Last updated: February 16, 2026
Water Doesn’t Read Your Site Plan—It Follows the Grade
After a heavy rain the homeowner notices a puddle growing against the foundation wall. The yard looked flat, and nobody checked the percent grade before the landscaper finished. Turns out the grade runs at 0.8 % toward the house instead of away from it. Fixing the problem now means regrading 30 ft of lawn, replacing sod, and rerouting a downspout—work that a five-minute slope check would have prevented.
This calculator converts rise and run into percent grade, degrees, and ratio, and compares the result against common drainage and driveway targets. Enter two elevations and the distance between them, or plug in a target slope to find out how much rise or run you need. It handles the trigonometry; you still need a tape measure, a laser level, and your local code book.
Percent, Degrees, Ratio: Three Labels for the Same Slope
Percent grade is rise divided by run, multiplied by 100. A 6 ft rise over 80 ft of run is 7.5 %. Degrees come from the arctangent of the same fraction: arctan(6 ÷ 80) ≈ 4.3°. Ratio expresses the relationship as 1 : N—here, 1 : 13.3 (one foot of rise for every 13.3 ft of run).
Grade (%) = (Rise ÷ Run) × 100
Angle (°) = arctan(Rise ÷ Run)
Ratio = 1 : (Run ÷ Rise)
For gentle slopes the percent number and the degree number are close (5 % ≈ 2.9°). They diverge as slopes get steeper: 100 % grade is 45°, not 100°. Mixing the two up on a grading plan is one of the fastest ways to order the wrong amount of fill or build a driveway that’s too steep to use in winter.
Drainage and Driveway Targets: How Much Slope Is Enough?
Different surfaces need different grades. Too flat and water ponds; too steep and you get erosion or an unusable driveway. The table below covers the ranges most codes and field guides agree on.
| Surface | Target grade | Why |
|---|---|---|
| Lawn / yard away from foundation | 2–5 % | Keeps water moving without visible erosion |
| Patio or flatwork | 1–2 % | Drains without making furniture rock |
| Swale or ditch | 1–4 % | Moves runoff to an outlet; over 4 % needs lining |
| Residential driveway | 2–12 % | Under 2 % ponds; above 12 % is icy in winter |
| ADA-compliant ramp | ≤ 8.33 % (1 : 12) | Federal maximum; landings every 30 ft |
| Parking lot cross-slope | 1–2 % | Drains toward inlets without tilting cars noticeably |
These are starting-point guidelines, not code citations. Your jurisdiction may specify tighter ranges, especially for ADA ramps and stormwater swales. Always verify with local ordinances.
Backyard Drainage Check: 80 Feet, 6-Inch Rise, One Soggy Foundation
Site: Backyard runs 80 ft from the foundation wall to the property line. A laser level shows the ground is 6 inches (0.5 ft) higher at the property line than at the foundation.
- Current grade: 0.5 ÷ 80 × 100 = 0.625 % (toward the house—negative direction)
- Drainage target: minimum 2 % away from the foundation for the first 10 ft, then ≥ 1 % to the property line
- Required rise in first 10 ft: 0.02 × 10 = 0.2 ft (2.4 in) drop away from the wall
- Angle equivalent: arctan(0.02) ≈ 1.15°
Right now the yard slopes the wrong way at 0.625 %. To fix it, the first 10 ft must be regraded to fall at least 2.4 in away from the house. The remaining 70 ft can stay gentler (1 % is fine), but the overall direction must reverse. That means adding fill against the foundation side and smoothing it out—or installing a French drain if regrading is not practical.
Slopes That Look Fine Until They Aren’t
- Driveway cross-slope vs longitudinal slope. A driveway at 8 % longitudinal grade may drain well downhill, but if the cross-slope is flat, water sheets across the full width instead of flowing to the edges. Cross-slope should be 1–2 % toward one side or crowned in the middle. Miss this and the surface freezes into a skating rink.
- ADA ramps that exceed 8.33 %. An 8.33 % slope (1 : 12) is the federal maximum for wheelchair ramps. A 3 ft rise needs at least 36 ft of run. Shorten the run to 30 ft and the grade hits 10 %—non-compliant and an injury risk. The calculator flags this instantly, but you need to know the rule exists before you check.
- Ponding from too-flat grading. Anything below about 1 % on turf looks flat and acts flat—water sits instead of draining. On clay-heavy soil the problem is worse because infiltration is almost zero. If your laser shows 0.5 % and the soil is clay, assume that section will pond.
- Slope direction that reverses at a midpoint. Checking grade only at the two endpoints misses a low spot in the middle. The grade from A to B reads 2 %, but a belly at 40 ft traps runoff and creates a permanent wet spot. Measure at least one midpoint to catch hidden dips.
What the Calculator Leaves Out
The tool divides rise by run and applies arctan. It does not model soil permeability, runoff volume, pipe sizing, or multi-directional slopes. It assumes a straight line between your two measurement points—real terrain has dips, crowns, and transitions that only a topographic survey captures. Conversions use standard factors (1 ft = 12 in; 1 m ≈ 3.281 ft). For any grading that changes drainage patterns or requires a permit, have a licensed civil engineer or surveyor confirm the design.
Need to check whether a slope is stable enough to leave unretained? Run the safe-gradient checker. Estimating cut-and-fill volumes for a grading project? Try the mass-balance calculator. Planning a retaining wall where the grade is too steep to hold? Use the retaining wall estimator.