Why isn't a 6/12 pitch 22.5 degrees?

Because roof angle is not linear. A 12/12 pitch really is 45°, but you can't just halve that — the angle comes from arctan(rise ÷ run), not from splitting 45° in proportion. A 6/12 pitch = arctan(6 ÷ 12) = 26.57°, not 22.5°. The pitch that actually lands near 22.5° is 5/12 (22.62°). Enter rise 6 and run 12 in the mini calculator above to see 26.57° for yourself.

What's the easiest way to measure roof pitch without climbing on the roof?

Two methods skip the roof entirely. From the attic: hold a level against the underside of a rafter, mark 12 inches along it, then measure straight up to the rafter. That rise is your pitch (rise:12). It is the most accurate because no shingles are in the way. From the ground: stand back until you can see the roof from the side, then estimate the angle from a sight line or a side-on photo. The attic method is the most accurate; the ground method is the fastest.

How accurate is measuring roof pitch with a phone app?

A phone inclinometer is usually within ±2–3° of the true angle — close enough to choose materials or estimate costs, but not precise enough for permit drawings or exact rafter cuts. For the steadiest reading, rest the phone's volume-button edge flat against the roof surface or a rafter instead of holding it in mid-air, and take the reading two or three times.

Why doesn't my speed square reading match a protractor?

A speed square measures the angle between the square and the rafter, which is the complementary angle — it adds up to 90° with the roof angle you expected, so the two won't match. Don't read the degree scale and compare it to pitch directly. Instead, use the scale marked 'Common' on the square, which gives the pitch (X:12) reading you actually want.

How do I measure roof pitch with a tape measure or a level?

Use both together. On the roof or in the attic, hold a level so it is horizontal for a 12-inch run, then use a tape to measure the vertical rise at the far end. That rise in inches is the pitch as rise:12. The attic version is the most accurate because you measure against the rafter, not the shingles. Enter those two numbers in the mini calculator above to check the pitch and angle.

Do I need fall protection to measure my roof pitch?

OSHA requires fall protection for roof work on slopes of 4:12 or steeper, and most homes are at least that steep. That is why measuring from the attic or from the ground is safer — you get the pitch without stepping onto the slope at all. If you must go up, use a harness and work only in dry, calm conditions.

How to Measure Roof Pitch Accurately

There are five reliable ways to measure roof pitch — from the attic, from the ground, or up on the roof, using a tape measure, a speed square, or your phone. Follow the steps below, then check your numbers with the mini calculator.

Mini Roof Pitch Calculator

Enter the rise and run you measured (in inches) to get the pitch, angle, and slope right away.

Rise (vertical height)

Run (horizontal distance)

Pitch

6:12

Angle

26.57°

Slope

50.0%

Run is horizontal, not along the shingles

The number you need is a level run. A tape laid on the roof measures slope length, not run, and that reading understates the pitch.

Correct measurement: Rise is vertical. Run is horizontal. Together they describe one roof plane as a right triangle.

Common measurement mistake: Measuring along the roof surface gives slope length, not horizontal run, and makes the pitch look flatter than it is.

5 Ways to Measure Your Roof Pitch

Each method suits a different situation — how much you can reach the roof, the tools on hand, and how precise you need to be. They run from the most hands-on to the most convenient.

Method 1

Direct

From the Roof Surface

Measuring on the roof itself gives a direct reading and is handy during an inspection or repair — but it puts you on the slope, so safety comes first.

1Lay a 12-inch level flat on the roof and center the bubble.
2Keep one end touching the surface so the level stays horizontal.
3From the far end (the 12-inch mark), measure straight down to the roof.
4That vertical distance in inches is your rise — the pitch is rise:12.

On any roof of 4:12 or steeper, OSHA calls for fall protection. Wear a harness, or switch to the attic or ground method instead.

Method 2

Most Accurate

From the Attic

Reading the pitch off the underside of the rafters takes the shingles and felt out of the equation, so the number is cleaner — and you never leave the house.

1Press a level horizontally against the bottom edge of a rafter.
2Measure 12 inches along the level from where it meets the rafter.
3At the 12-inch mark, measure straight up to the underside of the rafter.
4That rise gives you the pitch as rise:12.

Measure against the rafter's bottom face, not a brace or strut. No ladder and no slope make this the safest method as well as the most accurate.

Method 3

No Climbing

From the Ground

When you cannot reach the roof, estimate the pitch from the ground. Stand back and read the roof line from the side, or from a clear side-on photo. You do not need to touch the roof.

1Stand back until you can see the full triangle of the roof from the side.
2Sight the roof line by eye, or take a clear side-on photo.
3Estimate the angle of that roof line from horizontal.
4Convert that angle to pitch with the calculator above.

Expect about 2 to 5 degrees of error. This is a quick estimate from a distance, not a contact reading. Use it when the roof is too steep to walk.

Method 4

Fast

With a Speed Square

A speed square has pitch markings built in, so a framer can read a rafter angle in seconds as long as you read the right scale.

1Place the square's pivot on the edge of a rafter.
2Swing the square until the long edge is level.
3Read where the rafter edge crosses the Common scale.
4That Common-scale number is the pitch as X:12.

The degree scale shows the complementary angle, so it will not match a protractor or this calculator. Read the Common scale for pitch. That mix-up trips people up constantly.

Method 5

Always On Hand

With an iPhone or Phone App

Put a phone edge against the roof surface or a rafter and read the inclination. The iPhone Measure app includes a level, and free clinometer apps do the same on Android.

1Open the Measure app (iPhone) or a clinometer app (Android).
2Switch to the level or angle screen.
3Rest the phone's volume-button edge flat against the roof or a rafter.
4Read the angle, then convert it to pitch with the calculator above.

The phone edge must touch the roof or rafter. Holding it up by eye is a different, rougher method. A contact reading is usually within about 2 to 3 degrees.

Measurement Methods Compared

A side-by-side look at what each method needs, how precise it is, and when to reach for it.

Method	Tools Needed	Typical Accuracy	Safety	Best For
From the roof surface	12-inch level + tape measure	High (±1°)	Low — you're on the slope	Inspections or repairs already on the roof
From the attic	Level + tape measure	Highest (±1°)	High — indoors	The most accurate reading without climbing
From the ground	Side view or photo	Rough (±2 to 5°)	Highest: feet on the ground	Quick estimates without touching the roof
Speed square	Speed (rafter) square	Good (±1–2°)	Medium — at the rafter	Fast reads during framing
Phone inclinometer	iPhone Measure or clinometer app	Good (±2 to 3°)	Medium to high	A contact reading with a phone you already have

You have a pitch. What next?

Check the number on this page, measure again if it looks off, then use it to plan area or look up a class.

Enter the rise and run you just measured in the mini calculator on this page to check pitch, angle, and slope.
If the number looks off, pick another of the five methods and measure again.
To turn this pitch into roof area or material quantities, use the roof slope calculator.
To look up pitch classes or area multipliers, open the roof pitch chart.