Roof Pitch Calculator
Convert a roof's rise and run into pitch in degrees and as a ratio, and get the rafter length and roof slope factor.
How steep the roof is, and how long the rafters are.
How the roof pitch calculator works
Roof pitch is rise over run. Builders quote it as degrees (arctan of rise ÷ run), as a ratio such as 4:12 (rise per 12 units of run, common in the US) or as a percentage. The rafter length is the hypotenuse, √(rise² + run²), and the slope factor (rafter ÷ run) converts a plan area into a roof surface area for tiles or sheeting.
A 22.5° roof has a slope factor of 1.08 — the roof surface is 8% larger than the floor plan below it.
Formula: pitch = arctan(rise ÷ run) · rafter = √(rise² + run²)
Worked examples
| Inputs | Pitch | Note |
|---|---|---|
| Rise 1 m over run 4 m | 14 ° | 14° — 3:12 |
| Rise 2 m over run 4 m | 26.6 ° | 26.6° — 6:12 |
| 4:12 pitch (rise 4 ft over 12 ft) | 18.4 ° | 18.4° |
FAQFrequently asked questions
How do I convert a ratio pitch to degrees?
arctan(rise ÷ run). 4:12 is arctan(4 ÷ 12) = 18.4°; 6:12 is 26.6°; 12:12 is 45°.
What pitch do roof tiles need?
Most tiles need at least 15–22.5° depending on the profile; metal sheeting can go as low as 1–5°. The manufacturer states the minimum.
What is the slope factor for?
Multiply the plan area by it to get the roof surface area for ordering tiles, sheets or sarking. At 30° the factor is 1.155.
Does the rafter length include the overhang?
No — add the eaves overhang along the slope, and the ridge and birdsmouth allowances your framing method needs.
Where these figures come from
- The Engineering ToolBox — material densities, coverage and unit data — bulk densities, concrete at 2,400 kg/m³, board and sheet sizes
- Brick Development Association — brick sizes and quantities — 215 × 102.5 × 65 mm, 60 bricks per m²
- UK Government — Approved Documents (Building Regulations) — Building Regulations guidance
Last checked: September 2026. Coverage rates and material sizes are typical manufacturer figures — check the product you buy.