rest one end of a straight 4-ft level on the high ground, lift the other end until the bubble centers, and measure the gap beneath it. Gap in inches / 48 inches x 100 = percent grade.
What grade should ground have away from a house?
Building code (IRC R401.3) calls for a minimum 5% slope away from foundations for the first 10 feet - that's 6 inches of fall - where site constraints allow.