Roof Pitch Calculator — Degrees, Ratio & Percentage

Our roof pitch calculator determines the angle of your roof slope from either rise and run measurements, or from the total span and ridge height. It displays the result in degrees, as a ratio and as a percentage gradient.

The formula

Pitch (degrees) = arctan(Rise ÷ Run)
Pitch (%) = (Rise ÷ Run) × 100
Pitch ratio = (Rise ÷ Run) × 12 : 12

If using span and ridge height: Run = Span ÷ 2, then apply the same formula.

UK roofing normally states pitch in degrees. The X:12 ratio (rise per 12 units of run) is a US/imperial convention — we show it for cross-reference.

Common UK pitch ranges

• Under 10°: Very low pitch — flat roof membrane required (EPDM, GRP, built-up felt)
• 10-15°: Low pitch — specialist low-pitch tiles or metal sheeting only
• 15-22.5°: Low-moderate pitch — interlocking concrete tiles, some large-format slates
• 22.5-30°: Moderate pitch — most interlocking tiles and slates
• 30-45°: Standard UK pitch — all tile types suitable
• 45-60°: Steep pitch — traditional look, excellent weather protection

The calculator also shows the rafter length (the sloping distance from eaves to ridge), which is useful for ordering materials like battens and felt.