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How the flooring calculator works
Flooring is sold by the pack covering a fixed area, so the calculation is area, then packs, then round up. A 5 m × 4 m room is 20 m²; add the default 10% for cuts and the last row and you need to cover 22 m². At 2.2 m² per pack that is exactly ten packs — but if your room measures 20.1 m², or the packs you like hold 2.1 m², the same job jumps to eleven. The allowance is what stops you running out one row from the door.
The spare figure next to the pack count matters more than it looks. Ten packs cover 22 m² against a 20 m² room, so you finish with roughly 2 m² of offcuts and unopened boards. Keep them: laminate and LVT are produced in batches with slightly different shades, and a repair two years later needs a matching plank, not a similar one. Leave the spare packs flat and dry, not in a cold garage where the edges will swell.
Laminate and vinyl are not the same job
Laminate and engineered wood are floating floors. They need underlay — foam, fibre or an acoustic board — and over a concrete or screed subfloor they need a damp-proof membrane as well, otherwise moisture wicks up through the slab and the joints swell within months. Underlay is cut and lapped like the flooring, so the calculator asks for the same area plus the waste allowance, then converts it to rolls.
Vinyl, whether it is LVT planks, tiles or sheet, is far less forgiving of a lumpy subfloor, because it is thin and flexible and any bump telegraphs straight through to the surface. Over timber boards the normal fix is 6 mm plywood, screwed down and with the joints staggered and filled. Over concrete it is a levelling compound. That is why the shopping list swaps the membrane line for plywood sheets when you switch to vinyl — the sheet count is worked out from 2440 × 1220 mm boards, rounded up.
The details that decide whether it looks right
Leave a 10 mm expansion gap all the way round the room and cover it with scotia or beading — a floating floor moves, and a floor laid tight to the skirting will buckle in a warm room. The calculator works the beading out from the room perimeter, in 2.4 m lengths, and adds the doorways you will need to trim around.
Underlay should be taped at the joints and lapped rather than butted, and the flooring needs to acclimatise in the room for about 48 hours before it goes down. Lay the boards running along the longest wall so the joints do not line up like a corridor, stagger them by at least 300 mm, and order the flooring and underlay together so the batch and thickness match.
Flooring FAQs
How many packs of laminate do I need for a 5 m × 4 m room?
Ten packs at 2.2 m² per pack: 20 m² plus a 10% waste allowance is 22 m², and 22 ÷ 2.2 = 10 exactly. Check the pack area though — at 2.1 m² per pack you would need eleven, because packs are only sold whole.
How much underlay do I need?
Roughly the same figure as the flooring, including the waste allowance, because underlay is cut and lapped the same way. That 5 m × 4 m room needs about 22 m², which is two 15 m² rolls with a couple of square metres left over for patching.
Do I need a damp-proof membrane under laminate?
Yes, if the floor is concrete or screed. A DPM goes down before the underlay and stops moisture coming up through the slab and swelling the joints. Over timber boards you do not need one — but you do need to check the boards are flat and screwed down first.
What is different about laying vinyl?
Vinyl is thin, so it shows every imperfection in the subfloor. Over boards that means 6 mm plywood screwed down and joint-filled; over concrete a levelling compound. It also wants a thin, dense underlay rather than a thick foam one — a squashy underlay under LVT makes the joints work loose.
How much waste should I allow for a cut room?
10% is the standard allowance, and it is usually enough. Go to 15% if the room has more than two doorways, has awkward chimney breasts or alcoves, or if you are laying diagonally. Add the offcuts to the spare rather than throwing them away — the last row you cut is usually the first place a repair is needed.
How do I work out the scotia or beading?
Measure the room perimeter — twice the length plus twice the width — and divide by the length of each piece you are buying, typically 2.4 m. For a 5 m × 4 m room that is 18 m of skirting line, so eight lengths, plus a little extra at each doorway threshold.