Slope calculator
Work out the percent grade, angle and inches of fall per foot for yard drainage, patios and walks.
Slope
- Inches per foot
- Ratio
- Next to a house
How to use it
Enter how many inches the ground drops and the distance in feet it drops over. To measure, stretch a string between two stakes, level it with a line level, and measure from the string down to the ground at each stake: the difference is the drop.
How it's calculated
- Percent grade = drop ÷ (distance × 12) × 100.
- Inches per foot = drop ÷ distance.
- Angle = arctan(drop ÷ (distance × 12)).
| Fall per foot | Percent grade | Fall over 10 ft |
|---|---|---|
| 1/8 in | 1.04% | 1.25 in |
| 1/4 in | 2.08% | 2.5 in |
| 1/2 in | 4.17% | 5 in |
| 0.6 in | 5% | 6 in |
| 1 in | 8.33% | 10 in |
How much slope is enough next to a house?
The International Residential Code (R401.3) says the ground should fall at least 6 in within the first 10 ft from a foundation, a 5% grade. Paving within those 10 ft (a patio, walk or driveway) should slope away from the house at 2% or more, about ¼ in per foot.
The calculator's "Next to a house" line checks your slope against those two figures.
Tips
- Set stakes first, dig second. Mark the finished height on each stake and work to the string.
- Check after rain. Puddles against the foundation mean the slope isn't doing its job.