A 5 by 4.5 metre room is 22.5 square metres. At 2.2 square metres a box, that is 10.2 boxes, so you buy 11. Add a 10 per cent wastage allowance and the requirement becomes 24.75 square metres, which is 11.25 boxes, so you buy 12.
The area never changed. The order went up by a box because of two separate effects that both round in the same direction, and neither of them appears in the measurement.
Wastage is not contingency
Wastage is not padding for mistakes. It is the material that physically cannot end up on the floor: the offcut left when a board is cut to length, the part of a tile lost to the wall line, the extra needed to keep a pattern aligned across a joint.
That is why the allowance varies with the job rather than the material. A plain rectangular room with no pattern might need five per cent. A room with alcoves, angles and a repeating pattern can need considerably more, because every cut creates a piece too small to use elsewhere.
Everything is sold in whole units
The second rounding is the packaging. Flooring comes in boxes, paint in tins, concrete in cubic metres or bags. You cannot order 11.25 boxes.
Paint shows this clearly. A standard 5 by 4 metre room with a 2.7 metre ceiling, two doors and two windows, at two coats and 12 square metres a litre, needs 7 litres. Tins do not come in sevens, so the paint calculator reports 2 by 4 litre tins: 8 litres bought to apply 7.
The leftover is not waste in the same sense as an offcut. It is the difference between what the job needs and what the shelf sells, and on a small job it can be a substantial share of the cost.
Order once
The practical reason to allow properly rather than trim the order is dye lots and batch variation. Flooring, tiles and paint are manufactured in batches, and a second order placed a fortnight later can be a visibly different shade. Running short is not a matter of one more trip; it can mean a visible line across the room.
The same logic applies to concrete for a different reason. A 4 by 3 metre slab at 100 mm is 1.2 cubic metres net, and the concrete calculator adds a 10 per cent allowance to give 1.32. A pour that runs out partway through does not resume cleanly, because the first batch has already started curing.
What to check before ordering
Take the coverage figure from the actual product rather than a generic one, since it varies between ranges and is printed on the packaging. Confirm whether a quoted coverage already includes an allowance, because some manufacturers state a realistic figure and others state a laboratory one. Then round up, and expect to have some left over.
The flooring calculator, paint calculator and decking calculator all take the coverage and the allowance as inputs rather than assuming them, so the figure you get out reflects the product you are actually buying.