Rafter length calculator
Lay out a common rafter from span or run and pitch: line length, ridge deduction, tail, overall length, plumb and seat cut angles and the birdsmouth heights, with an SVG side view.
Span up to 100 m (328 ft) or run up to 50 m, overhang from 0 to 5 m, pitch above 0 up to 48 per 12, 75 degrees or 400%, ridge thickness from 0 to 500 mm, and rafter depth and seat length up to 1 m. Calculated on your device; download a JSON report, a results CSV and an SVG side view. This calculator uses the measurement fields only. Results update as you type. Up to 1,000,000 characters.
Your result
How to use Rafter length calculator
- Enter the building span or the run of one rafter, the level overhang and the pitch.
- Add the ridge board thickness only if there is a ridge board; leave it empty to measure to the ridge centre line.
- Enter the rafter depth and seat length for the birdsmouth heights, or clear them to skip.
- Review the lengths and cut angles, then download the JSON report, the CSV or the SVG side view.
Useful next steps
Where each length is measured
Every length runs along the measuring line: a line parallel to the rafter edges through the birdsmouth corner at the outside face of the wall, called the building line. Run is level, from the building line to the ridge centre line. Because the ridge cut and the tail cut are both plumb, the top edge and the uncut bottom edge measure the same as the measuring line between those cuts, so you can hook your tape on either edge from plumb mark to plumb mark.
Line length and ridge deduction
Line length is the run times √(1 + (rise ÷ run)²), the same multiplier a framing square table gives per foot of run (Ask the Builder, Tim Carter, Cutting a Roof Rafter, checked 16 October 2026). A 24 ft span at 6/12 gives a 12 ft run and a 13.4164 ft line length to the ridge centre. A ridge board is deducted only when you enter its thickness: half of it comes off the run, measured level, so a 1.5 in ridge shortens the rafter by 0.8385 in along its length, not exactly 0.75 in.
Cut angles
The roof angle is arctan(rise ÷ run) (OpenStax Precalculus 2e, 5.4 Right Triangle Trigonometry, checked 16 October 2026). The plumb cut is 90° minus the roof angle from the rafter edge, and the seat cut equals the roof angle from the edge. On a miter saw, with the rafter flat and an edge on the fence and 0 degrees a square cut, the plumb cut is set to the roof angle and the seat cut to 90° minus it. With a framing square, hold the rise on the tongue and 12 on the blade along the edge.
Birdsmouth, tail and limits
The heel cut height is the seat length times rise ÷ run, and the height above plate is the rafter depth measured plumb minus that heel. The tail is the level overhang times the same multiplier, ending in a plumb cut. The calculator does not size rafters, check spans, check notch limits or replace your building code. Conversions use NIST SP 811.