Calculate roof pitch and rafter length

Convert measured roof rise and horizontal run into degrees, percent slope, pitch ratio, and straight rafter length.

freeworks offlinenothing uploaded

ToolRoof Pitch Calculator

Input

Output

—

Put this on your own site

The frame below runs the same code as this page, in the reader's own browser. Nothing is sent to us, and nothing is sent to you.

AccentBackground

Pick a dark background and the text and panels follow it, so the frame stays readable on a dark page.

How it works

Pitch is arctangent of rise ÷ run, percent slope is the same ratio × 100, and rafter length is the hypotenuse √(rise² + run²). A horizontal eave extension continues at the same pitch, so the slope length scales by total horizontal run divided by structural run.

Formula

pitch degrees = arctan(rise ÷ run) × 180 ÷ π

Run means wall plate to ridge, usually half the full span of a symmetric gable roof.
Eave overhang defaults to zero when only the structural wall-plate-to-ridge triangle is required.

Worked example

A 2m rise over a 4m run

With a 300mm eave overhang

Input

											Rise m: 2
Run m: 4
Eave overhang m: 0.3

Output

												Pitch degrees: 26.57
Pitch percent: 50
Pitch ratio: 1:2
Rafter length m: 4.47
Rafter length with overhang m: 4.81

When to use this

Rafter set-out uses half-span run, roofing checks compare degrees with a product’s minimum pitch, and MCS solar assessments combine inclination with roof azimuth.

Edge cases

Using full building span as one side’s run halves the rise-to-run ratio and reports a roof much flatter than its measured geometry.
A 300 mm horizontal eave extension follows the same slope, so both horizontal and vertical components extend proportionally instead of holding ridge rise against the longer run.
Straight hypotenuse length omits ridge cuts, birdsmouth, fascia details, and stock-end allowance, which remain part of the carpentry layout.