Calculate slope from rise and run and see a reverse (negative) slope as a signed grade, angle in degrees and ratio, or solve for a missing rise or run.

Rise is positive uphill and negative downhill in your chosen reference direction. Run is horizontal.

Reverse Slope Formula

The reverse slope calculator converts vertical change and horizontal distance into a slope percentage. In grading and drainage work, a reverse slope usually means the surface falls opposite the intended direction of flow or away from the expected grade. The calculation itself is straightforward: divide the rise by the run, then convert the result to a percent.

S = (R) / (Ru) × 100
  • S = slope or grade, expressed as a percent
  • R = rise, fall, or total vertical change
  • Ru = run, or horizontal distance

If rise and run are measured in the same unit, the units cancel out. The calculator converts the separately selected rise and run units before dividing; handwritten calculations must first convert both lengths to the same unit.

Useful Rearrangements

If you know the slope and need to solve for rise or run, these forms are helpful:

R = (S × Ru) / (100)
Ru = (R × 100) / (S)

If you need the slope as an angle instead of a percent grade, use:

θ = tan⁻¹((R) / (Ru))

How to Calculate Reverse Slope

  1. Measure the vertical change of the surface.
  2. Measure the horizontal run over which that change occurs.
  3. Divide rise by run.
  4. Multiply by 100 to convert the ratio to a percent grade.
  5. Interpret the direction based on your project reference line. If the surface falls opposite the intended direction, it is considered a reverse slope.

When sign convention matters, some users record a descending reverse slope as a negative grade. Others report only the magnitude and describe the direction separately. Either approach can work as long as it is consistent with the drawing, plan set, or field notes.

Example

If the rise is 5 units and the run is 20 units, the slope is:

S = (5) / (20) × 100 = 25%

A 25% slope means the surface changes 25 units vertically for every 100 units of horizontal distance. If that change is opposite the intended drainage direction, it would be interpreted as a 25% reverse slope.

How to Read the Result

Result Meaning
0% Perfectly flat surface with no vertical change
Magnitude below 10% Relatively shallow geometrically; suitability depends on the application
Magnitude of 10% to 50% Moderate to steep slope depending on the application
100% Rise equals run, which corresponds to a 45° line
Magnitude above 100% Very steep grade where vertical change exceeds horizontal distance

Where Reverse Slope Calculations Are Commonly Used

  • Site grading: checking whether a yard, pad, or embankment drains in the intended direction
  • Sidewalks and patios: confirming surfaces do not send water back toward a structure
  • Roofing: identifying areas that may trap or redirect water
  • Road and driveway design: evaluating transitions, crowns, and drainage paths
  • Drainage systems: verifying trenches, swales, and channels maintain proper fall
  • Landscape construction: checking retaining areas, planter edges, and hardscape surfaces

Common Input Mistakes

  • Mixing units: selecting units that do not match the measured lengths, or dividing mixed units without converting first
  • Confusing percent grade with degrees: a 25% slope is not the same as 25°
  • Using sloped length instead of horizontal run: run must be horizontal distance, not surface distance
  • Ignoring direction: general slope results retain the rise sign; your reference direction determines whether the grade is reverse
  • Entering a run of zero: slope is undefined when horizontal distance is zero

Quick Conversion Note

Percent slope and slope ratio are closely related. A slope of 25% means:

(R) / (Ru) = (25) / (100) = (1) / (4)

So the surface changes 1 unit vertically for every 4 units horizontally.

Ramp results compare geometry with an entered slope target; they do not establish ADA compliance. The default 1:12 running slope and 30-inch rise per run come from the U.S. Access Board ramp guide. Width, cross slope, landings, handrails, edge protection, surfaces and applicable exceptions require separate review. Drainage and gutter targets are project inputs, not universal design standards.

Practical Tips

  • Measure over a long enough run to reduce small field errors.
  • Use the same reference points for both rise and run.
  • For drainage checks, verify both the amount of slope and the direction of flow.
  • On plans, label whether the value is a standard grade, negative grade, or reverse slope to avoid confusion.

Reverse Slope FAQ

Is reverse slope the same as negative slope?
A reverse slope is often represented as a negative slope relative to a chosen direction, but in practice the term usually emphasizes that the surface falls the wrong way or opposite the intended flow path.

Can slope be greater than 100%?
Yes. A slope above 100% means the vertical change is larger than the horizontal run.

Why is horizontal run used instead of sloped length?
Percent grade is defined from vertical change divided by horizontal distance. Using the sloped surface length will produce the wrong result.

What happens if the run is zero?
The slope is undefined because division by zero is not possible.