Rafter Length Calculator

The rafter length calculator adds the overhang to the horizontal run, then multiplies by the pitch's area multiplier, the square root of (rise² + 12²) divided by 12. It returns the rafter length and the total rise from the wall plate to the ridge cut.

Last reviewed September 2026Method & sources Report an error

Not to scale.

Rafter Length

14.53 ft

Breakdown
Rafter Length	14.53 ft
Total Rise (wall plate to ridge cut)	6.50 ft

How to Calculate Rafter Length

A common rafter follows the hypotenuse of a right triangle formed by its horizontal run and vertical rise. That is the same Pythagorean relationship the Roof Pitch calculator's area multiplier is built from. Adding the overhang to the run first gives the rafter's total horizontal span from the ridge centerline to the eave's outer edge. Multiplying that total run by the pitch's area multiplier (`√(rise² + 12²) ÷ 12`) gives the rafter's actual on-the-slope length.

Example: 12 ft run to the ridge centerline, 1 ft overhang, 6

A 12 ft run to the ridge centerline, 1 ft overhang, 6:12 pitch: 1. Total run: 12 + 1 = 13 ft 2. Area multiplier (6:12): √(6² + 12²) ÷ 12 = 1.118 3. Rafter length: 13 × 1.118 = 14.53 ft 4. Total rise: (13 × 6) ÷ 12 = 6.5 ft

Rafter Length: Compare Scenarios

Scenario	Rafter Length	Total Rise (wall plate to ridge cut)
Small (8 ft run shed roof, 4:12)	9.49 ft	3.00 ft
Medium (12 ft run gable roof, 6:12)	14.53 ft	6.50 ft
Large (20 ft run garage roof, 8:12)	26.44 ft	14.67 ft

What Is Rafter Length?

A rafter length calculator converts a roof's horizontal run, overhang, and pitch into the length of a common rafter. That's the board that spans from the ridge down to (and past) the exterior wall. It uses the same rise-and-run geometry as the Roof Pitch calculator, applied to a straight-line rafter length instead of a surface area multiplier.

Rafter Length: Pros and Cons

Pros

Includes overhang directly in the run, rather than requiring a separate calculation for the eave extension
Uses the same pitch geometry as the Roof Pitch and Roofing Material calculators, so the figures stay consistent across a full roof-framing project
Reports total rise alongside rafter length, useful for laying out the ridge board height

Cons

Doesn't subtract for the ridge board's thickness at the top cut or account for the birdsmouth notch at the bottom cut. Both slightly shorten the actual board needed versus this calculator's theoretical centerline-to-centerline length
Calculates a common rafter only. Hip and valley rafters follow different geometry (a 3D diagonal rather than a simple 2D triangle) and aren't covered here
Doesn't determine what rafter size or spacing your span needs structurally. That's a span-table or engineering question outside this calculator's scope

Rafter Length: What a Pro Checks

Account for the ridge board's thickness and the birdsmouth notch separately. Both shorten the actual rafter you cut slightly versus this calculator's theoretical centerline-to-centerline length.
This is for a common rafter only. A hip or valley rafter runs diagonally in three dimensions and needs different geometry, not this straight rise-and-run triangle.
This gives you a length, not a size. Check a span table or a structural engineer for what rafter size and spacing your actual span needs to carry.

Frequently Asked Questions

How do I calculate rafter length?

Add your overhang to the horizontal run (from the ridge centerline to the wall), then multiply by the pitch's area multiplier, √(rise² + 12²) ÷ 12. A 12 ft run with a 1 ft overhang at 6:12 pitch gives a rafter length of about 14.53 ft.

Does this account for the ridge board and birdsmouth cut?

No. This gives the theoretical rafter length along the centerline. In practice, framers typically account for the ridge board's thickness (shortening the rafter slightly at the top) and the birdsmouth notch cut where the rafter sits on the wall plate. Both are minor adjustments beyond this calculator's geometry.

What is the run for a rafter calculation?

Run is the horizontal distance the rafter spans. It's typically half the building's width, to the ridge centerline, for a simple gable roof. Add any overhang separately, since the eave extends the rafter past the exterior wall.

Does this work for hip or valley rafters?

No. Hip and valley rafters run diagonally in three dimensions rather than following a simple 2D right triangle. They need a different geometric approach than the one this calculator uses for a common rafter.

How does pitch affect rafter length?

A steeper pitch makes the rafter longer for the same horizontal run, since more of its length goes into rising rather than running horizontally. A 12:12 pitch rafter is about 41% longer than the horizontal run alone, versus about 12% longer at a 6:12 pitch.

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