Flooring Underlayment Calculator

How much underlayment do I need?

This flooring underlayment calculator divides your room area by 100 square feet, the coverage of one standard roll, and rounds up: rolls = (length × width) ÷ 100, rounded up. A 15 × 12 ft room is 180 sq ft, which is 1.8 rolls, so you buy 2 rolls. At $30 each that is $60.

Enter the room length and width in feet and the price of one roll. The result shows whole rolls and the total cost.

The formula in plain words

1. Area = length × width, in square feet.
2. Rolls = area ÷ 100.
3. Round up to a whole roll. You cannot buy 0.8 of one.
4. Cost = rolls × price per roll.

Rounding up is why small rooms seem to waste a lot. A 45 sq ft bathroom still needs one full roll. The leftover can go in the next room.

Worked examples

1. Default: 15 ft × 12 ft room, $30 per roll

• Area: 15 × 12 = 180 sq ft
• Rolls: 180 ÷ 100 = 1.8, rounded up to 2 rolls
• Cost: 2 × $30 = $60.00

2. Open living and dining area: 24 ft × 18 ft, $30 per roll

• Area: 24 × 18 = 432 sq ft
• Rolls: 432 ÷ 100 = 4.32, rounded up to 5 rolls
• Cost: 5 × $30 = $150.00

Doing several rooms? Add the areas first, then divide. Three 12 × 11 ft bedrooms run one at a time come to 2 + 2 + 2 = 6 rolls. Added together they are 396 sq ft, which is only 4 rolls, because offcuts carry from room to room.

Underlayment types and where they go

Type	Typical use	Notes
Basic foam	Laminate over wood subfloors	Cushions and evens minor bumps
Foam with film vapor barrier	Laminate or engineered wood over concrete	Combines cushion and moisture protection
Cork	Floating floors where sound matters	Denser, often sold in smaller rolls
Felt or fiber	Laminate and engineered wood, sound control	Heavier than foam
Thin LVP-rated underlayment	Vinyl plank without an attached pad	Must be approved by the plank maker
None	Attached-pad vinyl plank, glue-down floors	A second layer can void the warranty

Rolls needed by room size

Room size	Area	100 sq ft rolls
5 × 9 ft (bath)	45 sq ft	1
10 × 10 ft	100 sq ft	1
12 × 12 ft	144 sq ft	2
15 × 12 ft	180 sq ft	2
20 × 15 ft	300 sq ft	3
24 × 18 ft	432 sq ft	5

How to measure the room

• Measure wall to wall at the widest points, in feet.
• Include closets that get the same floor.
• For an L-shaped room, split it into two rectangles, add the areas, then enter one rectangle with that total, such as length 1 × width equal to the total.
• Skip cabinets and fixed islands that the floor will not run under.

Common mistakes

• Doubling up under padded plank. Too soft a base can strain the click joints.
• Overlapping seams. Most foam is butted edge to edge and taped, not overlapped. Follow the roll’s instructions.
• Skipping moisture protection on concrete. Check what your flooring maker requires.
• Running it up the wall. Trim at the floor so baseboard sits right.

Related calculators

Count the flooring itself with the vinyl plank calculator, the hardwood flooring calculator, or the general flooring calculator. To price a full laminate job, see the laminate flooring cost calculator.