Underlay calculator: how much underlay do I need?
Floor area in strips of your roll width plus 5 % for cuts, rounded up to whole rolls. Underlay strips are butted and taped, never overlapped; a separate vapour barrier film on concrete or screed overlaps and turns up at the walls.
Underlay calculator
Each room is counted in strips of your roll width; the rounding to whole rolls happens once, over all rooms. Roll, pack and film sizes are typical values: enter the ones on your product.
Typical size: check the label of your product
You need
Enter the length and width of a room.
Rolls of underlay by room size
Foam roll of 1 × 15 m on concrete or screed, with vapour barrier film in rolls of 2 × 25 m:
| Room | Underlay rolls | Seam tape | Film rolls (film area) |
|---|---|---|---|
| 3 × 3 m (9 m²) | 1 | 6 m | 1 (12.4 m²) |
| 4 × 3 m (12 m²) | 1 | 9 m | 1 (16.4 m²) |
| 4 × 4 m (16 m²) | 2 | 12 m | 1 (24.6 m²) |
| 5 × 4 m (20 m²) | 2 | 16 m | 1 (24.6 m²) |
| 6 × 4 m (24 m²) | 2 | 20 m | 1 (32.8 m²) |
| 6 × 5 m (30 m²) | 3 | 25 m | 1 (36.6 m²) |
| 8 × 5 m (40 m²) | 3 | 35 m | 1 (48.6 m²) |
Includes 5 % for cuts, counted in the direction that needs more underlay; film with 20 cm overlap and 5 cm turned up at the walls. On a wooden subfloor the film is left out.
Worked example: a 5 × 4 m living room on screed
- Area: 5 × 4 m = 20 m², foam underlay in rolls of 1 × 15 m, vapour barrier film in rolls of 2 × 25 m.
- Rolled out across the planks: 5 strips of 4 m, 20 m of underlay. Of the two directions the one that needs more is counted.
- Plus 5 % for strip ends and door frames: 21 m ÷ 15 m per roll = 1.4, you order 2 rolls.
- The strips are butted, not overlapped: 4 seams, about 16 m of seam tape.
- Film: 3 strips of 4.1 m, each overlapping the next by 20 cm and turned up 5 cm at the walls, 24.6 m² of film; film rolls to order: 1, about 9 m of tape along the overlaps.
Which underlay suits your floor and how to lay it is in the guide how to install floor underlay, and when film belongs under it in how to install a vapour barrier. For the floor on top, the laminate flooring calculator and the vinyl flooring calculator give the packs.
Frequently asked questions
- How much underlay do I need for a 20 m² (215 sq ft) room?
- For a 5 × 4 m room: 20 m² plus 5 % for strip ends and door frames, so 21 m² of underlay, which is 2 rolls of 1 × 15 m and about 16 m of seam tape. A 16 × 13 ft room (208 sq ft) in rolls of 3 × 33.3 ft takes 84 ft of strips with the 5 %, so 3 rolls and about 64 ft of tape. Nothing is added for overlap.
- Do I add extra for overlap?
- Not for the underlay: its strips are butted edge to edge and the seams taped. A separate vapour barrier film is different: each strip overlaps the next by at least 20 cm (8 in) and turns up about 5 cm (2 in) at the walls, so you need clearly more film than floor area, in the 5 × 4 m example 24.6 m² of film for 20 m² of floor.
- When do I need a vapour barrier under the underlay?
- On every mineral subfloor: concrete, cement or anhydrite screed and old tiles hold residual moisture. Use film with a water vapour diffusion resistance of at least 75 m, for example polyethylene from 0.15 mm, unless the underlay has the film attached. Wooden boards, chipboard and joist floors get no film, because the wood has to release moisture.
- Does underlay with an attached vapour barrier need fewer rolls?
- No, the same number: both kinds are butted, so the roll count follows the strips. You save the separate film, but the seams then have to be sealed vapour-tight along their full length with the matching tape, because the tape is the moisture seal. On a wooden subfloor choose an underlay without film.
- My planks already have a pad attached. Do I still need underlay?
- No. Manufacturers do not allow a second soft layer under planks with an attached pad, because the planks give under load and the click joints break or open. On concrete or screed the vapour barrier film is still needed, usually 0.2 mm polyethylene.
Plan the floor that goes on the underlay
Upload your floor plan and the free planner works out the exact packs and waste before you order.
create a layout planLaying rules: our guide to installing underlay, which follows the manufacturers' instructions. Roll, pack and film sizes are typical values, not products.