Roof Pitch Calculator
Enter the roof pitch and building size to get the angle, rafter length and roof area.
What this tool does
Roof pitch is the rise in inches for every 12 inches of horizontal run. A 6/12 pitch is an angle of 26.57° and a slope factor of 1.118, so a rafter spanning 13 ft of run is 14.5 ft long. Enter your pitch and the building size above to see the angle, rafter length and surface area.
Converts a rise-per-12 pitch to an angle, a percent slope and a slope factor.
Calculates rafter length, ridge height and the surface area of a simple gable roof.
How to use it
- Enter the rise per 12 inches of run.
- Enter the building width, length and overhang.
- Read the angle and slope factor.
- Use the rafter length and area for materials.
Why use this calculator
Turns a roof pitch such as 6/12 into an angle, a percent slope and a slope factor, and gives the rafter length, ridge height and roof surface area. A roof is larger than the footprint of the building, and the steeper it is, the larger it gets. The slope factor gives the true length and area, which matters for ordering rafters and roofing.
Why we built it
We built the roof pitch calculator to give the angle, slope factor, rafter length and roof area from a pitch like 6/12. The stair calculator applies similar maths to staircases and the slope calculator handles any gradient.
Who this tool is for
- Homeowners planning roofing or extensions
- Builders and roofers
- Students converting pitch to degrees
When to use it
- Planning a new roof or an extension.
- Estimating shingles or sheets.
- Converting between pitch and degrees.
- Choosing rafter stock.
How it works
Slope factor = √(1 + (rise ÷ 12)²). The angle is arctan(rise ÷ 12). Rafter length = (half the width + overhang) × slope factor. For a gable roof, surface area = 2 × rafter length × (building length + 2 × overhang).
Example: 6/12 pitch on a 24 ft × 40 ft building with a 1 ft overhang. Run = 12 + 1 = 13 ft. Rafter = 13 × 1.118 = 14.53 ft. Area = 2 × 14.53 × 42 = 1,221 sq ft, or 12.2 roofing squares.
A worked example
These are the values the calculator opens with, so you can see exactly what it does with them. Change any value on the page and the result updates; the figures below come from the same calculation.
| Input | Value |
|---|---|
| Rise per 12 inches of run | 6 |
| Building width | 24 |
| Building length | 40 |
| Overhang | 1 |
With those inputs, the calculator returns:
- Roof angle: 26.57°
- Pitch: 6:12
- Percent slope: 50%
- Slope factor (multiply a plan area by this): 1.118
- Rafter length, including overhang: 14.53 ft
- Ridge height above the walls: 6 ft
- Roof surface area (gable roof): 1,221 sq ft
- Roofing squares (100 sq ft): 12.2
What you give it, and what you get back
| Item | Details |
|---|---|
| Rise per 12 | Inches of rise for each 12 inches of run. |
| Width, length, overhang | The building footprint and eaves overhang in feet. |
| Output | Angle, pitch, percent slope, slope factor, rafter length, ridge height, area and squares. |
Issues you may run into
| What you see | Why it happens | What to do |
|---|---|---|
| Area is higher than the plan area | The roof surface is larger than its footprint. | This is expected; use the slope factor. |
| Hip or complex roofs | Only a simple gable is modelled. | Add the areas of each plane. |
| No allowance for waste | Shingles need extra for cuts. | Add 10-15%. |
Assumptions and limitations
- Simple two-plane gable roof.
- No waste or flashing allowance.
Frequently asked questions
What does 6/12 pitch mean?
The roof rises 6 inches for every 12 inches of horizontal run, an angle of about 26.6°.
How do I convert pitch to degrees?
Take the arctangent of rise divided by 12.
What is a roofing square?
An area of 100 square feet, the unit shingles are priced in.
How do I find rafter length?
Multiply the horizontal run by the slope factor.
What is a low-slope roof?
Generally less than 3/12, which needs special roofing materials.
Is my information sent anywhere?
No. The calculation runs in your browser and nothing leaves the page.
Sources and accuracy
Pitch is rise divided by run, the slope angle is its arctangent, and rafter length follows the Pythagorean theorem. Roof framing dimensions and assembly rules are covered by the International Residential Code as published by the International Code Council and the American Wood Council.
Figures last checked against the source on . Standards and rates can change — if you are relying on this for something that matters, confirm against the source above. Think a figure is wrong or out of date? Send a correction.