Add 10% waste for straight layouts and 15% or more for diagonal or herringbone patterns.
Waste allowance
Add 10% for straight-set tile and 15% for diagonal layouts or large-format tile. Breakage and cuts around toilets, vents and corners use more than you expect, and matching the same dye lot later is hard.
Worked example
A 60 sq ft bathroom floor with 12 × 12 in tiles needs 60 tiles; with 10% waste, 66. At 10 tiles per box, buy 7 boxes. Grout coverage depends on joint width and tile size, so check the grout bag's chart.
Frequently asked questions
How much extra tile should I buy?
10% for straight lays; 15%+ for diagonal.
How do I convert tile size to square feet?
Multiply the length and width in inches and divide by 144: a 12 × 24 in tile is 2 sq ft.
Should I include the grout lines?
Not for estimating; the waste allowance covers it.