Underlayment Calculator

How it works

The calculator finds the floor area, adds your extra allowance and divides by the area one roll actually covers. If the seams overlap, the overlap is taken off the width of each roll first.

Floor area = Length × Width  or  entered directly
Area incl. extra = Floor area × (1 + Extra ÷ 100)
Usable coverage per roll = Coverage per roll × (1 − Seam overlap ÷ Roll width)
Rolls = ⌈ Area incl. extra ÷ Usable coverage per roll ⌉
Area you buy = Rolls × Coverage per roll

⌈ ⌉ means rounding up to the next whole roll. Rounding happens only at the end, so a room that needs 1.8 rolls is shown as 2 rolls, not rounded at each step.

What underlayment does under a floating floor

Laminate, many vinyl plank products and floating engineered wood are not glued or nailed down; the planks lock to each other and rest on the subfloor. Underlayment is the thin layer of foam, felt, fiber or cork between the two. Depending on the product it cushions the floor, evens out small subfloor imperfections, reduces noise and can include a vapor barrier for concrete subfloors. Which of those you need – and whether the flooring allows a separate underlayment at all – is set by the flooring manufacturer, so read the installation instructions for your floor before you buy.

This calculator doesn’t choose a product. It answers the practical question: how many rolls of the underlayment you picked will cover your floor.

How to measure

Measure the room wall to wall at floor level. For a rectangle, enter length and width. For an L-shaped room, an open plan or several rooms done at once, split the floor into rectangles, add their areas and switch to “Known floor area”. Include closets, hallways and the floor under doorways if the new floor continues there – the underlayment goes everywhere the floating floor goes.

Use the area of the floor itself, not the area on your flooring order. A flooring order usually already includes a waste allowance for cut planks, and adding underlayment waste on top of that double counts.

Reading the roll label

Underlayment labels usually give the roll as width × length and the coverage in square feet or square meters. MP Global, for example, lists QuietWalk in 3 ft × 33.4 ft rolls (100 sq ft) and 6 ft × 60 ft rolls (360 sq ft); Quick-Step lists its Uniclic underlay at 15 m² per roll. Those figures are the example values here. Replace them with your product’s figure, because roll sizes differ between brands and even between versions of the same product.

If you compare prices, compare per square foot or per square meter – a larger roll often costs more but less per unit of area. The price field accepts a price per roll or per unit of area.

Butted or overlapped seams

This is the detail that changes the count. Some underlayments are butted edge to edge and taped; the QuietWalk spec sheet, for example, says to roll the underlayment out vapor barrier side up and “butt seams together”. Other products have a film flap that laps over the next strip, or instructions that call for an overlap. Every inch of overlap is an inch of roll width that doesn’t add floor coverage.

If your product overlaps, open More options and enter the overlap and the roll width. A 3 ft wide roll with a 6 in overlap covers only five sixths of its label area: a 100 sq ft roll covers about 83.3 sq ft of floor. On a 300 sq ft floor that is 4 rolls instead of 3. The calculator takes the overlap off the width of every roll; it doesn’t add anything for overlaps where one strip ends and the next begins, so add a little extra if your product requires those.

Extra allowance

Each strip is cut to length at the walls and trimmed around door casings, pipes and cabinets. Short leftovers can often start the next strip, but not always. The calculator starts at 0 % so you can see the true minimum; add an allowance that suits your room – more for rooms with many corners and obstacles, less for a plain rectangle. The extra is applied to the floor area before the rolls are counted, then the count is rounded up.

Rounding and overlap in numbers

A 15 × 12 ft room has 180 sq ft of floor. With 100 sq ft rolls and butted seams, 180 ÷ 100 = 1.8, so you buy 2 rolls and get 200 sq ft of underlayment. The extra 20 sq ft is what rounding up to a whole roll leaves over, and it gives some room for cuts. If the same room had a product that overlapped 6 in on a 3 ft roll, each roll would cover about 83.3 sq ft, and 180 ÷ 83.3 = 2.16 – you’d need 3 rolls.

Edges, gaps and finishing

Floating floors need an expansion gap at the walls. Pergo’s laminate installation guide, for example, says expansion gaps are mandatory on the entire perimeter of the floor. How the underlayment is cut at that edge depends on the product – the QuietWalk spec sheet says its perimeter edges need to be 1/2 to 3/4 in from the wall – so follow the instructions rather than running the underlayment up the wall by habit. The gap is later covered by baseboard or by quarter round; the quarter round calculator counts the pieces for that finishing step.

Worked example

Common mistakes

• Using the floor area of the flooring order. That figure usually includes a waste allowance for planks; underlayment needs its own count based on the floor itself.
• Ignoring seam overlap. If the product’s instructions say to overlap strips, every roll covers less than its label figure – enter the overlap and the roll width.
• Mixing up roll sizes. The same underlayment is often sold in more than one roll size, for example QuietWalk in 100 sq ft and 360 sq ft rolls, so check which size the price refers to.
• Forgetting closets, hallways and the floor under doorways where the new floor continues. Underlayment goes everywhere the floating floor goes.
• Buying underlayment for a floor that already has a pad attached without checking the flooring instructions. Whether an extra layer is allowed is up to the flooring manufacturer.

Assumptions and limits

• In room mode, the room is a rectangle; the area is length × width.
• Rolls are counted from total area and rounded up to whole rolls. Individual strips and their offcuts are not planned.
• A seam overlap is taken off the width of every roll. Overlaps at the ends of strips and edges turned up the wall are not included.
• The extra allowance is added to the floor area before rolls are counted.
• Underlayment tape, vapor barrier film and transition strips are not included.

Frequently asked questions

Sources

• QuietWalk acoustical flooring underlayment – product page — MP Global Products. Underlayment for laminate and floating wood: "Available in 3′ x 33.4′ (100 sq ft) or 6′ x 60′ (360 sq ft) Rolls" (US default uses 100 sq ft, 3 ft wide).
• QuietWalk Spec Sheet (June 2025) — MP Global Products. Seams of this product are butted, not overlapped: "Roll out underlayment vapor barrier side up and butt seams together. Perimeter edges need to be 1/2” to 3/4” from the wall."
• Uniclic 15 m² underlay (QSUDLUN15) – product page — Quick-Step (Unilin). Laminate underlay roll: "Height 3 mm M² per roll 15 m²" (metric default uses 15 m²).
• NIST Handbook 44, Appendix C – General Tables of Units of Measurement — National Institute of Standards and Technology. Exact unit definitions: 1 in = 2.54 cm, 1 yd³ = 27 ft³, 1 US gal = 231 in³, 1 lb = 0.45359237 kg.

Method reviewed November 3, 2026