How to Calculate Roof Pitch: 6 Methods That Actually Work

What roof pitch actually measures

Roof pitch is the vertical rise of a roof plane divided by its horizontal run, expressed against a fixed run of 12 inches. The convention exists because carpenters frame with a framing square whose long blade is graduated in inches: hold the square on the rafter, read 12 on the blade and the rise on the tongue, and you have the pitch without arithmetic. A roof that climbs 6 inches for every 12 inches of horizontal travel is a 6:12 roof, spoken as "six twelve" or "six in twelve".

Two things trip people up. First, run is horizontal distance, not distance along the roof surface — the sloping length is the rafter, and it is always longer than the run. Second, run is measured from the outside wall to the ridge centreline on a symmetrical gable, which is half the building span, not the whole span. Feeding a full 32-foot span into a rise-over-run formula halves the pitch you should have got.

Outside the United States the same slope is normally written as an angle in degrees or as a percentage grade. All three describe one number. Degrees come from the arctangent of rise divided by run, and percent is simply rise divided by run multiplied by 100. A 6:12 roof is 26.57 degrees and a 50 percent grade. Confusing degrees with percent is the single most common error in roofing forums, and it matters: a 45 percent grade is 24.2 degrees, while a 45 degree roof is a 100 percent grade.

Method 1 — level and tape measure (the standard)

This is the reference method that every other technique is checked against. You need a 12-inch or 24-inch spirit level, a tape measure and a pencil.

1. From a ladder at the gable end, or from a safe position on the roof, hold one end of the level against the roof surface at a point clear of ridge caps and hip flashing.
2. Bring the level to horizontal using the bubble, keeping that end firmly in contact with the roof.
3. Mark 12 inches along the level from the contact point. If you are using a 24-inch level, mark 24 inches and halve your final rise.
4. Measure vertically from that mark down to the roof surface. That distance is the rise.
5. Write the pair as rise:12. A rise of 5 inches over a 12-inch level is a 5:12 roof.

Take the reading in three places on the same plane — near the eave, at mid-slope and near the ridge. On a healthy roof the three numbers agree within about a quarter inch. If the mid-slope reading is noticeably shallower, the rafters are sagging and the roof needs structural attention before anyone orders material against the measurement.

Method 2 — measure from inside the attic

The attic method is safer, more accurate and works in any weather, which is why inspectors prefer it. You are measuring the rafter itself rather than the finished surface, so shingle thickness, ice-and-water membrane and cupped decking cannot bias the answer.

1. Press the end of a 12-inch level against the underside of a rafter, pointing toward the ridge, and level it.
2. Measure vertically from the 12-inch mark up to the rafter underside. That is the rise.
3. Repeat on a rafter on the opposite slope. On a symmetrical gable the two must match.

You can also derive the pitch from framing dimensions alone: measure the total rise from the top of the wall plate to the underside of the ridge board, measure half the span, and divide. A 7-foot rise over a 14-foot half-span is 0.5, which is 6:12. This is the most reliable approach on a roof with a thick built-up covering such as slate or clay tile, where surface measurements read a fraction of an inch flat.

Method 3 — speed square (fastest on site)

A speed square has a pivot point and two scales along its hypotenuse: COMMON (labelled in rise per foot) and HIP/VAL. Rest the pivot on the top edge of a rafter with the fence against the rafter, let the square hang, and pull it until the body of the square is plumb — or, more precisely, until a torpedo level laid on the square's edge reads level. The number on the COMMON scale aligned with the rafter edge is the pitch.

The speed square method is fast but has a resolution of about half a step. Use it to confirm a pitch you already suspect, then verify with a level and tape before ordering material. On a bare rafter you can also use the square directly to mark plumb and seat cuts once the pitch is known.

Method 4 — phone apps and digital angle finders

Every modern phone has a three-axis accelerometer that can be read as an inclinometer. Laid flat on a rafter or a straight board spanning the roof surface, a good app reads the angle to within about half a degree. A dedicated digital angle finder with a magnetic base is more repeatable and works on metal panel roofs where a phone slides.

The important step is converting the angle back to a framing pitch. Multiply the tangent of the angle by 12: tan(26.57°) × 12 = 6.0, a 6:12 roof. Angles that land between standard pitches are usually a measurement artefact, not a genuinely odd roof — 22.4 degrees is almost certainly a 5:12 roof measured across a shingle course.

Method 5 — from a photograph or satellite imagery

When you cannot get to the roof, a square-on photograph of the gable end gives a usable pitch. Stand well back so the lens is roughly level with the mid-height of the gable, shoot straight at the wall, and measure the triangle in any image editor: pixel rise divided by pixel run, multiplied by 12. Because you are taking a ratio, the scale cancels and you never need to know the real dimensions.

Accuracy depends entirely on the camera being perpendicular to the gable. A shot taken from a driveway at 20 degrees off-axis reports a pitch that can be two full steps out. Satellite and aerial roof-report services solve this with stereo imagery and typically quote pitch to the nearest half step, which is accurate enough for material take-off but not for cutting rafters.

Method 6 — calculate pitch from known dimensions

If you already hold plans, a truss drawing or a rafter length, no measuring is needed. Any two of rise, run and rafter length fix the triangle:

• Rise and run: pitch = (rise ÷ run) × 12.
• Rafter and run: rise = √(rafter² − run²), then apply the formula above.
• Rise and rafter: run = √(rafter² − rise²).
• Span and rise on a symmetrical gable: run = span ÷ 2.
• Angle alone: rise per 12 = tan(angle) × 12.

Worked example: a rafter measures 13 feet 5 inches (13.42 ft) along the top edge and the run from wall to ridge is 12 feet. Rise = √(13.42² − 12²) = √(180.1 − 144) = 6.01 feet. Pitch = (6.01 ÷ 12) × 12 = 6:12, an angle of 26.57 degrees. The pitch multiplier for that roof is 1.118, so a 1,600 square foot footprint carries 1,789 square feet of roof surface.

Conversion table for the pitches you will actually meet

The multiplier column is the number that turns a plan-view footprint into true sloped area, and it is why a steep roof costs more per square foot of house. Multiply the footprint area by the multiplier before ordering shingles, underlayment or panels.

Errors that ruin an otherwise good measurement

• Measuring over 6 inches instead of 12. Any error in the rise doubles when scaled to a 12-inch run.
• Using the full building span as run on a symmetrical gable, which reports half the true pitch.
• Measuring across the ridge cap or a hip, where the surface is not on either roof plane.
• Reading percent grade off an inclinometer and calling it degrees.
• Measuring a rafter tail below the birdsmouth, where an overhang may be furred to a shallower angle.
• Ignoring a sagged mid-span, then cutting new rafters to a pitch the ridge no longer holds.
• Rounding 4.8:12 down to 4:12 for a material order — that is a 1.5 percent area shortfall before waste.

When two methods disagree by more than a quarter step, trust the attic measurement. It is taken on structure rather than on covering, and structure is what the pitch is actually a property of.

Turning a measured pitch into the numbers you need

A pitch on its own rarely finishes a job. Once you have it, the same triangle gives rafter length, roof area, material quantities and the plumb and seat cut angles at the saw. Feed the measurement into the roof pitch calculator to get all of them at once, or work through the dedicated tools linked below. Every tool on this site shows the substituted numbers, so you can check the arithmetic against the formulas in this article rather than trusting a black box.

Frequently asked questions