Metric Unit Calculator
The formula, your substituted values, and the evaluated result are shown below.
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View offerAffiliate link — we may earn a commission at no extra cost to you.Calculate roof pitch as a percentage.
This is a planning estimate. Confirm dimensions, allowances, material specifications, applicable codes, and supplier requirements before purchasing or construction.
Calculate roof pitch as a percentage.
Roof pitch describes how steep a roof is. This calculator gives it as a percentage slope, which is the vertical rise divided by the horizontal run:
Slope (%) = rise ÷ run × 100
Rise and run just need to be in the same unit, such as both millimetres or both metres. The run is measured horizontally, from the outside of the wall to the point directly below the ridge. It is not the length along the rafter.
A gable roof rises 1.2 m from wall plate to ridge over a horizontal run of 4.8 m (half of a 9.6 m span).
Roofing products in Australia are usually specified in degrees, while US plans often use "rise in 12". To convert:
| Slope % | Degrees | Ratio (in 12) |
|---|---|---|
| 8.7% | 5° | 1.05 in 12 |
| 17.6% | 10° | 2.1 in 12 |
| 25% | 14° | 3 in 12 |
| 41.4% | 22.5° | 5 in 12 |
| 57.7% | 30° | 6.9 in 12 |
| 100% | 45° | 12 in 12 |
Every roofing material has a minimum pitch below which the maker won't guarantee it against leaks. Some metal sheet profiles are rated down to 1–3°. Concrete and terracotta tiles generally need much steeper roofs, often 15° or more, depending on the tile and the laps. Always check the manufacturer's installation guide before choosing a pitch. Pitch also drives rafter length, the roof area you have to buy, and how easily (and safely) the roof can be worked on.
From inside the roof space, hold a level horizontally against a rafter. Measure 1,000 mm along the level, then measure straight up to the underside of the rafter. That vertical measurement ÷ 1,000 × 100 is the slope in percent. It's much safer than climbing on the roof.
Pitched tiled roofs on houses are commonly around 20–25°. Skillion and metal roofs are often much lower, around 3–10°.
No. A 25% slope is only about 14°, while a 25° roof is about a 46.6% slope. Mixing the two up is a common and costly error.
Rafter length = √(rise² + run²), plus any eave overhang. For the example, √(1.2² + 4.8²) ≈ 4.95 m before the overhang.