Roof pitch calculator

Use this roof pitch calculator to convert a roof pitch to its angle in degrees, its percent grade, its slope ratio and its rafter factor. Pitch is written as rise per twelve inches of run: a 6/12 roof rises 6 inches for every foot of level run. Enter the pitch and all four values appear at once. Switch to DEGREES to go from an angle to a pitch, or to MEASURE to enter a rise and run you took from the roof itself.

Roof pitch to degrees conversion

Builders talk in ratios; saws and digital angle finders need degrees. The conversion is one arctangent: roof angle = atan(rise ÷ 12). A 4/12 pitch comes out at 18.4349°, a 6/12 pitch at 26.5651°, and a 12/12 pitch is exactly 45°. Only 12/12 gives an exact angle; every other pitch produces an irrational number, so the calculator marks every answer except 12/12 with ≈ to show the value is rounded.

Degrees to roof pitch

Going the other way is one tangent: rise per 12 = tan(angle) × 12. A 45° roof is exactly 12/12. A 30° roof is 6.93/12, which is framed as 7/12. 22.5° is 4.97/12, framed as 5/12, and 18° is 3.90/12, framed as 4/12. Roofs are framed to whole and half pitches, so the DEGREES mode gives the nearest half pitch beside the exact figure. Every whole pitch from 1/12 to 24/12 is on the roof pitch to degrees chart.

What is a good roof pitch for a shed?

Shed roof pitch depends on the roofing material. Asphalt shingles need at least 2/12, but most builders start at 3/12 for reliable drainage. Metal roofing can go as low as 1/12 with the right standing-seam profile. The pitch is worked out the same way for a shed as for a house.

Rafter factor and what it means for your cut list

The rafter factor (or roof pitch factor) is the number you multiply a horizontal run by to get the sloping length along the rafter. For a 6/12 pitch the rafter factor is 1.118034, so a 12 ft run gives a 13 ft 5 in common rafter (160.997 in), before the ridge deduction and overhang are counted. Enter the run above and the calculator works it out. The formula is √(144 + P²) ÷ 12 — the hypotenuse of a right triangle 12 in wide and P in tall, expressed per foot of run. The roof pitch multiplier table lists the factor for every pitch, with the hip and valley factor beside it.

Rafter length with an overhang, and for a shed roof

Add the overhang to the run before you multiply, measured level rather than along the slope. At 6/12, a 12 ft run with a 12 in overhang is 13 ft of run, so the rafter is 14 ft 6-7/16 in. A shed or lean-to roof has one slope, so its run is the full level distance the rafter covers, not half the building: a 10 ft wide shed at 4/12 needs a 10 ft 6-1/2 in rafter, or 11 ft 7-1/8 in with a 12 in overhang. For a diagonal that is not on a roof, use the diagonal calculator.

Percent grade and slope ratio

Roof pitch is written four ways, and which one you need depends on who is asking. Percent grade is rise ÷ run × 100, so a 6/12 roof is a 50% grade, the figure drainage drawings use. Slope ratio is written 1 : n, so 6/12 is 1 : 2. Degrees are what a saw or an angle finder reads. All four appear together above.

How to measure roof pitch from inside or outside

From inside the attic, hold a level against the underside of a rafter, mark 12 inches along it from where it touches the rafter, and measure straight up to the rafter — that is your rise per twelve. From a ladder at the eave, the method is the same against the roof surface. From the ground, take a square-on photograph of the gable end and measure the rise and run on the image: pitch is a ratio, so scaling does not change the answer. Round to the nearest half inch before you type it — roofs are framed to whole and half pitches.