How to Measure Slope and Incline (Degrees, % and Ratio)

Slope, incline, and grade: same idea, different words

When a surface isn’t level, it has slope. Slope is the math word for steepness; incline is the physical tilt you feel underfoot; grade is the word builders and road crews use for the same thing. In practice they’re interchangeable — a 5° incline, a 5° slope, and a 5° grade all describe one surface tilting five degrees off horizontal.

What matters is that steepness is a single quantity, and you can write it down in three different units. Picking the wrong unit — or mixing two of them — is where most slope mistakes happen, so it’s worth knowing all three before you measure anything.

The three ways to express slope

Degrees measure the angle directly, from 0° (perfectly level) to 90° (a vertical wall). This is the most intuitive unit and the one your phone reads natively. A gentle wheelchair ramp is under 5°; a steep staircase is around 35°.

Percent grade is rise divided by run, times 100. If a path climbs 8 units of height over 100 units of horizontal distance, that’s an 8% grade. Roads, trails, and drainage are almost always specified this way. The thing to remember: 100% grade is not vertical — it’s a 45° angle, where rise equals run. Percent grade keeps climbing past 100% as the angle approaches 90°.

Ratio (or “rise-in-run”) writes the same relationship as 1:12 or “1 in 12” — one unit up for every twelve units along. Ramps, drainage pipes, and accessibility codes love this format because it’s easy to lay out with a tape measure. A 1:12 ratio is the same as an 8.3% grade and about 4.8°.

All three describe one tilt. Converting between them is just trigonometry: percent = tan(degrees) × 100, and degrees = arctan(rise ÷ run).

Measuring slope with your phone

Your iPhone has an accelerometer and gyroscope — the same sensors behind Apple’s built-in level. They sense the direction of gravity, so the phone always knows how far it’s tilted from flat. That makes it a pocket inclinometer: lay it on a slope and the tilt it reads is the slope.

1. Open a level or inclinometer app and let it settle for a second.
2. Lay the phone flat on the surface you want to measure — along the direction of the slope, not across it. On a ramp or driveway, point the long edge up the grade.
3. Read the tilt. The screen shows the angle live in degrees as you set the phone down.
4. Switch units if you need to. A dedicated digital level app lets you flip the same reading between degrees, percent grade, and an X-in-12 ratio with a tap — no manual conversion.

For surfaces you can’t lay a phone against — like the slope of a roofline across the yard — point the camera at it and read the angle off the photo instead.

Measuring slope by hand: rise over run

No phone? Slope is just rise over run, and a tape measure and a level get you there.

1. Set a level horizontal line. Hold a straight board or a long spirit level out from the high point of the slope until the bubble centers. That board is your “run.”
2. Measure the run. Note the horizontal length of the board — say 24 inches.
3. Measure the rise. At the far end, measure straight down from the board to the surface — say 2 inches.
4. Divide. Rise ÷ run = 2 ÷ 24 = 0.083. Multiply by 100 for 8.3% grade, or read it as a 1:12 ratio. Take arctan(0.083) for 4.8°.

Longer boards give better numbers — a 12-inch offcut amplifies every wobble, while a 4-foot level averages out surface bumps.

A worked example: a wheelchair ramp

Accessibility guidelines cap a wheelchair ramp at a 1:12 ratio — one inch of rise for every twelve inches of run. Suppose a doorway sits 30 inches above the path. The minimum ramp run is 30 × 12 = 360 inches, or 30 feet of ramp.

Check the resulting slope three ways:

• Ratio: 30 rise over 360 run = 1:12. ✓
• Percent grade: 30 ÷ 360 × 100 = 8.3%.
• Degrees: arctan(30 ÷ 360) = arctan(0.083) ≈ 4.8°.

Lay a phone on the finished ramp and you should read right around 4.8° — confirming the build matches the code maximum. Anything steeper and the ramp gets unsafe to use unassisted, so 1:12 is a ceiling, not a target. Roofs work the same way, just expressed differently; see how to measure roof pitch or run the numbers through the roof pitch calculator.

Accuracy tips

A phone laid on a surface is good to about 1° for DIY work — plenty for ramps, driveways, shelving, and roof estimates, but not a substitute for a surveyor’s level on a job that demands certified grades.

• Calibrate first. Rest the phone on something you know is dead flat and zero it, so a small built-in offset doesn’t ride along into every reading.
• Measure along the slope, not diagonally across it — a phone canted sideways reads a shallower angle than the true grade.
• Bridge bumps. On rough concrete or gravel, set the phone on a flat board so it reads the overall slope, not one high pebble.
• Take a few readings at different spots and average them; real-world surfaces are rarely a single clean plane.

Get the unit right, set the phone down square, and read twice. That’s the whole job.