How Long Should Your Rafters Be?

Calculate rafter length from roof span, run, rise, and pitch for accurate roofing and framing measurements.

Rafter Length Calculator – Homes Remodeler

Inputs

Results update as you type. No Save.

I want to calculate…
Units
Calculation should include…
Rise & run
ft
ft
Roof pitch
ft
:12
ft
Leave blank to calculate. Or enter length + one other value to solve for rise/run.
Roof layout
ft
in
Truss cost
$ /hr

Rafter Planning Ideas

Size rafters from rise or pitch, then estimate how many trusses you need and what they may cost.

What you get

  • Rafter length from rise + run
  • Or from run + roof pitch
  • Truss count from roof length & spacing
  • Optional material + install cost

Helpful extras

  • Pitch as °, %, or x:12
  • Common 16″ / 24″ O.C. presets
  • Works both ways (solve any side)
  • Live truss diagram

How we calculate

  • rafter = √(rise² + run²)
  • rafter = run × √(pitch² + 1)
  • count = ⌈(L / spacing)⌉ + 1
  • cost = count × price + labor

Span is usually twice the run, so a 12 ft span → 6 ft run. At a 4:12 pitch, rafter ≈ 6 ft 4 in (before overhang). Always check lumber span tables and local codes.

Common on-center spacing is 16″ or 24″. Exact spacing depends on load, span, lumber size, and climate — follow your engineer or building code.