How to use the roof pitch calculator
- Choose what you know. Rise and run is the usual start. If you have an angle from an angle finder or a phone level, choose Degrees. If a plan gives the slope in percent, choose Percent.
- Type the rise. Leave the run empty and it counts as 12, so the rise you type is the pitch. Rise and run can be inches, feet or millimeters, as long as both use the same unit.
- Open Rafter length for the length of a rafter from the building span, or Roof area to turn a level area into the area along the roof.
The result shows the pitch as x/12, the roof pitch angle in degrees, the slope percent, the pitch multiplier and the rise per foot and per meter. Copy result puts a short summary on your clipboard.
How roof pitch is worked out
A roof slope is a right-angled triangle: the run is the level side, the rise is the upright side and the roof surface is the sloping side. Everything follows from rise divided by run:
| Figure | Formula | For 6 in rise in 12 in |
|---|---|---|
| Pitch | 12 x rise / run, written over 12 | 6/12 |
| Angle | arctan(rise / run) | 26.57° |
| Slope percent | 100 x rise / run | 50% |
| Pitch multiplier | square root of (1 + (rise / run)²) | 1.118 |
The run does not have to be 12. A rise of 2 ft over a run of 8 ft is 12 x 2 / 8 = 3/12. In metric, a rise of 500 mm in 1,000 mm is the same slope as 6/12.
Roof pitch chart: 1/12 to 24/12
This roof pitch chart is worked out by the same code as the calculator. The multiplier column is the roof pitch multiplier used for roof area.
| Pitch | Angle | Slope | Multiplier |
|---|---|---|---|
| 1/12 | 4.76° | 8.3% | 1.003 |
| 2/12 | 9.46° | 16.7% | 1.014 |
| 3/12 | 14.04° | 25% | 1.031 |
| 4/12 | 18.43° | 33.3% | 1.054 |
| 5/12 | 22.62° | 41.7% | 1.083 |
| 6/12 | 26.57° | 50% | 1.118 |
| 7/12 | 30.26° | 58.3% | 1.158 |
| 8/12 | 33.69° | 66.7% | 1.202 |
| 9/12 | 36.87° | 75% | 1.250 |
| 10/12 | 39.81° | 83.3% | 1.302 |
| 11/12 | 42.51° | 91.7% | 1.357 |
| 12/12 | 45.00° | 100% | 1.414 |
| 13/12 | 47.29° | 108.3% | 1.474 |
| 14/12 | 49.40° | 116.7% | 1.537 |
| 15/12 | 51.34° | 125% | 1.601 |
| 16/12 | 53.13° | 133.3% | 1.667 |
| 17/12 | 54.78° | 141.7% | 1.734 |
| 18/12 | 56.31° | 150% | 1.803 |
| 19/12 | 57.72° | 158.3% | 1.873 |
| 20/12 | 59.04° | 166.7% | 1.944 |
| 21/12 | 60.26° | 175% | 2.016 |
| 22/12 | 61.39° | 183.3% | 2.088 |
| 23/12 | 62.45° | 191.7% | 2.162 |
| 24/12 | 63.43° | 200% | 2.236 |
Roof pitch in degrees, and degrees to pitch
To turn a pitch into a roof angle, divide the rise by the run and take the arctangent: a 4/12 roof is arctan(4 / 12) = 18.43 degrees, and a 12/12 roof is exactly 45 degrees.
To go the other way, the pitch is 12 times the tangent of the angle. A 30 degree roof is 12 x tan(30°) = 6.93/12, just under 7/12. Choose Degrees above and the calculator also shows the nearest whole pitch, here 7/12. A phone level or angle finder held on a rafter gives this roof pitch angle directly.
Roof pitch for square footage: the multiplier
A sloping roof has more surface than the level area under it. Multiply the level area by the pitch multiplier to get the roof area. For a house with a 42 ft by 32 ft outline, overhangs included, the level area is 1,344 sq ft. At 6/12 the roof area is 1,344 x 1.118034 = 1,502.64 sq ft.
The same multiplier turns a level length into a sloping length, which is how the rafter length below is found. To go on to roofing squares and shingle bundles, use the roofing calculator.
Rafter length from the span
On a gable roof each rafter covers half the span. For a 24 ft span at 6/12 with a 12 inch overhang:
| Rafter run (half the span) | 12 ft |
| Ridge height above the walls (12 x 6/12) | 6 ft |
| Rafter length to the wall (12 x 1.118) | 13 ft 5 in |
| With the 1 ft overhang (13 x 1.118) | 14 ft 6 7/16 in |
These are line lengths along the rafter. The ridge board, the birdsmouth seat and the plumb cuts change the length you cut, so check them with your framing plan.
