Divide by 144, not 12 — the area trap
A foot is 12 inches, so it is tempting to divide by 12. But you are converting area, not length, and area squares the factor: 12 × 12 = 144. A 12×12 inch tile is exactly one square foot. Divide by 12 by mistake and every answer comes out 12 times too large.
Length converts by 12, area converts by 144
The same span of inches shrinks far more when it is area. Twelve inches is one foot; but 12 square inches is only a twelfth of a square foot, because the square-foot base (144) is twelve times the foot base.
144 inches of length is 12 feet, but 144 square inches is just one square foot — the same number, a very different result.
Common sizes at a glance
| Square inches | Square feet | What it is |
|---|---|---|
| 144 | 1.00 | 12×12 in tile |
| 216 | 1.50 | 12×18 in tile |
| 288 | 2.00 | 12×24 in tile |
| 324 | 2.25 | 18×18 in tile |
| 93.5 | 0.649 | US Letter sheet (8.5×11) |
| 500 | 3.472 | — |
| 1,000 | 6.944 | — |
For tiles you can skip square inches entirely: divide each side by 12, then multiply. An 18×18 in tile is (18÷12)² = 1.5² = 2.25 ft².
Common questions
How many square inches are in a square foot?
Exactly 144. A foot is 12 inches, and area squares that: 12 times 12 is 144. So you divide square inches by 144 to get square feet.
Why divide by 144 and not by 12?
Twelve converts a length (inches to feet). Area is two-dimensional, so you square the factor. Dividing by 12 instead of 144 leaves your answer 12 times too big.
How do I convert a tile size to square feet?
Multiply the tile length by its width in inches to get square inches, then divide by 144. An 18 by 18 inch tile is 324 square inches, which is 2.25 square feet.
