📐Rafter Length Calculator PRO
Common rafters, tails, ridge, counts, and board feet — IRC R802 framing.
Dimension Diagram — updates as you type
Half-span and pitch set the rise; the rafter is the hypotenuse plus the overhang tail.
1 Building & Pitch
2 Lumber & Pricing
Results
Rafter Cut List
Roof Geometry
Cost Breakdown
⚠️ Pre-Export Checklist
- Span measured wall-to-wall (run = half span)
- Pitch matches the plans (rise per 12 in run)
- Overhang tail included in lumber order
- Lumber size checked against span tables for snow load
- Ridge board one size deeper than rafters
How to Calculate Rafter Length
Show formulas and worked example ▼
Formulas
Run = Span ÷ 2, Rise= Run × Pitch ÷ 12
Slope = √(Run² + Rise²)
Tail= Overhang × Slope ÷ Run,Total = Slope + Tail
Pairs= (Length in inches ÷ Spacing) + 1, doubled, plus ridge
Worked Example
24 ft span, 30 ft long, 6/12 pitch, 2 ft overhang, 16 in o.c.:
- Run 12 ft, rise 6 ft → slope √(144+36) = 13.42 ft
- Tail: 2 × 13.42 ÷ 12 = 2.24 ft → 15.66 ft per rafter (buy 16 ft)
- Pairs: 360 ÷ 16 + 1 = 23.5 → 24 pairs = 48 rafters
- Total: 48 × 15.66 + 30 ridge = 782 lin ft
Common Mistakes to Avoid
- Using full span instead of half-span run
- Forgetting the overhang tail lumber
- Counting one roof side only
- Undersized lumber for the span and snow load
Drag to rotate — Scroll to zoom
Calculadoras Relacionadas
Frequently used together on the same project.
Client & Quote (prints on PDF & Excel)
Adjustments (applies to exports)
Contingency is added as a separate line on the exported cost summary. Waste % is printed as an advisory reminder.
Project notes (prints on PDF & Excel)
Result compare (A/B)
| Item | Pinned | Current | Change |
|---|---|---|---|
Saved rates auto-apply from your Price Library. Advanced details stay in your browser — nothing is uploaded except inside your own PDF/Excel exports.
Cutting rafters is the most math-intensive part of roof framing, and a wrong cut wastes expensive lumber. Our free rafter calculator turns span, pitch, and overhang into exact rafter lengths, tail cuts, ridge board size, and a full lumber list. It follows IRC R802 conventional framing rules, so the numbers match what an inspector expects to see on the roof.
Enter the building width (the span the rafters must cross), the building length, the roof pitch in twelfths, the eave overhang, and your rafter spacing. The calculator derives the rise from the pitch, applies the Pythagorean theorem for the slope length, extends the tail for the overhang, and counts rafter pairs plus the ridge board. Board feet are computed from nominal lumber sizes so you can price the package.
📖 Cómo Usar Esta Calculadora
Step 1: Enter Building Dimensions
Type the building width (span) and length in feet. The span is the distance the rafter pair must bridge.
Step 2: Set Roof Pitch
Enter pitch as rise over a 12-inch run — 6/12 is typical for homes, 4/12 for sheds, 10/12 and up for steep designs.
Step 3: Add Overhang and Spacing
Enter the horizontal eave overhang (usually 1 to 2 ft) and choose 16 or 24 in o.c. rafter spacing.
Step 4: Review Lengths and Lumber
Results show rafter slope length, tail length, total per-rafter length, rafter count, ridge board, total linear feet, board feet, and cost.
📐 Cómo Se Calcula el Cabio
Run equals half the span. Rise equals run times pitch divided by 12. The rafter slope length is √(run² + rise²). The tail (overhang) runs along the same slope, so tail = overhang × rafter ÷ run. Total rafter length is slope plus tail. Rafter pairs equal (building length in inches ÷ spacing) + 1, doubled for both roof sides, plus one ridge board the full building length. Board feet use nominal sizes: a 2x6 carries 1 board foot per linear foot.
⚠️ Errores Comunes a Evitar
Measuring span instead of run
✓ The triangle uses half the span (the run). A 24 ft span gives a 12 ft run — using 24 ft doubles the rise and the rafter length.
Forgetting the tail
✓ The overhang adds real lumber: a 2 ft overhang on a 6/12 pitch adds over 2.2 ft per rafter. Multiply by 40 rafters and that is 90 feet of lumber.
Counting one side only
✓ Rafters come in pairs. Always double the one-side count, then add the ridge board.
Wrong lumber size for the span
✓ Long spans need deeper rafters (2x8, 2x10). Check AWC span tables — an undersized rafter sags under snow load.
💡 Consejos Profesionales
Cut one test rafter and dry-fit it before cutting the whole package.
Mark the birdsmouth plumb cut and seat cut with a framing square to keep the math honest.
Order rafters 2 ft longer than calculated when in doubt — trimming is cheaper than scrapping.
Use a ridge board one size deeper than the rafters for solid nailing.
❓ Preguntas Frecuentes
Run is 12 ft, rise is 6 ft, slope length is √(144+36) = 13.42 ft. Add a 2 ft overhang tail (2 × 13.42 ÷ 12 = 2.24 ft) for 15.66 ft total — buy 16-footers.
Divide the building length in inches by the spacing and add one pair. A 24 ft building at 16 in o.c. needs (288 ÷ 16 + 1) = 19 pairs, or 38 rafters.
2x6 works for short spans at 16 in o.c.; spans over ~14 ft typically need 2x8 or larger. Always verify against AWC span tables for your snow load.
The ridge board runs the full building length, typically one size deeper than the rafters (e.g., 2x8 ridge with 2x6 rafters).
🏗️ Aplicaciones Reales
Rafter math shows up on garages, additions, sheds, cabins, and full-house roofs. Owner-builders cutting their first roof use this calculator to turn a scary framing-square problem into a cut list. Even experienced crews run the numbers to order lumber before the walls are up, since lead times on long 2x10s and 2x12s can stall a project.
Our free rafter length calculator computes common rafter slope length, overhang tails, ridge boards, rafter counts, board feet, and cost for any roof pitch. Use it for gable roofs on garages, homes, sheds, and cabins with 16 or 24 inch spacing. Enter span, length, pitch, and overhang for an instant IRC-based cut list.
✅Method, Sources & Review
Maintained by theBuildingCalc methods desk· Last reviewedSeptember 23, 2026. This calculator implements the standards and references below; every figure can be rechecked against the formulas and worked example on this page.
Results are estimates, not engineering advice. Verify against local codes and licensed professionals for structural, electrical, and gas work. See ourTerms. Spotted an error? Report it— corrections ship with the next review.