Rafter Length Calculator
Enter the building width, the rise in each foot of run, the ridge board thickness and the overhang, and the building length and spacing, and get the rafters to cut in their stock length; the working shows the rafter length with the ridge deduction and the overhang both visible, the plumb and seat angles and a hip.
Estimates for planning, not professional advice. Read the full notice.
Example: a gable roof
32 Rafter board, 16 ft
- Run after the ridge deduction 11 ft 11 1/4 in
- Rafter line length, wall to ridge 13 ft 4 3/16 in
- Rafter length with the overhang 14 ft 5 9/16 in
- Plumb cut angle, in degrees 26.57 degrees
- Seat cut angle, in degrees 63.43 degrees
Estimate for planning, from what you entered and the product data shown. Check product labels and your supplier before you buy.
Your project
Enter a size to see your total.
Each product is rounded up once for the whole project, so the total can be less than the items bought alone added together. A line counted from a cut layout (sheets, boards or tiles laid out piece by piece) is counted for each item and added.
Shopping list
| Item | Size | Buy |
|---|---|---|
| Rafter boards | Rafter board, 16 ft | 32 |
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Worked example: a gable roof
Show the working
- Run: half the width, less half the ridge board 24 ft ÷ 2 − 1 1/2 in ÷ 2 = 11 ft 11 1/4 in
- Line length: the run times the slope factor 11 ft 11 1/4 in × 1.118 = 13 ft 4 3/16 in
- With the overhang: the run plus the overhang, times the slope factor 12 ft 11 1/4 in × 1.118 = 14 ft 5 9/16 in
- Rafters: the building length over the spacing, rounded up, plus one, on both sides 16 per side × 2 sides = 32 rafters
- Stock length that covers the line, in whole steps 14 ft 5 9/16 in = 16 ft
Assumptions
The figures behind the example. Sourced figures link to their source below; a default is a starting value to check against your label or supplier.
| Assumption | Value | Details and changes |
|---|
Feet and inches, like 12' 6"
Rafter line length by run and pitch
Line length of a common rafter, with no ridge deduction and no overhang, by horizontal run and by rise in each foot
| Run | Rise 4 in 12 | Rise 6 in 12 | Rise 8 in 12 | Rise 10 in 12 | Rise 12 in 12 |
|---|---|---|---|---|---|
| 8 ft | 8 ft 5 3/16 in | 8 ft 11 5/16 in | 9 ft 7 3/8 in | 10 ft 4 15/16 in | 11 ft 3 3/4 in |
| 10 ft | 10 ft 6 1/2 in | 11 ft 2 3/16 in | 12 ft 1/4 in | 13 ft 3/16 in | 14 ft 1 11/16 in |
| 12 ft | 12 ft 7 13/16 in | 13 ft 5 in | 14 ft 5 1/16 in | 15 ft 7 7/16 in | 16 ft 11 5/8 in |
| 14 ft | 14 ft 9 1/16 in | 15 ft 7 13/16 in | 16 ft 9 15/16 in | 18 ft 2 11/16 in | 19 ft 9 9/16 in |
| 16 ft | 16 ft 10 3/8 in | 17 ft 10 11/16 in | 19 ft 2 3/4 in | 20 ft 9 15/16 in | 22 ft 7 1/2 in |
The run is the level distance from the wall to the ridge. Each figure is the run times the slope factor of that rise; the ridge, the overhang and the stock length are in the calculator above.
Questions
What is the rafter length formula?
The line length is the run times the slope factor, which is the square root of one plus the rise over twelve, squared. For the roof in the example the factor is 1.118. The worked example shows the run and each step.
How do I add the overhang to a rafter?
Add the overhang, measured level, to the run before you multiply by the slope factor. The worked example shows both lengths, with and without the overhang.
How long is a hip rafter?
A hip crosses the corner of two planes: its length is the square root of the run squared, the other plane's run squared and the height squared. The example line shows one for the run above; the calculator does not size the member.