Roofing
Rafter Length Calculator
Work out rafter length from run and pitch, with the overhang and the ridge allowance accounted for.
Result
This is the theoretical line length including overhang. It does not account for the plumb cut, birdsmouth seat or any bevel, cut one rafter as a pattern and test-fit it before cutting the rest.
Get a Free Business AuditWhat moves the number
- Run, measured to the centre of the ridge rather than to the outside of the wall.
- Pitch, which sets the diagonal length.
- Overhang beyond the wall plate, which is additional length.
Where this one goes wrong
Forgetting the ridge board thickness. Half the ridge thickness comes off the run at the top, and a rafter cut without allowing for it is long by that amount on every single one.
Questions
About this calculator
Where is the run measured from?
From the outside of the wall plate to the centre of the ridge. Half the ridge board thickness is then deducted, which is the step most commonly missed.
Does this include the overhang?
It should be added separately, because the overhang is a design decision rather than a function of the span. Add it to the calculated length before cutting.
What about the birdsmouth?
It is a cut rather than a length, but it affects the seat and the plumb cut positions. Mark from the calculated line rather than measuring from the cut.
The formula
rafter = √(run² + rise²) + overhang × slope multiplier
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How to use this calculator
- Building span (ft) (ft). Enter your own figure; the page starts at 30 ft.
- Roof pitch (rise per 12). Enter your own figure; the page starts at 6/12 (common).
- Overhang (inches) (in). Enter your own figure; the page starts at 18 in.
- Ridge board thickness (inches) (in). Enter your own figure; the page starts at 1.5 in.
The result updates as you type, so you can try a figure, see what it does to rafter length, and change it back. Nothing you enter is sent anywhere or saved.
A worked example
These are the figures the calculator opens with. Every number in the results column below is produced by the same calculation the page runs, so you can follow it through before putting your own figures in.
What goes in
| Input | Value |
|---|---|
| Building span (ft) | 30 ft |
| Roof pitch (rise per 12) | 6/12 (common) |
| Overhang (inches) | 18 in |
| Ridge board thickness (inches) | 1.5 in |
What comes out
| Result | Value |
|---|---|
| Rafter length main result | 18' 4.5625" |
| Rafter length (decimal ft) | 18.3777 |
| Horizontal run | 14.9375 |
| Total rise | 7.4688 |
Each common rafter is 18' 4.5625" (18.3777 ft) over a 14.9375 ft run with 7.4688 ft of rise.
How sensitive the answer is
Holding everything else steady and changing only building span (ft), this is what happens to rafter length. It is worth knowing which of your inputs the answer actually hinges on, and which barely move it.
| Building span (ft) (ft) | Rafter length |
|---|---|
| 15 ft | 9' 11.9375" |
| 22.5 ft | 14' 2.25" |
| 30 ft example above | 18' 4.5625" |
| 45 ft | 26' 9.125" |
| 60 ft | 35' 1.75" |
Every other input is held at the example value while this one changes, which is not how a real job behaves. Treat it as a guide to which number matters most, not as five separate quotes.
What each figure means
- Building span (ft) (ft)
- Measured in ft, starting at 30 ft.
- Roof pitch (rise per 12)
- Choose from 1/12 (very low), 2/12, 3/12, 4/12, 5/12, 6/12 (common), 7/12, 8/12, 9/12, 10/12, 12/12 (45°), 14/12 (steep).
- Overhang (inches) (in)
- Measured in in, between 0 and 60, starting at 18 in.
- Ridge board thickness (inches) (in)
- Measured in in, between 0 and 12, starting at 1.5 in.
What the results tell you
| Result | What it is |
|---|---|
| Rafter length | The headline result. A category or planning selection, rather than a measured quantity. |
| Rafter length (decimal ft) | A number. |
| Horizontal run | A number. |
| Total rise | A number. |
What this cannot see
Material quantities assume a clean rectangular plane and the waste factor you enter. Valleys, hips, dormers, penetrations and a steep pitch all raise real waste, and a tear-off frequently uncovers decking that was not in the quote.
Keep going
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