How to Measure Roof Pitch (From the Ground or the Roof)

Two ways pitch is written

Roofers describe pitch in two units, and you’ll hit both:

• Ratio (rise-in-run): how many inches the roof rises for every 12 inches it runs horizontally. Written 4:12, 6:12, “4-in-12,” etc. This is what tile and shingle specs use.
• Degrees: the actual angle of the slope. This is what an app or inclinometer shows directly.

They describe the same thing. A 4:12 roof and an 18.4° roof are identical — see the conversion table below.

Method 1 — Phone against the roof or a rafter (most accurate)

If you can safely reach the roof, or get into the attic:

1. Open an angle / inclinometer app.
2. Hold the phone with one long edge flat against the slope — the top of the roof deck, or the edge of a rafter in the attic.
3. Read the angle in degrees. Tap to switch units if you want X-in-12 or percent.

The attic approach is the safest accurate method: rafters share the roof’s exact slope, and you never leave the ground floor. The phone’s accelerometer reads the tilt the same way Apple’s Compass-app level does.

Method 2 — Level and tape measure (the manual classic)

No app? Use a 12-inch level and a tape:

1. Hold the level horizontal with one end touching the roof slope.
2. Keep it level (bubble centered), then measure straight down from the 12-inch mark to the roof surface.
3. That vertical distance is your rise per 12 inches of run. Measure 4 inches down → a 4:12 pitch.

This gives you the ratio directly; convert to degrees if needed.

Method 3 — From the ground, with a photo

Can’t or shouldn’t climb up? Stand back and take a side-on photo of the roofline (the more square-on, the better). Open it in a camera-based protractor, lay one arm along the horizontal eave line and the other along the slope, and read the angle. It’s an estimate — perspective skews it if you shoot at an angle — but it’s accurate enough to order materials or compare two roofs.

Converting pitch to degrees

The formula is the same arctan relationship every slope uses:

degrees = arctan(rise ÷ 12)

Pitch (rise:12)	Angle
1:12	4.8°
3:12	14.0°
4:12	18.4°
6:12	26.6°
9:12	36.9°
12:12	45.0°

Most apps do this conversion for you — measure once in degrees and read off the X-in-12 ratio without touching a calculator.

A note on safety

Never measure pitch from a roof in wet, windy, or icy conditions, and don’t lean off a ladder to reach a slope. The attic-rafter method and the ground-photo method both give you the number without putting you on the roof at all.