📐Rafter Length Calculator
Find rafter length, rise, plumb cut angle and the lumber length to buy from your roof's run and pitch.
Your roof
Each rafter is about
14.53 ft
rise 6.00 ft · plumb cut 26.6° · buy 16 ft boards
rise
6.00 ft
plumb cut °
26.6
stock length
16 ft
rafters
38
🛒 Shopping list
- Rafter boards (16 ft)38 boards
Rafter size, spacing and span must follow your local building code — check a span table before you buy.
How it works
Rise = run × pitch ÷ 12. Rafter length = √(run² + rise²), plus the overhang measured along the slope (overhang × √(1 + (pitch ÷ 12)²)). Plumb cut angle = arctan(pitch ÷ 12). The suggested board is the next standard length (8–24 ft). Rafter count = (building length ÷ spacing + 1) × 2 sides.
Important: rafter size, spacing and allowable span must follow your local building code.
Worked example
A 12 ft run at 6/12 pitch with a 1 ft overhang: rise = 6 ft, √(144 + 36) = 13.42 ft, plus 1.12 ft of overhang along the slope = 14.53 ft. Plumb cut 26.6° — buy 16 ft boards. A 24 ft building at 16 in spacing needs 38 rafters.
Questions people ask
Half the building's span, measured level from the outside wall to the center of the ridge.
More calculators you might like
Metal Roofing Calculator
Count metal roof panels, screws, ridge cap and trim for a gable roof.
Roof Pitch Calculator
Turn rise and run (or an angle) into roof pitch, degrees, slope percent and an area multiplier.
Roofing Calculator
Measure the footprint, pick your roof pitch, and get squares, shingle bundles and underlayment in seconds.
Email me my results
Pop in your email and keep your numbers handy at the store.
Last updated October 8, 2026. These results are friendly estimates to help you plan — always double-check before buying or making big decisions.