Roof Area Calculator
Calculate roof area and squares from your roof's dimensions and pitch with this free roof area calculator that runs in your browser.
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Minimum 0
Minimum 0
Before you rely on this: First-pass guide only. Verify safety-critical or regulated work against the relevant standards, your project requirements and a qualified professional.
How to use this calculator
- Enter the footprint area and the roof pitch (rise and run).
- Read the sloped roof area.
How it works
The slope factor is √(rise² + run²) ÷ run. Multiplying the flat footprint by it gives the actual sloped surface area.
Worked example
100 m² footprint, 4:12 pitch. Slope factor ≈ 1.054, so roof area ≈ 105 m².
Common mistakes
- Using the footprint as the roof area — a pitched roof is always larger.
- Forgetting overhangs, hips, valleys and waste when ordering material.
Frequently asked questions
How do I work out roof area?
Take the plan (footprint) area and divide by the cosine of the pitch angle, since a sloped roof covers more than its footprint.
Why is roof area larger than the floor area?
The slope adds length to each rafter, so a pitched roof always has more surface than the flat area beneath it — steeper pitches add more.
What's a roof pitch like 4:12?
It rises 4 units for every 12 of horizontal run — enter 4 as rise and 12 as run.
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Cite this tool
Zerdly. (2026). Roof Area Calculator. Retrieved from https://www.zerdly.com/tools/roof-area-calculator/
Tip: Enter any known values to calculate the remaining results.
All calculations run in your browser. Your inputs are never saved or transmitted.