Calculators · Roof Pitch Calculator

Roofing

Roof Pitch Calculator

Enter rise and run, an angle or a slope percentage to get the pitch in x/12, the angle, the pitch factor and the rafter length.

Rise

The vertical height.

Run

The horizontal distance, not measured along the slope.

Overhang (optional)

Measured horizontally. Added to the run for the rafter length.

Check the IRC minimum slope (optional)

Pick a roof covering to compare your pitch with the code minimum.

How to calculate roof pitch

Roof pitch compares how much a roof rises with how far it runs. The rise is the vertical height. The run is the horizontal distance, measured level, from the outside of the wall to the point directly under the ridge. In the United States the pitch is quoted for a run of 12 inches, so a roof that rises 6 inches over 12 inches is a 6/12 roof. All the other measures come from the same ratio, which is the tangent of the roof angle:

What you get	Formula
Pitch (x/12)	rise ÷ run × 12
Angle	arctangent of (rise ÷ run)
Slope in percent	rise ÷ run × 100
Pitch factor	√(1 + (rise ÷ run)²)
Rafter length	(run + overhang) × pitch factor

The pitch factor is the number you multiply a horizontal length by to get the length along the slope. It also multiplies a floor-plan area to give the roof area. To turn your footprint into roof area, roofing squares and shingle bundles, use the roofing calculator. This page only handles the pitch itself and the rafter.

Example: a 28 ft wide house with the ridge 7 ft above the wall

The house is 28 ft wide. The ridge sits 7 ft above the top of the walls. The roof has a 1 ft overhang. In the calculator, use the tab "Total rise and span".

Step	Operation	Result
Run	28 ÷ 2	14 ft
Pitch	7 ÷ 14 × 12	6/12
Angle	arctangent of (7 ÷ 14)	26.57°
Slope	7 ÷ 14 × 100	50%
Pitch factor	√(1 + 0.5²)	1.1180
Rafter, no overhang	14 × 1.1180	15.652 ft (15 ft 7 13/16 in)
Overhang along the slope	1 × 1.1180	1.118 ft (1 ft 1 7/16 in)
Rafter with overhang	(14 + 1) × 1.1180	16.771 ft (16 ft 9 1/4 in)

Roof pitch chart from 1/12 to 12/12

Pitch	Angle	Slope	Pitch factor	Rafter for 12 ft of run
1/12	4.76°	8.3%	1.0035	12.04 ft
2/12	9.46°	16.7%	1.0138	12.17 ft
3/12	14.04°	25.0%	1.0308	12.37 ft
4/12	18.43°	33.3%	1.0541	12.65 ft
5/12	22.62°	41.7%	1.0833	13.00 ft
6/12	26.57°	50.0%	1.1180	13.42 ft
7/12	30.26°	58.3%	1.1577	13.89 ft
8/12	33.69°	66.7%	1.2019	14.42 ft
9/12	36.87°	75.0%	1.2500	15.00 ft
10/12	39.81°	83.3%	1.3017	15.62 ft
11/12	42.51°	91.7%	1.3566	16.28 ft
12/12	45.00°	100.0%	1.4142	16.97 ft

The last column is the theoretical rafter length for a horizontal run of 12 ft. For another run, multiply the pitch factor by that run.

Degrees to roof pitch

If you only know the angle, for example from a digital level, this table gives the matching pitch. The calculator handles any other angle in the "Angle" tab.

Angle	Pitch	Slope	Pitch factor
5°	1.05/12	8.7%	1.0038
10°	2.12/12	17.6%	1.0154
15°	3.22/12	26.8%	1.0353
20°	4.37/12	36.4%	1.0642
25°	5.60/12	46.6%	1.1034
30°	6.93/12	57.7%	1.1547
35°	8.40/12	70.0%	1.2208
40°	10.07/12	83.9%	1.3054
45°	12.00/12	100.0%	1.4142

How to measure roof pitch

The easiest way is from the attic, with a level and a tape. Hold the level horizontal against the underside of a rafter. Mark 12 inches along the level, measure straight up or down from that mark to the rafter, and that distance in inches is the first number of the pitch. If it measures 6 inches, the roof is 6/12.

If you have the plans, the rise and the span are usually on the elevation drawing. Use the tab "Total rise and span" for that. If you only have an angle from a digital level, use the tab "Angle". Work from the attic or the eave rather than walking on the roof.