Lean-to roof pitch
A lean-to (shed) roof slopes one way, from a high wall to a low one. Its rise is the drop from the high side to the low side and its run is the level depth, so choose Lean-to under Rafter length. A lean-to 8 ft deep that drops 2 ft is 3/12, or 14.04 degrees, and its rafter is 8 ft 2 15/16 in before any overhang.
Shallow lean-to roofs often fall below 2/12. The International Residential Code says "Asphalt shingles shall be used only on roof slopes of 2 units vertical in 12 units horizontal (17-percent slope) or greater", so a flatter lean-to needs a different covering.
How to measure roof pitch safely
Do not climb onto a roof to measure it. Measure from inside the attic, or on the ground where you can reach a sloping part safely:
- In the attic, hold a level against the underside of a rafter so that one end touches the rafter and the bubble is centered.
- Mark 12 inches along the level from the end that touches.
- Measure straight up from the 12 inch mark to the underside of the rafter. That number is the rise, so 6 inches means a 6/12 pitch.
The same level and tape method works on a porch or shed rafter you can reach from the ground. The pitch is also often written on the building plans or roof truss drawings. Anything on the roof itself is a job for a roofer.
Low slope vs steep slope roofs
The 2024 International Residential Code's energy chapter (N1101.6) defines a low slope roof as one "less than 2 units vertical in 12 units horizontal (17 percent slope)", and a steep slope roof as 2/12 or steeper. What decides the roof covering is the minimum slope the code sets for each one (section R905):
| Roof covering | Lowest slope allowed |
|---|---|
| Built-up and modified bitumen roofing | 1/4 unit in 12, "for drainage" (1/8 for coal-tar built-up roofs) |
| Asphalt shingles | 2/12, "(17-percent slope) or greater" |
| Clay and concrete tile | 2 1/2/12 |
| Wood shingles and wood shakes | 3/12 |
| Slate shingles | 4/12 |
For asphalt shingles from 2/12 up to 4/12, R905.2.2 adds that "double underlayment application is required". The calculator warns you in that range. Local codes can differ, and the shingle maker's instructions apply too.
Roof pitch in Excel or Google Sheets
With the rise in A2 and the run in B2, these formulas work in both Excel and Google Sheets:
| To get | Formula |
|---|---|
| Pitch (rise per 12) | =12*A2/B2 |
| Angle in degrees | =DEGREES(ATAN(A2/B2)) |
| Slope percent | =A2/B2*100 |
| Pitch multiplier | =SQRT(1+(A2/B2)^2) |
| Pitch from an angle in C2 | =12*TAN(RADIANS(C2)) |
ATAN gives its answer in radians: Microsoft says to "use the DEGREES function" to express it in degrees, and Google says "Use the DEGREES function to convert the result of ATAN into degrees". TAN works the other way and needs radians, which is why RADIANS goes inside it.
Questions and answers
What is a 6/12 roof pitch in degrees?
A 6/12 pitch is 26.57 degrees. The roof rises 6 inches for every 12 inches of level run, a 50% slope, with a multiplier of 1.118.
What angle is a 4/12 roof pitch?
A 4/12 roof is 18.43 degrees, a 33.3% slope. Its multiplier is 1.054, so 1,000 sq ft of level area becomes about 1,054 sq ft of roof.
Is a 12/12 roof pitch 45 degrees?
Yes. When the rise equals the run, the angle is exactly 45 degrees and the slope is 100%. The multiplier is 1.414, the square root of 2.
What is the minimum roof pitch for shingles?
The International Residential Code allows asphalt shingles on slopes of 2/12 or steeper, with a double layer of underlayment from 2/12 up to 4/12. Below 2/12 you need a low slope roof covering.
How do I find my roof pitch without going on the roof?
Use a level and tape measure on a rafter in the attic: mark 12 inches along the level and measure up to the rafter. Building plans and truss drawings often state the pitch too.
What is a roof pitch multiplier used for?
It turns level measurements into sloping ones. Multiply the level area under the roof by it to get roof area, or a level run by it to get rafter length.
Sources
Checked on 8 October 2026. If a rule has changed, please tell us.
- ICC: 2024 International Residential Code, Chapter 11 (N1101.6 definitions)
- ICC: 2024 International Residential Code, Chapter 9 Roof Assemblies (R905)
- Microsoft Support: ATAN function
- Microsoft Support: DEGREES function
- Microsoft Support: SQRT function
- Microsoft Support: TAN function
- Microsoft Support: RADIANS function
- Google Docs Editors Help: ATAN
- Google Docs Editors Help: DEGREES
- Google Docs Editors Help: SQRT
- Google Docs Editors Help: TAN