Formula and method
The calculator applies the visible measurements in one consistent unit, performs the quantity calculation, and rounds only the final buying quantity where whole packs or bags are required.
Assumptions and limitations
- Enter the actual sloped roof dimensions, not floor footprint.
- Valleys, hips and complex cuts need a larger allowance.
Worked example
The pre-filled values form a complete example. Select Calculate to see the result, then replace each value with your own measurements. Check the result against the chosen product’s coverage, yield or application instructions before buying.
Method reviewed: 27 August 2026