Should I buy an extra box beyond the waste allowance?

For a floor you expect to live with for years, yes. Waste covers the cuts you can predict; an extra box covers the plank the delivery driver drops and the range being discontinued before you notice a damaged board.

Does the waste allowance change with plank length?

Yes, though not dramatically. Long planks produce longer offcuts that are harder to use up at the ends of rows, so a long-plank floor in a narrow room wastes more than the same area in a wide one.

Do I measure under the doorways?

Measure the finished floor area including thresholds you intend to cover. If the flooring stops at the door line, stop there too — but door reveals and alcoves are easy to forget and each one costs a plank.

Home & construction

Flooring Calculator

Enter the room area and the coverage printed on the box to see how many boxes to buy.

The link carries your figures, so it reopens on exactly these numbers.

What each waste allowance adds

Flooring is sold in whole boxes, so the allowance moves in steps rather than smoothly — sometimes 5% and 10% buy exactly the same delivery.

Waste allowance	Boxes to buy	Area including waste (ft²)	Spare coverage left (ft²)
None	14	280	0
5%A plain rectangle laid parallel to the walls.	15	294	20
10%The usual allowance.	16	308	40
15%Diagonal runs, or an awkward outline.	17	322	60
20%	17	336	60

The formula

area with waste = area × (1 + waste ÷ 100)

boxes = ⌈ area with waste ÷ coverage per box ⌉

spare = boxes × coverage per box − area

Worked example

280 ft² room, boxes covering 20 ft², 10% waste

280 × 1.10 = 308 ft²
308 ÷ 20 = 15.4 → 16 boxes
16 × 20 = 320 ft² covered
320 − 280 = 40 ft² spare

Where this goes wrong

Trusting a coverage figure you did not read off the box

Box coverage varies widely between ranges — 20 ft² and 24 ft² boxes sit side by side in the same aisle. Assume 24 when yours is 20 and a 308 ft² job comes out at 13 boxes instead of 16, three boxes short. This calculator asks you for the number rather than guessing it.

Questions

Should I buy an extra box beyond the waste allowance?: For a floor you expect to live with for years, yes. Waste covers the cuts you can predict; an extra box covers the plank the delivery driver drops and the range being discontinued before you notice a damaged board.
Does the waste allowance change with plank length?: Yes, though not dramatically. Long planks produce longer offcuts that are harder to use up at the ends of rows, so a long-plank floor in a narrow room wastes more than the same area in a wide one.
Do I measure under the doorways?: Measure the finished floor area including thresholds you intend to cover. If the flooring stops at the door line, stop there too — but door reveals and alcoves are easy to forget and each one costs a plank.