Rafter Length Calculator
Enter the span, pitch, overhang and ridge board and get the common rafter length in feet and inches, the rise, the roof angle and the stock length that covers it.
Outside of wall plate to outside.
1.5 for a 2× ridge, 0 for none.
Common rafters on a gable roof with the same pitch both sides. The result updates as you type.
Common rafter length
14' 5-9/16" (14.46 ft)
13' 4-3/16" plate to ridge + 1' 1-7/16" tail · fits a 16 ft board
| Run (half the span) | 12 ft |
| Run after half-ridge deduction | 11.938 ft |
| Total rise at the ridge centre | 6' 0" (6 ft) |
| Rafter length per foot of run | 13.42 in |
| Roof angle | 26.57° (50% slope) |
| Plumb cut / seat cut on a framing square | 6 on the blade, 12 on the tongue |
What this page cannot tell you
Rafter size, species, grade or allowed span — those come from the span tables in your building code and the loads where you build. Lengths are theoretical line lengths along the rafter's measuring line before the birdsmouth and plumb cuts; cut one pattern rafter and test it before cutting the rest.
How to calculate common rafter length
- Measure the span. From the outside of one wall plate to the outside of the other.
- Enter the pitch. Inches of rise per 12 in of run, e.g. 6 for a 6/12 roof.
- Enter the overhang. The level distance the tail runs past the wall.
- Enter the ridge thickness. 1.5 in for a 2× ridge board; the page takes half of it off the run.
- Read the length. Plate-to-ridge length, tail length and total, in feet and inches.
10 situations where a rafter length calculator helps
Shed roof framing
Get the rafter length for a gable shed before buying lumber.
Choosing stock length
The result names the shortest even-foot board that covers each rafter.
Garage or addition
Check rafter lengths for a new gable before ordering a load.
Comparing pitches
Change the pitch to see how much longer each rafter and the total rise get.
Adding an overhang
See what a deeper eave adds to every rafter.
Cross-checking a framing square
The length per foot of run matches the figure on a rafter table.
Porch roof
A small gable over a porch uses the same calculation.
Checking a plan's numbers
Verify the rafter lengths on a set of drawings.
Teaching carpentry
Each step — run, rise, unit length — is shown, so students can follow it.
Ridge height
The total rise tells you how high the ridge sits above the plates.
Where the method comes from
The method is from Ira S. Griffith's 'Carpentry' (§17, 'To Find the Length of a Common Rafter'), a public-domain textbook: the length of a common rafter is the hypotenuse of a right triangle whose legs are the rise and the run. Because the ratio of rise to run is fixed for a pitch, the length per foot of run is a constant, √(12² + rise²) inches, and the rafter length is that constant times the run in feet. Griffith's own example — 25 ft span, quarter pitch, 13.42 in per foot of run, 13.98 ft — is reproduced exactly by this page.
Griffith adds the run of the tail to the run of the rafter when there is an overhang; this page does the same. The ridge board deduction takes half the ridge thickness off the level run, so the rafter ends at the face of the ridge rather than at its centre.
Limits: lengths are along the rafter's measuring line, before the birdsmouth, plumb cut and tail cut are laid out. The page does not size rafters or check spans — that comes from the span tables in your building code and the snow, wind and dead loads where you build. Cut and test one pattern rafter first.
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Sources
- Ira S. Griffith — Carpentry, §17 To Find the Length of a Common Rafter — Length = run × length per foot of run; 25 ft span example
Frequently asked questions
How do I calculate rafter length?
Multiply the run in feet by √(144 + rise²) ÷ 12. For a 24 ft span at 6/12: run 12 ft × 13.416 ÷ 12 = 13.42 ft before overhang and ridge deduction.
What is the run of a rafter?
Half the span — the level distance from the outside of the wall plate to the centre of the ridge.
Why subtract half the ridge board?
The rafter stops at the face of the ridge board, so the level run is shorter by half its thickness.
Does it include the overhang?
Yes. Enter the level overhang; its length along the slope is shown and added to the total.
What does 6/12 pitch mean?
6 inches of rise for every 12 inches of level run — about 26.6°.
Can this tell me what size rafter I need?
No. Size and spacing come from the rafter span tables in your building code for your loads and lumber grade.
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