How to Calculate Roof Pitch

Roof pitch comes from two measurements: vertical rise and horizontal run. Start with an accessible attic or a drawing when possible, then express the result as X/12, degrees, percent slope, and a pitch multiplier.

Already have the measurements? Enter them in the roof pitch calculator →

FIELD GUIDE PC-07 · US CUSTOMARY

What roof pitch means

In US customary construction, an X/12 roof pitch tells you how many inches the roof rises for every 12 inches of horizontal run. A 6/12 roof rises 6 inches while moving 12 inches horizontally. Run is the level distance, not the length along the roof surface. Because pitch is a ratio, the same 6-to-12 relationship also applies to 6 feet of rise over 12 feet of run. The small triangle above shows the three parts: run is horizontal, rise is vertical, and the sloped edge follows the rafter or roof surface.

Method 1: Measure from the attic

Example: 6 in rise over a level 12 in run = 6/12 pitch. Measure to the rafter underside; the level thickness is not part of either measurement.

An accessible attic often gives a cleaner measurement because the rafter underside is straight and the roofing layers are out of the way. Place one end of a level against the underside of a common rafter. Hold the level horizontally, with its length running inward from the rafter, and center the bubble. Mark a point 12 inches from the rafter contact point. At that mark, measure vertically up to the underside of the same rafter. That distance is the rise per 12 inches of run.

Use the level’s upper surface for both the contact point and the start of the vertical reading; do not add the thickness of the level. Keep both measurements on one rafter face. Do not measure to roof sheathing at one end and to the rafter at the other, because the offset changes the result. If framing blocks a full 12-inch run, measure a shorter usable triangle, divide rise by run, and multiply by 12. For example, 3 inches of rise over 6 inches of run scales to 6/12. Avoid truss webs, collar ties, and valley or hip rafters; they do not represent the common roof plane in the same way.

Method 2: Measure from the roof or a ladder

This method uses a 12-inch level and a tape measure at an exposed roof edge. Work from a securely positioned ladder and stay off wet, icy, or unstable roofing. You are making a small right triangle, so the level supplies the 12-inch run and the tape supplies the rise.

Set the horizontal run
Set the uphill end of the level on the roof and extend the level down the slope toward the eave, not sideways along it. Adjust it until the bubble is centered. If the level is longer than 12 inches, make a clear mark exactly 12 inches from the contact point.
Measure the rise
At the point 12 inches downhill from the roof contact, measure straight down from the underside of the level to the roof surface. Keep the tape perpendicular to the level. Do not follow the shingle angle, and do not include the thickness of the level in the reading.
Record the pitch
The vertical reading is the first number in the pitch. A 5-inch gap at 12 inches of horizontal run is 5/12. Measure twice at a clean, straight section; curled shingles, a thick drip edge, or a sagging gutter can shift a small field reading.

Method 3: From a blueprint or a known angle

Construction drawings may label the roof directly with a slope triangle or a note such as 6:12. Read the vertical number as rise and the 12 as run. If the plan gives ridge height and plate height instead, subtract the two elevations to get rise, then use the horizontal distance from the wall line to the ridge centerline as run. Check the drawing scale and written dimensions before measuring the printed page; reduced or resized sheets can make a ruler reading unreliable.

When a digital level, inclinometer, or plan gives only an angle, convert degrees back to X/12 with rise per 12 = tan(angle) × 12. An angle near 26.6 degrees produces tan(26.6°) × 12, or approximately 6, so the pitch is approximately 6/12. Round only after the conversion, then match the result to the intended plan or standard pitch. To compare common rounded values, see the full chart.

The math: ratio, degrees, percent, and multiplier

Start with rise and run measured in the same unit. If the run is not 12, normalize the ratio with rise per 12 = (rise ÷ run) × 12. The angle is degrees = atan(rise ÷ run) × 180 ÷ π. Slope percent is (rise ÷ run) × 100. The pitch multiplier is √(1 + (rise ÷ run)²), which is the sloped length for one unit of horizontal run and is useful when converting projected area to sloped roof area. These formulas and the site’s display-rounding rules are documented on the methodology page.

Worked example: a 6/12 roof

A measured rise of 6 inches over a 12-inch run gives 6 ÷ 12 = 0.5. For the angle, atan(0.5) equals about 26.565 degrees, displayed as 26.6°. For slope percent, 0.5 × 100 = 50%. For the multiplier, √(1 + 0.5²) = √1.25 = 1.118 when rounded to three decimals. Therefore, a simple 1,000-square-foot horizontal projection at a uniform 6/12 pitch corresponds to about 1,118 square feet of sloped surface before waste. The multiplier changes area; it does not add a waste allowance or account for separate roof planes.

Check the field result

Enter the same rise and run in the pitch calculator to verify the ratio, angle, percent, and multiplier together. Keep the unrounded ratio through the calculation and round only the displayed result; rounding an angle first and converting it back can move the X/12 value slightly.

Common roof-pitch measuring mistakes

Most wrong answers come from the setup, not the trigonometry. Check the triangle before trusting a conversion.

Mixing inches and feet

Rise and run must use the same unit before division. Six inches over one foot is 6 inches over 12 inches, or 6/12—not 6/1. Convert both measurements first and write the units beside them.

Using total span instead of run

Building span is the full horizontal width between exterior supports. Run is one side of the roof triangle. On a centered, symmetrical gable, run is typically half the span. Using the full span with one side’s rise cuts the calculated pitch in half.

Letting the level follow the roof

The run line must be truly horizontal. Center the bubble before taking the vertical reading, confirm that the tool is not resting on a shingle ridge or debris, and measure at the exact 12-inch mark. A level that is only close to level builds its own error into the rise.

What to do with your pitch

Pitch is an input to the next estimate, not a complete roof measurement. Use the pitch multiplier with a horizontal projection to estimate sloped surface area. Follow the ground-measurement steps to calculate roof square footage without measuring on the roof. For a single right-triangle member, use the rafter calculator to solve a geometric rafter length from pitch and run. For a broader footprint, waste, roofing squares, and bundle workflow, continue to the roofing calculator.

Complex roofs need separate measurements for planes with different pitches, plus careful handling of hips, valleys, dormers, overhangs, and penetrations. A calculated surface area is a planning quantity, not a structural design, cut list, code check, or final order. Confirm material coverage and waste with the actual product and installer.

How to calculate roof pitch FAQ

Short answers for checking field measurements and pitch conversions.

How can I measure roof pitch without getting on the roof?

Measure from an accessible attic. Hold a level against the underside of a rafter, mark a 12-inch horizontal run, and measure vertically from that mark to the rafter. The vertical distance in inches is the rise in an X/12 pitch.

Do I need a 12-inch level to measure roof pitch?

No, but a 12-inch level makes the reading direct. With a longer level, mark exactly 12 inches from one end and measure at that mark. With any other run, divide rise by run and multiply by 12.

What angle is a 6/12 roof pitch?

A 6/12 roof pitch is about 26.6 degrees. It also equals a 50% slope and has a pitch multiplier of about 1.118.

How do I convert a roof angle in degrees to X/12 pitch?

Take the tangent of the angle and multiply by 12. For example, tan(26.6 degrees) × 12 is approximately 6, so the result is approximately 6/12.

Should I use the building span or the roof run?

Use run, not total span. On a symmetrical gable roof, run is usually half the building span because it is the horizontal distance from the outside wall line to the ridge centerline.

Check the 6/12 example

Open the worked values in PitchCalc, then replace them with your own field measurements to calculate your pitch →