Underlayment Calculator
Turn your room square footage into the number of underlayment rolls to buy — the cushioning, sound-damping layer that goes under a floating laminate, engineered or luxury-vinyl floor.
Calculator
Covering 200 sq ft with 100 sq ft rolls needs about 2 rolls. Underlayment cushions a floating floor and evens minor subfloor differences. Confirm the roll coverage on your product’s spec sheet — it varies by brand and thickness.
Underlayment is the thin resilient layer rolled out between the subfloor and a floating floor (click-lock laminate, engineered wood or many luxury-vinyl planks). It softens footfall, quiets the room, smooths out minor subfloor imperfections and — in a foam-plus-film product — adds a light moisture break. Nail-down and full-glue floors usually skip it, so check what your specific floor calls for before buying.
Because underlayment is sold by the roll, the only real question is how many rolls cover the room. That is a pure division rounded up to a whole roll. Buy a little past the exact number: you lose some coverage to seam overlaps, wall trimming and the odd damaged end. For the coverage typicals used here see the underlayment, leveler & vapor coverage table, and for the wider workflow the guide Underlayment & vapor barrier: what & how much.
Formula
rolls = ceil( area_sqft ÷ roll_coverage_sqft )
- area_sqft — the floor area you are covering, in square feet.
- roll_coverage_sqft — how many square feet one roll covers (from the label; ~100 sq ft is common).
- ceil() — round up, because you buy whole rolls.
The result is a purchase quantity, not a cut list — it does not subtract the small waste from overlapping seams or trimming to the walls, so treat it as the minimum and round up if you are close to a boundary.
Worked example
A 200 sq ft bedroom, underlayment rated at 100 sq ft per roll:
ceil(200 ÷ 100) = ceil(2.0) = 2 rolls
Two rolls land exactly on the area, so there is no slack for overlap or trimming. If the label lists a required overlap at the seams, or your room is a hair over 200 sq ft, buy a third roll — the leftover is cheap insurance against stopping the job one strip short.
Underlayment in practice
Match the underlayment to the floor and the subfloor. Over a concrete slab, use a product with a built-in moisture film or add a separate vapor barrier (see the moisture & vapor-barrier coverage calculator); over a wood subfloor, a plain foam or felt is usually enough. Some luxury-vinyl planks arrive with padding pre-attached — adding a second layer under those can void the warranty, so read the install sheet first.
Coverage varies by brand and thickness. A dense 2 mm foam roll and a thick cork roll cover very different square footages, so the 100 sq ft default is only a starting point — overwrite it with the number printed on your roll. Underlayment does not fix a bad subfloor: it evens out minor unevenness, not dips or humps. Flatten those first with a self-leveling compound or by sanding high spots.
Overlap and tape the seams. Butt or overlap the edges exactly as the maker specifies and tape them so the sheet stays put while you lay the floor. That small overlap is why buying a touch extra beats buying exactly to the calculated number.
Frequently asked questions
How many rolls of underlayment do I need for 200 sq ft?
With a roll rated at 100 sq ft, ceil(200 ÷ 100) = 2 rolls. Because that lands exactly on the area with no slack for seam overlaps or trimming, many installers round up to 3 for a room right at 200 sq ft.
Do I need underlayment under vinyl plank?
It depends on the plank. Many luxury-vinyl (LVP) products already have padding attached — adding another layer can void the warranty. Bare click-vinyl over concrete usually wants a thin vinyl-rated underlayment with a moisture film. Always follow the plank's install sheet.
What roll coverage should I enter?
Use the square footage printed on your product's label, not a guess. Foam rolls often cover ~100 sq ft, but cork, felt and combination foam-plus-film rolls differ widely. The 100 sq ft default is only a placeholder to overwrite.
Does the calculator subtract seam overlap?
No. It returns whole rolls to cover the bare area. Seam overlaps and wall trimming eat into real coverage, so if the result lands exactly on your area, buy one more roll.
Is underlayment the same as a vapor barrier?
Not always. Some underlayments include a moisture film that doubles as a vapor break; plain foam or felt does not. Over concrete or below grade you generally want a dedicated barrier — size it with the moisture & vapor-barrier coverage calculator.
Can underlayment level an uneven floor?
No. It smooths only minor surface texture. Real dips or humps need a self-leveling compound or spot-sanding first — a floating floor over an uneven subfloor will flex, gap and creak.