Calculate the exact cut length of common rafters from your building span, roof pitch, ridge thickness, and overhang. Results in feet-inches and decimal feet. Free — no signup required.
Total outside-to-outside width of the building (not half).
Horizontal projection beyond the wall — not the rafter length of the overhang.
Multiply your horizontal run by the multiplier to get rafter length.
| Pitch | Multiplier | Angle |
|---|---|---|
| 3:12 | 1.031 | 14.0° |
| 4:12 | 1.054 | 18.4° |
| 5:12 | 1.083 | 22.6° |
| 6:12 | 1.118 | 26.6° |
| 7:12 | 1.158 | 30.3° |
| 8:12 | 1.202 | 33.7° |
| 9:12 | 1.250 | 36.9° |
| 10:12 | 1.302 | 39.8° |
| 12:12 | 1.414 | 45.0° |
A 28-foot wide garage with a 6:12 pitch, 1.5-inch ridge board, and 16-inch overhang: run = (28 × 12 / 2) − (1.5 / 2) = 168 − 0.75 = 167.25 inches. Pitch multiplier for 6:12 = √(36 + 144) / 12 = 1.118. Rafter length = 167.25 × 1.118 = 186.9 inches. Overhang addition = 16 × 1.118 = 17.9 inches. Total = 186.9 + 17.9 = 204.8 inches = 17 ft 0⅞ in. Order 18-foot lumber.
The pitch multiplier converts horizontal run to actual rafter length along the slope. For 6:12 pitch, it's √(6² + 12²) / 12 = 1.118. Multiply your run by this to get the rafter length.
The run is the horizontal distance from the outside of the wall to the centerline of the ridge, minus half the ridge board thickness. The rafter sits on the wall plate and meets the ridge at this point.
The bird's mouth (notch at the wall plate) doesn't change the calculated rafter length — it's cut into the existing rafter. Add 1–2 inches to your lumber order length for the seat cut and plumb cut waste.
A common rafter runs perpendicular from the ridge board to the top plate. Hip and valley rafters run at 45° and need different multipliers (1.414 × common for a square hip).
Ridge height = run × (pitch rise / 12). For a 12-foot run at 6:12 pitch, ridge = 12 × 0.5 = 6 feet above the top plate.
Typical Results
💡 Always cut one test rafter and dry-fit it before cutting the full set. Small errors in span measurement compound across all rafters.
Common Mistakes