Metric Rafter Calculator Guide

Why a Metric Rafter Calculator? UK, Australia & Europe

Most online rafter calculators default to imperial units — feet, inches, and the X/12 pitch convention. But builders in the UK, Australia, New Zealand, and across continental Europe work in millimetres and metres, with pitch expressed in degrees rather than rise-per-foot. A metric rafter calculator removes the constant unit conversion that introduces errors: a 3.6 m run is entered as 3.6, not 11'-9-3/4", and a 22.5° pitch is entered directly rather than looked up as "4.9/12".

This guide covers the full metric workflow: how to calculate rafter length in metric, how to convert between degree pitch and X/12, the UK and Australia rafter size conventions, and an imperial-to-metric conversion table for cross-border projects. The Pythagorean math is identical — only the units and pitch expression change.

How to Calculate Rafter Length in Metric

The metric rafter length formula is the same right-triangle hypotenuse as the imperial version, just with metres or millimetres as the unit:

Notice the key difference from the imperial workflow: pitch is entered in degrees (22.5°), not as X/12. The calculator above handles this directly — type the degree pitch and the run in metres, and you get line length, rise, and total length in both metres and millimetres, with an imperial cross-check for reference.

Pitch in Degrees vs X/12: The UK & Australia Convention

In the US, pitch is expressed as X/12 (inches of rise per foot of run). In the UK and Australia, pitch is expressed in degrees from horizontal. The conversion is straightforward:

A common point of confusion: Australia frequently uses 30° as a standard residential pitch, which does not map to a clean X/12 value (it is ≈ 6.9/12). This is exactly why the metric/degree system is preferred outside the US — round-degree pitches (20°, 25°, 30°, 35°, 45°) are easier to set on a protractor or digital level than fractional X/12 values.

How to Calculate Rafter Length and Angle in Metric

When you calculate rafter length and angle in metric together, the angle is a direct input (in degrees), so you skip the X/12-to-degrees conversion that imperial users must perform. The full workflow:

Metric Rafter Size Guide: UK & Australia

UK and Australia use different actual dressed sizes from US nominal lumber. UK rafters are typically 47 mm thick (C16/C24 grade), while Australia uses 38 mm thick MGP10/MGP12 pine. The table below maps the common metric sizes to their imperial equivalents and typical span ranges.

To calculate rafter size UK, start with the clear span, spacing (400 mm or 600 mm centres), pitch, and roof load, then consult the TRADA Span Tables or Building Regulations Approved Document Afor the correct cross-section. In Australia, refer to AS 1684 Span Tables for your wind and roof load category. For a full span chart with bending-check methodology, see our Rafter Span and Size Guide.

Imperial ↔ Metric Conversion Table

For cross-border projects or working with imperial span tables, use these exact conversion factors. Always carry full precision through the calculation and round only the final displayed result.

Regional Pitch Conventions: Australia vs UK vs US

Two pitfalls catch metric builders most often. First, using a US span table with metric timber without converting actual depths — a US 2×8 span table assumes 7.25" (184 mm) actual depth, but an Australia 38×184 mm is the same depth, so the table transfers cleanly. A UK 47×150 mm, however, is only 150 mm deep (5.9"), which is shallower than a 2×8 — so the US 2×8 span table would over-rate it. Second, forgetting that UK/Australia rafter spacing is often 600 mm (24") rather than 400 mm (16"), which reduces allowable span by roughly 20%. Always match the spacing to the span table you are reading.

Frequently Asked Questions