Underlayment calculator
Buy underlayment by floor area: length × width, plus about 5% for trimming, divided by the roll's coverage and rounded up. A 12 × 15 ft room is 180 sq ft, so with 100 sq ft rolls you need 2. Over concrete, add 6 mil polyethylene with seams overlapped at least 8 in unless the underlayment has its own barrier.
Updated · By the Interior Redesign AI editorial team · Sources cited on the page
Assumptions: a 5% trim allowance, and an example 10 × 25 ft poly roll; enter the sizes printed on your rolls.
Underlayment to buy
2 rolls
189 sq ft with trim allowance, for 180 sq ft of floor.
| Floor area | 180 sq ft |
|---|---|
| To cover, with allowance | 189 sq ft |
| Underlayment rolls | 2 |
- OKButt the pad seams tight and tape them; do not overlap the pad. Keep it 1/2-3/4 in off the walls.
- CheckOn a concrete slab? Turn on the barrier above, unless your underlayment or planks have one attached.
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Key numbers
- Typical foam or felt roll
- 100 sq ft (QuietWalk rolls are cut to a minimum of 100 sq ft)
- Underlayment seams
- butted tight, not overlapped, and taped (QuietWalk instructions)
- Vapor barrier on concrete
- 6 mil polyethylene, seams overlapped 8 in or more (Quick-Step Studio guide)
- Gap at the walls
- keep the pad 1/2-3/4 in off the wall (QuietWalk)
- Trim allowance
- 5% by default (our assumption; raise it for many small rooms or closets)
Worked example
A 12 × 15 ft room over a concrete slab
100 sq ft rolls of underlayment without an attached barrier, 5% trim allowance, poly in 10 ft wide rolls.
- Floor area: 12 × 15 = 180 sq ft; with 5% = 189 sq ft.
- Underlayment: 189 ÷ 100 → 2 rolls.
- Poly: 12 ft is wider than one 10 ft strip, so 2 strips overlapping 8 in, each 15 ft long = 300 sq ft.
- Poly rolls: 300 ÷ (10 × 25) → 2 rolls, with 1 taped seam.
Buy 2 rolls of underlayment and 300 sq ft of 6 mil poly for this slab.
How we calculate
Underlayment pads are laid edge to edge with butted, taped seams, so the area you need is the floor area. We add a trim allowance (5% by default, an assumption) for cut-offs at walls and doorways, divide by the coverage printed on the roll and round up.
On a concrete slab, many floating-floor guides call for a vapor barrier. The Quick-Step Studio laminate guide specifies 6 mil polyethylene with seams overlapped 8 in or more. We lay strips the long way across the room, each new strip overlapping the last by 8 in, and count the poly in square feet and rolls. The 10 × 25 ft poly roll is an example size; enter the one on your roll.
Skip the separate barrier when your underlayment or plank already has one attached, and follow your flooring maker's guide: some forbid a second barrier under their attached pad. Slab moisture limits and test methods also come from the flooring maker.
Estimates, sources
Planning estimates and published guidelines, not quotes, designs or code rulings. Figures were checked on each source on the date shown. Local prices, your home and your jurisdiction's adopted code decide the real answer.
- QuietWalk underlayment installation instructions (MP Global Products, as of Oct 26, 2026): rolls cut to a minimum of 100 sq ft; seams butted together flush, not overlapped, and taped; vapor barrier film side up; edges 1/2-3/4 in from the wall; slab vapor emission over 5 lb per 1,000 sq ft in 24 h needs a sealer or polyethylene sheeting
- Quick-Step Studio laminate flooring installation instructions (Quick-Step (hosted by Lowe's), as of Oct 26, 2026): concrete subfloors: install a 6 mil (0.15 mm) virgin polyethylene vapor barrier with seams overlapped 8 in or more; 3/8 in expansion space on all sides
Underlayment rolls by room size
With 100 sq ft rolls and a 5% trim allowance.
| Room | Floor area | Rolls |
|---|---|---|
| 10 × 10 ft | 100 sq ft | 2 |
| 12 × 12 ft | 144 sq ft | 2 |
| 12 × 15 ft | 180 sq ft | 2 |
| 14 × 18 ft | 252 sq ft | 3 |
| 20 × 20 ft | 400 sq ft | 5 |
| 20 × 30 ft | 600 sq ft | 7 |
Do you need underlayment at all?
Floating laminate and engineered floors usually go over a pad unless the planks come with one attached. Glue-down floors do not use one, and click vinyl makers often limit you to an approved pad or none. Read the installation guide for your exact floor before you buy: it names the allowed underlayments and whether a vapor barrier is required.
How to use this tool
Measure the floor
Length and width of each room; add closets.
Enter the roll
Coverage printed on the roll, usually 100 sq ft.
Check the subfloor
Concrete slab without a barrier on the pad? Turn on the poly barrier.
Read what to buy
Rolls of underlayment, and poly square feet and rolls.
Questions
How much underlayment do I need for 1,000 sq ft?
About 1,050 sq ft with a 5% trim allowance: 11 rolls of 100 sq ft.
Do underlayment seams overlap?
Foam and felt pads are usually butted tight and taped, not overlapped (QuietWalk's instructions say so). A separate poly vapor barrier is overlapped, 8 in or more in the Quick-Step guide.
Do I need a vapor barrier on concrete?
Usually, for floating floors over a slab, unless the underlayment or plank has one attached. Follow your flooring maker's guide; some forbid a second barrier.
Can I put underlayment under vinyl plank?
Only what the vinyl maker approves. Planks with an attached pad often allow nothing extra; the installation guide lists what is allowed.
How far should underlayment go up the wall?
It should not: QuietWalk asks for the edges to stop 1/2-3/4 in from the wall.
What size is a roll of underlayment?
Many foam and felt rolls cover 100 sq ft; some contractor rolls are larger. Enter the coverage printed on your roll.
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