Slope Calculator
A slope calculator divides rise by run: grade % = rise ÷ run × 100. A 2 ft rise over 100 ft is a 2% grade, 1.15 degrees, or a 1:50 ratio.
Slope calculator: grade, angle, and ratio from rise and run
A slope calculator starts with two measurements: how far something climbs (rise) and how far it travels sideways (run). Divide rise by run and multiply by 100 for the percent grade. A 2 ft rise over a 100 ft run is a 2 percent grade, which is 1.15 degrees, or 1 foot of rise for every 50 feet of run.
The tool shows all three forms at once because contractors, engineers, and codes each use a different one.
The formula in plain words
Picture rolling a ball down a ramp. The rise is how far the ball drops. The run is how far it rolls along the ground. The ratio of those two numbers is the slope.
- Grade % = (rise ÷ run) × 100
- Angle = arctan(rise ÷ run), in degrees
- Ratio = run ÷ rise, written as 1 : X
Rise and run must use the same unit. Feet over feet works. Inches over feet does not until you convert.
Worked examples
- Lawn swale, 2 ft of fall over 100 ft
- Grade: (2 ÷ 100) × 100 = 2%
- Angle: arctan(0.02) = 1.15 degrees
- Ratio: 100 ÷ 2 = 1 : 50
- Driveway, 1.5 ft of climb over 12.5 ft
- Grade: (1.5 ÷ 12.5) × 100 = 12%
- Angle: arctan(0.12) = 6.84 degrees
- Ratio: 12.5 ÷ 1.5 = 1 : 8.3
Example 2 is a steep driveway. A car climbing it must gain 1.44 inches for every foot, and the bottom of the driveway may scrape at the transition. Local rules on maximum driveway grade vary, so check yours.
Slope reference table
| Grade | Angle | Ratio | Fall per foot of run | Typical use |
|---|---|---|---|---|
| 1% | 0.57° | 1 : 100 | 0.12 in | Very gentle lawn grading |
| 2% | 1.15° | 1 : 50 | 0.24 in | Common minimum for paved drainage |
| 5% | 2.86° | 1 : 20 | 0.60 in | Walkway before ramp rules apply |
| 8.33% | 4.76° | 1 : 12 | 1.00 in | Steepest accessible ramp ratio |
| 12% | 6.84° | 1 : 8.3 | 1.44 in | Steep driveway territory |
| 15% | 8.53° | 1 : 6.7 | 1.80 in | Very steep; confirm local limits |
How to measure rise and run
- Use a level line. Stretch a string between stakes at the high and low points and level it, then measure the drop to the ground.
- Measure run horizontally. A tape laid along a slope reads slightly long.
- Take several readings. A yard rarely falls evenly, so measure at three points and look for the low spot.
- Use a laser for long runs. Over 50 feet, a string sags and adds error.
- Know your target. If you want a 2 percent fall over 30 feet, the total drop is 30 × 0.02 = 0.6 ft, about 7.2 inches.
Common mistakes
- Mixing units. Entering rise in inches and run in feet produces a number 12 times too large.
- Mistaking degrees for percent. A 45 degree slope is a 100 percent grade, not 45.
- Swapping rise and run. The calculator divides rise by run, so a large result means you reversed them.
- Measuring along the slope. That counts the sloped surface, not the horizontal distance.
- Ignoring where water goes. A correct slope that ends at a wall still floods the wall.
Percent grade versus degrees
Percent and degrees look alike at low slopes and drift apart as the ground gets steeper. A 5 percent grade is 2.86 degrees, a 10 percent grade is 5.71 degrees, and a 50 percent grade is 26.57 degrees.
At 100 percent the rise equals the run, which is 45 degrees. So a “30 degree slope” is not a 30 percent grade. It is about 57.7 percent. When you read a spec, check which unit it uses before you compare it to your measurement.
Related calculators
For accessible ramps, the ramp calculator applies landing rules. The driveway slope calculator is tuned to inches of rise, and the drainage slope calculator sets fall for pipes and swales. After regrading, price the dirt work with the excavation cost calculator.
Frequently asked questions
What is the difference between grade, angle, and ratio?
They are three ways to state the same slope. Grade is rise over run as a percent (2 percent). Angle is the same slope in degrees (about 1.15 degrees for a 2 percent grade). Ratio expresses it as 1 in X (1:50). This calculator shows all three from one measurement.
What slope do I need for drainage?
A common rule of thumb is a minimum of about 2 percent, a quarter inch per foot, to move water off hard surfaces and away from foundations. Check your local code or a drainage guide for your exact case. Too flat and water ponds; too steep and it erodes.
What grade is too steep for a driveway?
Limits vary by town, and many local rules cap residential driveway grade somewhere around 12 to 15 percent. Steep grades risk scraping and traction problems, especially on ice. Check local code, and break long steep runs with a flatter landing near the garage and the street.
How do I measure rise and run on site?
Set a level line from the high point to the low point. Measure the horizontal distance (run) and the vertical drop (rise) between the two ends. A string level, laser level, or transit makes this accurate over long distances.
How does percent grade relate to inches per foot?
Multiply the percent by 0.12 to get inches of fall per foot of run. One percent is about 0.12 inch per foot, 2 percent is about 0.24 (roughly a quarter inch), and 8.33 percent is a full inch per foot.
Is the run measured along the slope or horizontally?
Horizontally. The calculator expects level distance. If you measure along the sloped surface, the error stays small on gentle grades but grows on steep ones, so use a level line or subtract using the rise.
Estimates only. Verify quantities with your supplier before purchasing.