Short answer: the boxes you buy cover more than your floor for two different reasons. One is the allowance you chose for cuts and mistakes. The other is that boxes come in fixed sizes, so the last box is rounded up. Keep them apart, because only the first one is a decision.
Three numbers to keep
| Number | Example | Where it comes from |
|---|---|---|
| Floor you measured | 120 sq ft | 12 ft × 10 ft |
| With your allowance | 132 sq ft | 120 × 1.10 (10% is your choice, not a rule) |
| Coverage you buy | 141.84 sq ft | 6 boxes × 23.64 sq ft per box |
The difference between the last two lines (9.84 sq ft) is whole-box rounding. The difference between the first and last (21.84 sq ft) is everything left over. The calculator shows both lines so you can see which part you controlled.
Changing the allowance does not always change the boxes
| Allowance | Area with allowance | Boxes | Coverage bought | Of which whole-box rounding |
|---|---|---|---|---|
| 0% | 120.00 | 6 | 141.84 | 21.84 |
| 5% | 126.00 | 6 | 141.84 | 15.84 |
| 10% | 132.00 | 6 | 141.84 | 9.84 |
| 15% | 138.00 | 6 | 141.84 | 3.84 |
For this room and this box size, any allowance from 0% to 15% still lands on 6 boxes; the rounding absorbs it. The allowance only changes how much of the surplus is “yours”. With a different room or box size the boundary moves, so run your own numbers before deciding.
Where 23.64 comes from
It is the “Sq. Ft. Per Carton” printed for one real 7 in × 48 in luxury vinyl plank (Shaw Distinction Plus, style 2045V-07066), used here only to show the arithmetic. Other products pack different areas per box; copy the figure from the box you will buy. That manufacturer’s own calculator adds 15% for installation errors, which is their default, not this site’s recommendation.
Questions people ask
Which allowance should I use?
This site does not recommend one. Manufacturer and installer guidance differs by product and layout (straight lay, diagonal, herringbone need different amounts). Choose it with the product instructions, then use the table above to see whether it changes the box count.
Does the leftover mean I bought too much?
Not necessarily. Whole-box rounding is unavoidable, and a spare box of the same dye lot is often useful for repairs.
L-shaped room?
If the same product goes everywhere, add the areas first and run the total once, because rounding up twice can cost a box. Otherwise calculate each rectangle separately and add the boxes. Do not overlap the rectangles.
Floors worked through by the carton
Three worked examples on whole-carton rounding.
- A 12-by-15 living room, 4 cartons or 11 — the same floor from the same manufacturer, decided by the square feet printed on the box.
- An L-shaped room, split in two — splitting the floor into two rectangles is the easy part; adding the waste allowance twice instead of once costs a carton.
- Closet plus hallway: one order or two — added together and bought once they take 2 to 4 cartons; bought separately, two of the six cartons cost an extra box.
Numbers in the tables were calculated by hand and checked against the site’s calculation code and its automated tests on 2026-09-08. They describe the examples shown, not any specific product.
Sources: Shaw Floors product specification. Arithmetic: see how Wall Math calculates.