Underlayment calculator: how much underlayment do I need?
Floor area in strips of your roll width plus 5 % for cuts, rounded up to whole rolls. Underlayment strips are butted and taped, never overlapped; a separate vapor barrier film on concrete or screed overlaps and turns up at the walls.
Underlayment 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 underlayment by room size
Foam roll of 3 × 33.3 ft on concrete or screed, with vapor barrier film in rolls of 10 × 25 ft:
| Room | Underlayment rolls | Seam tape | Film rolls (film area) |
|---|---|---|---|
| 10 × 10 ft (100 sq ft) | 2 | 30 ft | 1 (206.6 sq ft) |
| 12 × 10 ft (120 sq ft) | 2 | 36 ft | 1 (206.6 sq ft) |
| 14 × 12 ft (168 sq ft) | 2 | 48 ft | 1 (246.6 sq ft) |
| 16 × 13 ft (208 sq ft) | 3 | 64 ft | 2 (266.6 sq ft) |
| 20 × 12 ft (240 sq ft) | 3 | 72 ft | 2 (369.8 sq ft) |
| 20 × 15 ft (300 sq ft) | 4 | 90 ft | 2 (406.6 sq ft) |
| 25 × 16 ft (400 sq ft) | 5 | 125 ft | 2 (489.8 sq ft) |
Includes 5 % for cuts, counted in the direction that needs more underlayment; film with 8 in overlap and 2 in turned up at the walls. On a wooden subfloor the film is left out.
Worked example: a 16 × 13 ft living room on screed
- Area: 16 × 13 ft = 208 sq ft, foam underlayment in rolls of 3 × 33.3 ft, vapor barrier film in rolls of 10 × 25 ft.
- Rolled out across the planks: 5 strips of 16 ft, 80 ft of underlayment. Of the two directions the one that needs more is counted.
- Plus 5 % for strip ends and door frames: 84 ft ÷ 33.3 ft per roll = 2.52, you order 3 rolls.
- The strips are butted, not overlapped: 4 seams, about 64 ft of seam tape.
- Film: 2 strips of 13.3 ft, each overlapping the next by 8 in and turned up 2 in at the walls, 266.6 sq ft of film; film rolls to order: 2, about 14 ft of tape along the overlaps.
Which underlayment suits your floor and how to lay it is in the guide how to install floor underlayment, and when film belongs under it in how to install a vapor barrier. For the floor on top, the laminate flooring calculator and the vinyl flooring calculator give the packs.
Frequently asked questions
- How much underlayment 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 underlayment, 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 underlayment: its strips are butted edge to edge and the seams taped. A separate vapor 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 vapor barrier under the underlayment?
- On every mineral subfloor: concrete, cement or anhydrite screed and old tiles hold residual moisture. Use film with a water vapor diffusion resistance of at least 75 m, for example polyethylene from 0.15 mm, unless the underlayment has the film attached. Wooden boards, chipboard and joist floors get no film, because the wood has to release moisture.
- Does underlayment with an attached vapor 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 vapor-tight along their full length with the matching tape, because the tape is the moisture seal. On a wooden subfloor choose an underlayment without film.
- My planks already have a pad attached. Do I still need underlayment?
- 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 vapor barrier film is still needed, usually 0.2 mm polyethylene.
Plan the floor that goes on the underlayment
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 underlayment, which follows the manufacturers' instructions. Roll, pack and film sizes are typical values, not products.