How the underlayment calculator works
Underlay goes down in one continuous layer, edge to edge, under laminate, engineered wood or luxury vinyl. It is not overlapped, so the quantity is simply the floor area plus an allowance for offcuts at walls, door frames and alcoves. That area is then divided by what one roll or board covers.
Rolls or boards = length × width × (1 + waste %) ÷ coverage per roll or boardThe vapour barrier is different: polyethylene film is laid with overlaps of about 200 mm (8 in) at every seam and turned up the walls by about 50 mm (2 in), then trimmed after the skirting goes on. The calculator adds 15% to the floor area to account for both. Imperial rooms are converted with 1 ft² = 0.0929 m² so that metric and US product sizes can be mixed.
Worked example
A 5 × 4 m bedroom with a concrete slab gets laminate. The floor is 20 m²; with 10% waste you need 22 m² of underlay, so three 10 m² rolls (two would leave you 2 m² short). Because the slab can release moisture, a 0.2 mm polyethylene film goes underneath: 20 × 1.15 = 23 m², which is a single 25 m² roll of film. Switch to wood-fibre boards of 790 × 590 mm and the same 22 m² takes 22 ÷ 0.466 = 47.2, so 48 boards. In the US, a 16 × 13 ft room (208 ft²) at 10% needs 229 ft², three 100 ft² rolls.
Common underlay types
| Type | Typical format | Thickness | Best under |
|---|---|---|---|
| Polyethylene foam | Rolls of 10–15 m² or 100 ft² | 2–3 mm | Budget laminate on level floors |
| Foam with film attached | Rolls of 10 m² or 100 ft² | 2–3 mm | Laminate over concrete, no separate film |
| Wood-fibre boards | 790 × 590 mm, packs of about 7 m² | 5–7 mm | Evening out small dips, warmth, sound |
| Cork | Rolls of 10 m² or 100–200 ft² | 2–6 mm | Sound reduction, engineered wood |
| Dense acoustic LVT underlay | Rolls of 10 m² or 100 ft² | 1–1.5 mm | Click vinyl, where thicker pads flex the joints |
| Plywood or hardboard panels | 4 × 8 ft or 1220 × 2440 mm | 5–6 mm (1/4 in) | Sheet vinyl and tile on uneven timber subfloors |
Mistakes that cost a floor
- Doubling up moisture films. If the underlay already has a film, do not add another; two films can trap water between them.
- Using a thick pad under click vinyl. Most rigid-core planks allow only 1–1.5 mm of dense underlay. Anything softer lets the locking joints flex and break. Check the flooring instructions.
- Expecting underlay to fix a bad subfloor. It absorbs dips of a millimetre or two, not a slope. Level hollows over about 3 mm per 2 m first.
- Taping too little. Tape every seam of foam and film so the layer cannot shift while you lay the floor.
- Stacking layers on underfloor heating. Each layer adds thermal resistance; choose an underlay rated for heated floors.
Frequently asked questions
How much underlayment do I need for a 12x12 room?
A 12 × 12 ft room is 144 ft²; with 10% waste you need about 158 ft², which is two 100 ft² rolls of underlayment.
How many square metres are in a roll of underlay?
Most laminate underlay rolls hold 10 or 15 m² in Europe and 100 ft² (9.3 m²) in North America; the exact figure is printed on the roll.
Do I need a vapor barrier under laminate?
Over concrete, screed or a ground-floor slab, yes: a 0.2 mm (6 mil) polyethylene film or an underlay with built-in film. Over a timber subfloor a separate film is normally not used.
Should underlayment seams overlap?
No, foam and fibre underlay is butted edge to edge and taped. Only the separate plastic vapour barrier overlaps, by about 200 mm (8 in).
How many underlay boards per square metre?
Boards of 790 × 590 mm cover 0.466 m² each, so you need about 2.15 boards per m², or 2.4 with 10% waste.
Last reviewed: 2026-09-27 · How we calculate