Minimum roof pitch by roof covering

Every roof covering has a lowest pitch it can be installed on. These are the minimum slopes in Chapter 9 of the International Residential Code (IRC), which we read in the 2024 edition and the 2021 edition in October 2026. The values are the same in both. The percentages are calculated here from the pitch.

Roof covering	Minimum pitch	Slope
Mineral-surfaced roll roofing	1/12	8.3%
Asphalt shingles	2/12	16.7%
Clay and concrete tile	2 1/2 in 12	20.8%
Metal roof shingles	3/12	25.0%
Wood shingles and shakes	3/12	25.0%
Slate shingles	4/12	33.3%
Metal panels, lapped seams, no sealant	3/12	25.0%
Metal panels, lapped seams with sealant	1/2 in 12	4.2%
Standing-seam metal roof	1/4 in 12	2.1%
Modified bitumen, single-ply and built-up (not coal-tar)	1/4 in 12	2.1%

For asphalt shingles, the same section requires a double layer of underlayment between 2/12 and 4/12. For clay and concrete tile it requires it between 2 1/2 in 12 and 4/12. Coal-tar built-up roofs have their own, lower minimum that is not in the table. A manufacturer can ask for more pitch than the code does, and your town or state may use a different edition, so confirm both before you buy. Use the menu in the calculator to compare your own pitch with these values.

How the rafter length is calculated

The rafter, the rise and the horizontal run form a right triangle, so the rafter is the hypotenuse, which is the square root of run² + rise². That is the same as the run times the pitch factor. With a 12 ft run at 6/12 the rise is 6 ft, and √(12² + 6²) = 13.416 ft. The overhang counts as extra run, measured horizontally from the wall, which is why the calculator multiplies (run + overhang) by the factor.

The number is a theoretical length along the slope. A real rafter also gives up half the thickness of the ridge board at the top and has a birdsmouth cut where it sits on the wall, and the tail can be cut plumb or square. None of that is in the result, so mark and cut from the layout on site, not from this figure alone.

Frequently asked questions

What is roof pitch?

Roof pitch is how steep a roof is. In the United States it is written as the rise in inches for every 12 inches of horizontal run, such as 6/12 or 6:12. A 6/12 roof rises 6 inches for every 12 inches it runs, which is an angle of 26.57 degrees and a slope of 50%.

How do I calculate roof pitch?

Divide the rise by the run and multiply by 12. A roof that rises 7 ft over a run of 14 ft has a pitch of 7 ÷ 14 × 12 = 6, so it is a 6/12 roof. To get the angle, take the arctangent of rise ÷ run, which is 26.57 degrees in this example.

How steep is a 12/12 roof pitch?

A 12/12 roof rises 12 inches for every 12 inches of run, so the angle is 45 degrees and the slope is 100%. Its pitch factor is 1.4142, which means the sloped surface is 41.42% longer than the horizontal distance under it.

Is a 1/12 roof pitch OK?

It depends on the roof covering. A 1/12 pitch is 8.3% or 4.76 degrees. In the IRC (2021 and 2024 editions) asphalt shingles are limited to roof slopes of 2/12 or more, so 1/12 is below that minimum. The same code sets 1/4 in 12 as the minimum design slope for modified bitumen, single-ply and standing-seam metal roofs, and says mineral-surfaced roll roofing cannot go below 1/12. Your local code and the manufacturer instructions have the final word.

How do I convert degrees to a roof pitch?

Take the tangent of the angle and multiply by 12. For 30 degrees, tan 30° = 0.5774, and 0.5774 × 12 = 6.93, so it is about a 7/12 pitch. Choose the angle tab in the calculator and it does this for you. A 5 degree roof is only about 1.05/12.

How do I find the length of a rafter?

Multiply the run by the pitch factor, which is the square root of 1 plus (rise ÷ run) squared. With a run of 12 ft at 6/12, the rafter is 12 × 1.1180 = 13.416 ft, or about 13 ft 5 in. If there is an overhang, add it to the run first. The result is a theoretical length and does not include the ridge board, the birdsmouth cut or any plumb cut.

Can this calculator measure my roof pitch from a photo or an app?

No. It does not read photos, addresses or satellite images. You type the measurements and the math runs in your browser, so nothing you enter is sent anywhere. To measure the pitch yourself, hold a level horizontal against the underside of a rafter in the attic, mark 12 inches along the level, and measure straight up or down from that mark to the rafter. That distance in inches is the first number of the pitch, so 6 inches means a 6/12 roof. If you have the plans, use the total rise and the building span from the elevation drawing, or type the angle from a digital level.