The formula
Room area = length × width. Tile face area = the tile's own length × width (converted to the same units as the room). Tiles needed = room area ÷ tile face area, with a wastage allowance added on top for cuts at edges, corners, and breakage during handling and installation — commonly 10%, sometimes higher for rooms with many corners or diagonal laying patterns.
A worked example
A 4m × 3m room (12 m²) tiled with 60×60cm tiles, at 10% wastage: needs 37 tiles. The exact same 12 m² room tiled with smaller 30×30cm tiles instead, same wastage: needs 147 tiles — almost exactly 4 times as many, even though the tiles are only half the length and width of the larger ones.
Why halving the tile size roughly quadruples the count
A tile's face area scales with the square of its side length — a 30×30cm tile has a face area of 0.09 m², exactly a quarter of a 60×60cm tile's 0.36 m² face area. Since tile count is inversely proportional to face area, a tile with a quarter of the face area needs roughly four times as many pieces to cover the identical floor. This "smaller tile, disproportionately more pieces" relationship surprises people who intuitively expect a "half-size tile, double the pieces" outcome instead.
Why this matters for both cost and labor estimation
More pieces means more individual cuts, more grout lines, and generally more installation labor time — smaller tiles aren't just a per-unit cost consideration but a labor-time one as well, since laying nearly 4x as many pieces for the identical floor area takes meaningfully longer even at the same skill level. This is a genuine, often underestimated cost factor when choosing a smaller tile size purely for its aesthetic, without factoring in the installation-time consequence.
Why the wastage percentage itself often needs to be higher for smaller tiles or complex layouts
Smaller tiles generally need proportionally more cuts at room edges relative to their own size (a cut edge piece "wastes" a larger fraction of a small tile than of a large one), and diagonal or patterned layouts increase waste further regardless of tile size. A 10% wastage default is a reasonable general estimate for straightforward rectangular layouts with mid-sized tiles, but should be increased for smaller tiles, irregular rooms, or non-standard laying patterns.