Rafter Length Calculator
Exact common, hip and valley rafter lengths from span, pitch and overhang — in decimal feet, feet and inches, or metres.
Measurements
Results
- Common rafter (with overhang)
- 18.448 ft
- Rafter to wall plate
- 16.771 ft
- Hip / valley rafter
- 22.500 ft
- Rafter multiplier
- 1.1180
- Angle
- 26.57°
Rafter in feet & inches: 18 ft 5 6/16 in
Steep-ish — use roof jacks and soft-soled boots
Everything else this roof gives you
- Rise at ridge
- 7.50 ft
- Run (half span)
- 15.00 ft
- Run + overhang
- 16.50 ft
- Rafter multiplier
- 1.1180
- Hip / valley rafter
- 22.500 ft
- Plumb cut
- 63.43°
- Seat (birdsmouth) cut
- 26.57°
- Roof area
- 1,476 sq ft
- Squares (100 sq ft)
- 14.8
- Shingle bundles (3/sq)
- 45
Squares and bundles assume US shingle sizing, so they are quoted in square feet even when you work in metres.
Quick answer
A rafter is the hypotenuse of the roof triangle, so its length is √(rise² + run²). On a 6/12 roof with a 15 ft run the common rafter is 16.77 ft to the wall plate, and a 1.5 ft overhang takes it to 18.45 ft (18 ft 5 in) of timber.
rafter = √(rise² + run²) = run × √(1 + slope²)
How to use the rafter length calculator
- 1Enter the pitch as rise per 12, degrees or percent grade — whichever you have on the drawing.
- 2Type the building span; the run used for the rafter is half of it.
- 3Add the eave overhang so the rafter tail past the wall plate is included.
- 4Read the common rafter length, the length to the wall plate, and the hip or valley rafter length.
- 5Use the feet-and-inches conversion and cut angles to mark the timber before you cut.
Rafter multipliers by pitch
Multiply your run (including overhang) by the multiplier for a fast rafter length in the field.
| Pitch | Angle | Grade | Rafter multiplier |
|---|---|---|---|
| 1/12 | 4.76° | 8.3% | 1.0035 |
| 2/12 | 9.46° | 16.7% | 1.0138 |
| 3/12 | 14.04° | 25.0% | 1.0308 |
| 4/12 | 18.43° | 33.3% | 1.0541 |
| 5/12 | 22.62° | 41.7% | 1.0833 |
| 6/12 | 26.57° | 50.0% | 1.1180 |
| 7/12 | 30.26° | 58.3% | 1.1577 |
| 8/12 | 33.69° | 66.7% | 1.2019 |
| 9/12 | 36.87° | 75.0% | 1.2500 |
| 10/12 | 39.81° | 83.3% | 1.3017 |
| 11/12 | 42.51° | 91.7% | 1.3566 |
| 12/12 | 45.00° | 100.0% | 1.4142 |
| 14/12 | 49.40° | 116.7% | 1.5366 |
| 16/12 | 53.13° | 133.3% | 1.6667 |
| 18/12 | 56.31° | 150.0% | 1.8028 |
Which rafter length do you need?
- Common rafter: ridge to wall plate at full pitch — the figure used for ordering timber.
- Overall length: common rafter plus the sloped tail over the overhang, what you actually cut.
- Hip / valley rafter: the diagonal member at a corner; longer than a common rafter and cut with a double cheek at the top.
- Jack rafters: shortened commons running into a hip; each one drops by the common multiplier × spacing (16 in centres at 6/12 drops about 17.9 in per jack).
Frequently asked questions
How do you calculate rafter length?
Rafter length is the hypotenuse of the rise and run: length = √(rise² + run²). Shortcut: multiply the run by the rafter multiplier for your pitch — 1.1180 at 6/12, 1.2019 at 8/12, 1.4142 at 12/12.
Does rafter length include the overhang?
It should if you are ordering timber. Add the horizontal overhang to the run before multiplying, which is exactly what the overhang field here does. The tool also shows the shorter length to the wall plate for the birdsmouth position.
Is the run half the span?
Yes for a symmetrical gable: the run of each common rafter is half the wall-to-wall span, minus half the ridge board thickness if you want theoretical precision. For an asymmetric roof, enter twice the actual run in the span field.
How do I calculate a hip or valley rafter length?
Hip and valley rafters run diagonally, so their run is the diagonal of the two plan runs. For an equal-pitch hip, length = run × √(2 + slope²) — the calculator reports this alongside the common rafter.
How much extra should I add when ordering rafters?
Order the next standard length above the calculated figure and allow roughly 6 inches per rafter for the plumb and seat cuts, plus 5–10% on the total for waste and defects.
What size timber do I need for a rafter?
Span tables decide this from species, grade, spacing, snow load and the rafter's horizontal span — commonly 2×6 up to about 12 ft and 2×8 to 16 ft at 16 in centres. Use this calculator for length, and a local span table or engineer for the section.
What is the rafter multiplier?
It is √(1 + slope²) — the number that converts horizontal run into sloped length. It also converts a flat footprint area into true roof area, which is why roofers use the same figure for material take-offs.
How do I mark the birdsmouth?
Set the seat cut level (equal to the roof angle from the plumb line) and keep the seat depth to no more than one third of the rafter depth. Measure the rafter length to the outside face of the wall plate, then add the tail.