Understanding your result
The headline is the rolls of underlayment, with the floor area, the seams and the tape beside it, and the vapor barrier rolls when you added one. The table shows the allowance, the area needed and bought, the strips and seams, and the tape and barrier counts.
Rolls is the area with its allowance divided by the roll coverage, rounded up. The allowance is 5% for products that butt at the seams, which is most foam and felt, and 10% for products that overlap, such as rosin paper and some film underlayments; choose by the instructions on the roll. Strips and seams come from the room’s shape: the strips run the length of the room, their number is the width divided by the roll width, and the seams are one fewer. The tape is 10% over the seam length, rounded to rolls.
The vapor barrier is a separate 6 mil polyethylene sheet, needed over concrete and nowhere else, counted at 10% over the area for the 6 in overlaps and the turn-up at the walls. If your underlayment has a film bonded to it, you do not need the separate sheet; the notes say which is which. Underlayment goes under floating floors such as the ones the laminate calculator and the vinyl plank calculator count; for the moisture question on its own, the moisture barrier calculator covers barriers under any floor.
How we calculate this
When you give the floor as an area rather than a room, the strips and seams are estimated for a square room of that area, which is a fair middle case. The strips are assumed to run along the longer dimension, which is how installers lay them to keep seams few. The types in the list do not change the arithmetic; they change the defaults you should type and the notes you get, because a 3 mm foam, a 6 mm cork and a roll of rosin paper are bought in very different rolls. The underlayment guide compares them, and the standard sizes reference lists the roll sizes.
The assumptions behind the numbers
| Assumption | Default | Where it comes from |
|---|---|---|
| Roll coverage | 100 sq ft (10 m²), 3.33 ft (1 m) wide | Standard foam underlayment roll (40 in × 30 ft); 200 sq ft rolls are 44 in × 55 ft |
| Allowance | 5% butted, 10% lapped | Manufacturer installation instructions; rosin paper is lapped 4 in |
| Seam tape | 164 ft (50 m) rolls | Standard underlayment seam tape packaging; 100 ft (30 m) rolls are common |
| Vapor barrier | 6 mil polyethylene, 250 sq ft rolls (10 × 25 ft), 6 in overlaps | Flooring maker requirements over concrete; 6 mil is the thickness most specify |
| Rosin paper | 400 to 500 sq ft rolls, 36 in wide | Standard rosin and felt paper packaging |
| Cork | 6 mm, 200 sq ft rolls or 4 sq ft sheets | Cork underlayment product pages |
Assumptions last reviewed October 8, 2026.
The calculator does not judge whether a given underlayment is allowed under a given floor; that is in the flooring warranty, which names acceptable thicknesses and, for radiant floors, a maximum thermal resistance. It does not count the acoustic underlayment some condominium rules require by a sound rating, though the rolls are counted the same way, and it does not model the cove at the wall for a sheet barrier beyond the 10%. The subfloor prep guide explains what an underlayment can and cannot fix in a subfloor.
Two worked examples
A 20 × 15 ft room in 200 sq ft foam rolls
Foam, 200 sq ft rolls 3.67 ft (44 in) wide, 5% allowance, seams taped with 164 ft tape.
- Underlayment: 300 × 1.05 = 315 sq ft; 315 ÷ 200 = 1.6, rounded up to 2 rolls (400 sq ft)
- Strips: 15 ÷ 3.67 = 4.1, rounded up to 5 strips; seams 4 × 20 = 80 ft
- Tape: 80 × 1.1 = 88 ft, one 164 ft roll
Two large rolls, one roll of tape. The 85 sq ft over is the price of large rolls on a mid-sized room; a 200 plus a 100 would fit the 315 sq ft more closely if the store stocks both widths of the same product.
A 240 sq ft basement floor in combined foam-and-film
Area typed directly, foam with a built-in vapor barrier in 100 sq ft rolls, no separate poly, 100 ft tape.
- Underlayment: 240 × 1.05 = 252 sq ft; 252 ÷ 100 = 2.5, rounded up to 3 rolls
- Strips for a 15.5 ft square-equivalent width at 3.33 ft: 5 strips; seams 4 × 15.5 = 62 ft
- Tape: 62 × 1.1 = 68 ft, one 100 ft roll
The combined product does the vapor barrier’s job, so the separate 6 mil sheet is not ticked; the seams of a film-backed underlayment must be taped for the barrier to be continuous, which is why the tape is not optional here.
Where to find your inputs
Roll coverage and width. On the roll’s label: square feet and the roll’s width and length. Foam is 40 or 44 in wide; rosin paper 36 in.
Type. From the flooring’s installation instructions, which name the acceptable underlayments. When in doubt, the flooring maker’s own underlayment is the safe buy.
Allowance. 5% unless the roll’s instructions say to overlap the seams, then 10%.
Vapor barrier. Needed over concrete only. A roll of 6 mil polyethylene is labelled by its dimensions (10 × 25 ft is 250 sq ft).
Common mistakes
- Underlayment under an attached pad. Two cushions and the click joints fail. Barrier only, if on a slab.
- Foam as a vapor barrier. Plain foam passes moisture. Use a film-backed product or a 6 mil sheet.
- Forgetting the tape. Untaped seams let the barrier leak and the strips creep apart. Count the seams.
- Rosin paper on concrete. It is for nailed wood on plywood. Nothing is nailed to a slab.
- Running the strips across the room. More seams, more tape, more trimming. Run them along the longer dimension.
- Thick cushion for comfort. Thickness beyond the warranty limit breaks the floor. Comfort comes from the flooring, not the pad.
Questions people ask
- How many rolls of underlayment for 300 square feet?
- With 5% for trimming, 300 sq ft needs 315 sq ft of underlayment. That is 4 rolls of 100 sq ft or 2 rolls of 200 sq ft; the 200 sq ft rolls leave 85 sq ft over against 85 for the smaller rolls, so the choice is price, and a 100 plus a 200 is the tightest fit if the store sells both.
- How much seam tape do I need?
- One strip of tape per seam, and the seams are one fewer than the strips. A 15 ft wide room covered by 3.67 ft wide rolls takes 5 strips and 4 seams of the room's length. For a 20 ft long room that is 80 ft of seam, so one 164 ft roll or one 100 ft roll with a little spare.
- Which underlayment do I need for laminate?
- A 2 to 3 mm foam or a 3 mm felt, with a vapor barrier if the floor is on concrete; most laminate makers name the acceptable thickness and the maximum compressibility in the warranty. If the planks have an attached pad, no underlayment at all, only the vapor barrier on a slab.
- Do I need a vapor barrier under underlayment?
- Over a concrete slab, on grade or below, yes, either as a 6 mil polyethylene sheet lapped 6 inches and taped, or an underlayment with a film bonded to it. Over a wood subfloor above a dry space, no. Over a crawl space, the barrier belongs on the ground of the crawl space, not under the floor.
- What goes under nailed solid hardwood?
- Rosin paper or 15 lb asphalt-saturated felt, lapped 4 inches, as a slip sheet and dust barrier. It is not a cushion and not a vapor barrier. Rolls are large, 400 to 500 sq ft in 3 ft widths. Choose "rosin or felt paper" and enter your roll.
- Can I use thick cork or rubber under a floating floor?
- Only within the flooring maker's limits. Thick, soft underlayment lets a click floor flex at the joints and they break; 6 mm cork is accepted by many for sound control, rubber by fewer. Check the warranty's maximum thickness and compression before you order.