This calculator turns the two measurements you actually have on a job site — horizontal run and vertical rise — into the three numbers you need: rafter length, pitch, and angle. No carpenter’s square, no triangle math, no looking up tables.
How rafter length is calculated
Rafter length is the hypotenuse of a right triangle where the run and rise are the two legs:
rafter = √(run² + rise²)
The calculator also derives:
- Pitch = (rise ÷ run) × 12, expressed as X/12
- Angle = arctangent(rise ÷ run), in degrees
Everything is calculated in the unit you enter (feet, inches, meters) and converted to feet for the result.
What the result represents
The rafter length the calculator gives you is the plumb-cut to plumb-cut length — from the outside face of the wall plate (at the bird’s mouth) to the centerline of the ridge. It does not include:
- Eave overhang — add separately. For a 12" overhang at 4/12 pitch, add about 12.6" to the rafter length.
- Ridge board thickness — subtract half the ridge thickness if measuring to the ridge face instead of centerline. For a 1.5"-thick ridge, that’s 0.75".
- Bird’s mouth seat cut — doesn’t affect length, but affects where you mark the cut.
Common rafter pitch reference
| Pitch | Angle | Multiplier (×run) |
|---|---|---|
| 2/12 | 9.46° | 1.014 |
| 3/12 | 14.04° | 1.031 |
| 4/12 | 18.43° | 1.054 |
| 5/12 | 22.62° | 1.083 |
| 6/12 | 26.57° | 1.118 |
| 7/12 | 30.26° | 1.158 |
| 8/12 | 33.69° | 1.202 |
| 9/12 | 36.87° | 1.250 |
| 10/12 | 39.81° | 1.302 |
| 12/12 | 45° | 1.414 |
The “multiplier” is the ratio of rafter length to run. A 12 ft run at 6/12 pitch gives a rafter length of 12 × 1.118 = 13.42 ft.
When you can’t measure rise directly
If you know the building width and the desired pitch but haven’t framed anything yet:
- Run = building width ÷ 2 (for a symmetrical gable)
- Rise = run × (pitch ÷ 12)
- Plug into the calculator
Example: 24 ft wide building at 6/12 pitch → run = 12 ft, rise = 12 × (6/12) = 6 ft → rafter length = 13.42 ft (plus your overhang).
Hip rafters need a different calculator
For hip roofs, the hip rafter runs along the corner — its run is the diagonal of the building corner, not half the width. A square hip multiplies the common rafter run by √2 (1.414). Use the Hip Roof Rafter Calculator for hip rafters specifically — the geometry is different.
Frequently asked questions
What is rafter length vs run and rise?
Run is the horizontal distance from the outside wall to directly below the ridge — typically half the building width. Rise is how far the ridge sits above the wall plate. Rafter length is the diagonal — the actual length of lumber you'll cut. It's the hypotenuse of the run/rise right triangle.
How do I find the roof pitch from my measurements?
Pitch is expressed as rise per 12 inches of run. If your rise is 4 ft over a 12 ft run, pitch is 4/12 (read "four in twelve"). The calculator shows this in the result. Common pitches: 4/12 (low), 6/12 (medium), 9/12 (steep), 12/12 (45°).
Does this include overhang?
No — the calculation is from wall plate to ridge plumb cut only. Add your eave overhang (typically 12"–24") to the rafter length when ordering lumber. For a 12" overhang on a 4/12 pitch, add about 12.6" to the rafter length.
What's the difference between common rafters and hip rafters?
Common rafters run perpendicular to the wall plates — what this calculator handles. Hip rafters run diagonally from the corner of the building to the end of the ridge and are longer — use a hip rafter calculator and multiply your common rafter length by ~1.414 (for a square hip).
What lumber size do I need?
Rafter sizing depends on span, spacing, snow load, and lumber species. Rough rule for residential: 2×8 SPF at 16" OC handles ~12 ft span at 4/12 pitch. Always check IRC span tables for your specific load and pitch. This calculator computes geometry only, not structural sizing.