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
- All wall faces are treated as rectangles before openings are subtracted.
- The cutting allowance covers offcuts but not structural or fire-rating requirements.
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