MortarDesk
ceil(132 sq ft ÷ 20 sq ft per unit) = 7
Sources: NIST SP 811, Appendix B.9 (1 ft = 0.3048 m and 1 sq ft = 0.092 903 04 sq m, both exact) · NIST Handbook 44, Appendix C (1 sq yd = 9 sq ft; 1 acre = 43,560 sq ft). Area formulas: rectangle l×w, circle π(d/2)², triangle ½×base×height.
How It Works
The calculator multiplies the dimensions you enter into an area using the standard formula for the shape: length × width for a rectangle, the sum of two rectangles for an L-shape, π × (diameter ÷ 2)² for a circle and ½ × base × height for a triangle. Feet and inches are combined first (inches ÷ 12), so 12’-6” is treated as 12.5 ft.
Conversions use the exact definitions in NIST SP 811: 1 ft = 0.3048 m, so 1 sq ft = 0.092 903 04 sq m. Square yards and acres follow NIST Handbook 44 Appendix C: 1 sq yd = 9 sq ft and 1 acre = 43,560 sq ft.
The order count applies your waste percentage to the area and divides by the coverage printed on the box, rounding up to a whole unit: ceil(area × (1 + waste ÷ 100) ÷ coverage). Coverage is read in the same unit system as the dimensions. Waste allowances are your call; our waste factors guide covers the usual starting points.
Worked Example
A 12 ft × 10 ft room is 120 sq ft. In metric that is 120 × 0.09290304 = 11.148 sq m, or 13.33 sq yd, or 0.00275 acres. With 10% waste the area to buy for is 132 sq ft; if a box covers 20 sq ft, ceil(132 ÷ 20) = 7 boxes.
FAQ
How do I measure an L-shaped room?
Split it into two rectangles at the inside corner, measure each, and enter both pairs. The calculator adds the two areas; overlaps are avoided as long as the two rectangles do not share floor.
Why is the metric result not exactly a tenth of the imperial one?
Because a square foot is 0.09290304 sq m, not 0.1. The factor comes from the international foot, defined as exactly 0.3048 m.
Is the acre figure exact?
Yes for the arithmetic: an acre is defined as 43,560 sq ft in NIST Handbook 44. Handbook 44 notes that the U.S. survey foot was retired at the end of 2022, so acres here use the international foot.
What waste percentage should I use?
Ten percent is the common starting point for simple rectangular layouts and about fifteen for diagonal or patterned work. The right figure depends on the layout in front of you, so treat it as an allowance rather than a rule.
Can I share a calculation?
Yes. Copy link writes your inputs into the page address; anyone opening it sees the same result.
Sources
- NIST Guide to the SI, Appendix B.9 (SP 811) - exact factors: 1 ft = 0.3048 m, 1 sq ft = 0.09290304 sq m
- NIST Handbook 44 (2026), Appendix C, General Tables of Units of Measurement - 1 sq yd = 9 sq ft, 1 acre = 43,560 sq ft, retirement of the U.S. survey foot