Rafter calculator
A common rafter’s line length is the hypotenuse of total rise and total run. That length is what you shop for before birdsmouths and ridge thickness eat into the stock. Enter rise and run (or pitch + run), read line length and optional stock count. This does not size species, snow load, or collar ties.
+5% so you don't come up short
Common rafter line length only — not engineered framing or hip/valley math.
When to use
Shed roof shopping list; checking a cut list against 2×6 / 2×8 stock lengths; linking pitch math to a board length.
When not to
Hip/valley rafters (different lengths). Truss orders. Stamped engineered designs. Deck stairs → stair calculator.
Assumptions
Line length = √(rise² + run²). Pitch mode: rise = run × (pitch/12). Overhang adds to run if you include it. Stock count = ceil(length ÷ usable stock) with waste; ridge board thickness not auto-subtracted unless you shorten run.
Worked examples
Example 1
Run 12 ft, rise 6 ft (6/12)
→ length = √(12²+6²) = √180 ≈ 13.42 ft
Buy 14 ft stock or scarf — check yard lengths.
Example 2
Run 10 ft including overhang, 8/12 pitch
→ rise = 10×8/12 ≈ 6.67 ft → length ≈ 12.02 ft
Birdsmouth seating shortens the plumb length above the wall — leave pattern stock.
How it calculates
Hypotenuse of rise and run (plus overhang), times piece count.
- Enter rise + run, or pitch on 12 + run.
- Add overhang if it is part of the stock length.
- Set count and waste; read length and linear feet.
rise, run in the same units (feet here) length = √(rise² + run²) optional pieces × length → linear ft
Common traps
- Total run is to the ridge centerline math you chose — measure consistently.
- Overhang forgotten = short rafters.
- Hips are longer — do not use common length for hips.
- Code span tables decide depth; this page decides length to buy.
Next tool
Pitch / slope factors → roof pitch · roof slope.
Board feet → board foot calculator.
Stairs → stair calculator · stair stringer.
FAQ
√(rise² + run²) for a common rafter.
Related
Estimate only — not a contractor bid or engineered takeoff. Confirm bag yields and local codes before you pour or order.