This rafter calculator turns building width and roof pitch into common rafter length, hip rafter length, slope angle and the unit lengths carpenters actually mark out.
| Roof angle | – |
| Common rafter (to ridge centerline) | – |
| Common rafter incl. overhang tail | – |
| Hip rafter (square-plan corner) | – |
| Unit lengths (per ft of run) | – |
Lengths are line lengths to the ridge centerline — deduct half the ridge board’s thickness (¾″ for a standard 2× ridge) before cutting, and confirm birdsmouth seat cuts locally. Geometry first, saw second.
Everything Is the Unit Triangle, Scaled
Pitch itself (rise/12, degrees, percent) converts on our Roof Pitch Calculator. Part of our Construction Calculators collection.
Worked Example (Verified)
- 24 ft wide building, 6/12 pitch (defaults): run = 12 ft, angle = 26.57°, common unit length = 1.1180 ft/ft → common rafter 13.42 ft (13 ft 5 in); hip unit = 1.500 exactly → hip rafter 18.00 ft — the 6/12 hip is a gift of clean numbers
- Overhang: a 12″ horizontal tail adds 1 × 1.1180 = 1.12 ft of rafter — tails scale by the same unit length, measured horizontally then sloped
- Steeper sanity: at 12/12 (45°), unit = 1.4142 (√2) and the common for this building = 16.97 ft
Framing Discipline
- Theoretical length ≠ cut length: ridge deduction, birdsmouth depth and fascia details all consume their share — pattern one rafter, test-fit, then batch-cut.
- Hip math here assumes a square corner with equal pitches both sides; unequal pitches need the full irregular-hip treatment.
- Span tables and snow loads govern rafter SIZE (2×8 vs 2×10) — geometry gives length, engineering gives lumber; check local code tables.
- Rafters vs trusses: most modern production roofs are engineered trusses — this calculator serves stick-framed, shed, porch and addition work.
Related Tools
- Roof Pitch Calculator — rise/run/angle conversions
- Roofing Calculator — squares & bundles for the roof above
- Construction Calculators — the full collection
Frequently Asked Questions
How do you calculate rafter length?
Rafter length = run × unit length, where unit length = √(144 + pitch²)/12 per foot of run. For a 24 ft building at 6/12: run 12 ft × 1.1180 = 13.42 ft (13 ft 5 in) to the ridge centerline. This calculator adds overhang tails and hip rafters.
What is a unit length in rafter framing?
The rafter length per one foot of horizontal run — the hypotenuse of the pitch triangle: 1.1180 ft at 6/12, 1.4142 ft at 12/12. Framing squares are stamped with unit lengths because every rafter on the roof is the unit triangle scaled by its run.
How long is a hip rafter compared to a common?
Longer — it travels the diagonal: hip run = common run × √2, so its unit length = √(288 + pitch²)/12 (1.500 ft at 6/12). On the default building, hip = 18.00 ft vs common 13.42 ft. Equal pitches and a square corner assumed.
Do I subtract for the ridge board?
Yes — the calculator gives length to the ridge centerline. In practice most framers deduct ¾″ (half of a standard 1½″ ridge board, laid out along the measuring line) and verify with a pattern rafter before cutting the pile — the pattern-rafter step forgives small layout differences between methods.
What pitch is standard for houses?
4/12 to 8/12 covers most residential roofs; 6/12 (26.57°) is the walkable-with-care middle. Below 4/12 constrains shingle choices; above 9/12 climbs like a ladder and prices like one too.
Is this rafter calculator free?
Yes — free, instant, no sign-up.