How do I calculate rafter length?

Rafter length is the hypotenuse of the roof triangle: √(run² + rise²). If you only know the pitch, enter pitch and run — the calculator solves the rise first. In the field, multiply the per-metre figure in the table above by the building run.

What does 6/12 pitch mean?

Six units of rise for every 12 units of run — a 26.6° angle. The same roof can be described as 500 mm of rise per metre of run.

Does the rafter length include the overhang?

Enter the horizontal overhang separately and the demo converts it to sloped rafter length so you can add it on. A 400 mm horizontal overhang at 6/12 pitch becomes about 447 mm along the rafter.

What about the birdsmouth cut?

The birdsmouth — the seat and plumb notch that rests on the wall plate — is laid out from the rafter depth on site. The calculator gives lengths and angles; the notch depth stays a framing-detail decision.

Rafter Calculator for Roof Pitch and Length

How this rafter calculator works

A common rafter is a right triangle: rise is the vertical height, run is the horizontal span and the rafter length is the diagonal. StructCalc accepts any two of rise, run and pitch angle, then solves the missing value before calculating the rafter length.

Add a horizontal overhang when you need the extra rafter length beyond the wall line. The calculator converts the horizontal overhang into the sloped rafter length so the number is useful for cutting and layout.

Example roof pitch calculation

If you know the run and pitch angle, leave rise empty and StructCalc calculates the rise for you. If you know rise and run from the plans, leave pitch empty and the calculator returns the roof angle. This is useful when checking roof framing dimensions before laying out rafters.

Roof pitch reference

Rafter length per metre of run for the pitches that cover most residential roofs. Multiply the length figure by the run in metres: a 6 m run at 6/12 needs 6 × 1.118 ≈ 6.71 m of common rafter before any overhang.

Pitch	Angle	Rafter length per 1 m run
4/12	18.4°	1.054 m
6/12	26.6°	1.118 m
8/12	33.7°	1.202 m
10/12	39.8°	1.302 m
12/12	45.0°	1.414 m

Rafter calculator FAQ

How do I calculate rafter length?: Rafter length is the hypotenuse of the roof triangle: √(run² + rise²). If you only know the pitch, enter pitch and run — the calculator solves the rise first. In the field, multiply the per-metre figure in the table above by the building run.
What does 6/12 pitch mean?: Six units of rise for every 12 units of run — a 26.6° angle. The same roof can be described as 500 mm of rise per metre of run.
Does the rafter length include the overhang?: Enter the horizontal overhang separately and the demo converts it to sloped rafter length so you can add it on. A 400 mm horizontal overhang at 6/12 pitch becomes about 447 mm along the rafter.
What about the birdsmouth cut?: The birdsmouth — the seat and plumb notch that rests on the wall plate — is laid out from the rafter depth on site. The calculator gives lengths and angles; the notch depth stays a framing-detail decision.

Need this on site?

The Android app includes common rafter and hip or valley rafter workflows, plus stairs, balusters, raked walls, square checks and measurement tools. The mobile workflow is built for quick inputs, clear results and offline use at the jobsite.

Get StructCalc on Google Play