Slope & Grade Calculator
Convert between percent grade, run:rise ratio, angle in degrees, and rise over run — solve from whichever pair you have. The same slope gets named three different ways on a job (a 2% pad, a 12:1 ramp, a 1.15° laser setting), and this tool keeps them honest against each other for drainage falls, ramps, and site grading. Free, no login.
Inputs
Rise & run use any consistent unit (ft, in, m).
Slope
Enter values to calculate
How this calculator works
This is pure trigonometry — no code table behind it, and the page cites none. Everything derives from rise and run in the same units:
grade % = rise ÷ run × 100 · ratio = run ÷ rise : 1 · angle = atan(rise ÷ run)
Enter a grade or an angle instead and the calculator just inverts the formula to recover the rise for your run. Two things trip people up. First, the ratio here is run:rise (a 4:1 slope runs 4 ft for every 1 ft of drop) — the excavation convention; some civil drawings write it the other way, so check which side the “1” is on. Second, percent grade and degrees are not proportional: 100% grade is 45°, not 90°, because grade is a tangent. Below about 10% they track closely (grade ≈ 1.75 × degrees), then diverge fast.
Worked example
An entry ramp needs to climb 2.5 ft of rise in the 30 ft of run available:
- Grade: 2.5 ÷ 30 × 100 = 8.33%.
- Ratio: 30 ÷ 2.5 = 12:1 — exactly the 1:12 commonly cited as the accessibility maximum for ramp running slope, with zero margin.
- Angle: atan(2.5 ÷ 30) = 4.76°.
Enter the same slope above (Rise & Run mode, rise 2.5, run 30) to reproduce it. Since it lands dead on the limit, any as-built bump makes it fail — real designs buy margin with more run.
Reference: grade ↔ ratio ↔ degrees
The conversion ladder for common site slopes, computed by the same engine as the calculator. Notes are common practice waypoints, not code citations — thresholds that matter on your job come from your drawings and jurisdiction.
| Grade | Ratio (run:rise) | Angle | Note |
|---|---|---|---|
| 1% | 100:1 | 0.57° | typical minimum pavement drainage |
| 2% | 50:1 | 1.15° | common walk / patio cross-slope target |
| 5% | 20:1 | 2.86° | |
| 8.33% | 12:1 | 4.76° | 1:12 — commonly cited accessibility ramp maximum |
| 10% | 10:1 | 5.71° | |
| 15% | 6.67:1 | 8.53° | |
| 25% | 4:1 | 14.04° | |
| 33.33% | 3:1 | 18.43° | 3:1 — common mower / landscape limit |
| 50% | 2:1 | 26.57° | 2:1 — steep cut, verify soil will hold |
| 100% | 1:1 | 45° | 45° — 1:1 |
Frequently asked questions
How do I calculate percent grade?
Percent grade = rise ÷ run × 100, where run is the horizontal distance — 1 ft of fall over 100 ft of run is a 1% grade. Enter rise and run in any single consistent unit (both in feet, both in inches, both in meters) and the calculator returns the grade, the run:rise ratio, and the angle in degrees together.
What is a 2% slope in inches per foot?
2% of 12 inches is 0.24 in of fall per foot of run — just under the familiar 1/4 inch per foot, which is itself a 2.08% grade. That quarter-inch-per-foot figure is the common rule-of-thumb fall for flatwork and small-diameter drainage piping; for grading away from a building, the IRC calls for 6 inches of fall in the first 10 feet (5%), with exceptions where lot lines or impervious surfaces prevent it. Confirm the governing spec for your jurisdiction and scope.
What is the difference between percent grade and degrees?
They are different scales and do not track linearly: degrees = arctan(rise ÷ run), so a 100% grade is a 45° angle, not 90°. At shallow slopes the two feel interchangeable but are not — 2% is about 1.15°. The calculator always reports both so you can quote whichever the spec uses.
How do I read a slope ratio like 4:1?
This calculator writes ratios as run:rise — 4:1 means 4 units horizontal for every 1 unit vertical, a 25% grade. Watch the convention, because some standards flip it: the ADA ramp limit written 1:12 means 1 unit of rise per 12 of run (8.33%), which this tool would display as 12:1. When a spec cites a ratio, confirm which number is the run before you build to it.
Is run measured horizontally or along the slope?
Horizontally — a true plan-view distance, not tape stretched along the ground. On shallow grades the difference is negligible (at 5%, the slope length is only about 0.12% longer than the run), but on steep embankments taping along the surface overstates the run and makes the slope look flatter than it is. Pull level, or measure the slope distance and angle and let the trig give you the true rise and run.
Method: grade = rise ÷ run as a percent, ratio = run ÷ rise, angle from the inverse tangent — exact math with no code citation. The limits are where codes live, and they vary by context: accessibility standards for ramps and cross-slopes, excavation safety for temporary cut slopes by soil type, stormwater rules for minimum drainage falls. This tool converts between the three languages; it does not tell you what your jurisdiction allows — pull the governing standard for that. Slopes measured on grade with a torpedo level over a short base amplify error; longer bases and laser or level readings are more trustworthy.
Pair with the Drainage Pipe Slope Calculator for fall per run and the IPC minimum-grade check, the Roof Pitch Multiplier for the rise-in-12 roofing convention, or the trench slope chart for OSHA maximum excavation slopes by soil type.