Rafter Length Calculator
A 12-foot run at a 6/12 pitch needs a common rafter about 13 feet 5 inches long. Use this rafter length calculator to turn horizontal run and roof pitch into the rafter length you cut before ridge and bird's mouth adjustments. Enter the run, the pitch as rise per 12 inches, and an optional overhang to get the full board length.
Quick answer
Run is the horizontal distance from the outside wall to the ridge centerline, not the full building width.
What this tells you
- •Run is the horizontal distance from the outside wall to the ridge centerline, not the full building width.
- •Pitch is written as rise per 12 inches of run, such as 6/12 or 9/12.
- •Rafter length is the hypotenuse of the run and the rise that pitch creates, found with the Pythagorean relationship.
How to Use
- 1Measure the horizontal run in inches. On a simple gable roof this is usually half the building span.
- 2Enter the roof pitch as rise per 12 inches of run, for example 6 for a 6/12 pitch.
- 3Add an overhang in inches if the rafter needs to extend past the wall line at the eave. Leave it at 0 for the rafter length to the wall line only.
- 4Click Calculate to see rafter length in inches, feet, and feet and inches.
How It Works
Formula
Total Run = Run + Overhang
Rise = Total Run x (Pitch / 12)
Rafter Length = sqrt(Total Run^2 + Rise^2)The roof pitch sets a rise-to-run ratio. Multiplying the total horizontal run by pitch divided by 12 gives the vertical rise that run produces. The rafter itself is the hypotenuse of that rise and run, so squaring both, adding them, and taking the square root gives the straight-line rafter length. An overhang extends the horizontal run at the same pitch before that hypotenuse is taken, since the eave follows the same roof slope.
Calculation note: values are processed in the order shown above, using the current input units.
Worked Examples
12-foot run at 6/12 pitch, no overhang
A 144-inch run at a 6/12 pitch produces a 72-inch rise. The rafter length is the hypotenuse of 144 and 72, which comes out to 161.00 inches, or about 13 feet 5 inches.
10-foot run at 4/12 pitch
A 120-inch run at a 4/12 pitch produces a 40-inch rise. The hypotenuse of 120 and 40 is 126.49 inches, or about 10 feet 6.49 inches.
12-foot run at 6/12 pitch with an 18-inch overhang
Adding an 18-inch overhang extends the total run to 162 inches. At a 6/12 pitch that run produces an 81-inch rise, and the rafter length grows to 181.12 inches, or about 15 feet 1.12 inches.
Rafter Length per Foot of Run
Rafter length in inches for each foot of horizontal run at common roof pitches. Multiply by your run in feet.
| Pitch | Rafter Length per Foot of Run |
|---|---|
| 3/12 | 12.37 in |
| 4/12 | 12.65 in |
| 6/12 | 13.42 in |
| 8/12 | 14.42 in |
| 9/12 | 15.00 in |
| 12/12 | 16.97 in |
These figures assume a straight common rafter with no overhang. Multiply the per-foot figure by the run in feet, then add any overhang length separately.
What counts as run for a rafter?
Run is the horizontal distance the rafter covers, not the length of the board itself. On a simple gable roof, run is usually measured from the outside face of the wall to the centerline of the ridge, which is often half the total building span.
The rafter length this calculator returns is the theoretical line length along the top of the rafter, from the outer wall line to the ridge line. Actual lumber length is usually longer, since it needs to account for the ridge cut, the bird's mouth notch at the wall, and any tail past the overhang.
Pitch stays constant across the whole rafter, so extending the run with an overhang extends the rise by the same ratio. That is why this calculator adds the overhang to the run before finding the rafter length, instead of just adding overhang inches to the result.
Common mistakes
- Using the full building width as the run instead of the horizontal distance from wall to ridge.
- Entering rafter length or roof width in the run field instead of horizontal run.
- Forgetting that pitch is rise per 12 inches of run, not a percent or a decimal.
- Expecting the result to be the exact lumber cut length. It does not include the ridge cut, bird's mouth notch, or rafter tail trim.