Underlayment Calculator

How this is calculated

A ROLL DOES NOT COVER WHAT IT SAYS. The figure on the packaging is the nominal area of the material, and real coverage is lower for two ordinary reasons: seams overlap or carry a taped flap, and each run gets trimmed at the wall. Allowing about 5% for that is realistic for a rectangular room and low for a complicated one. THE ARITHMETIC: room area, plus waste, divided by (roll coverage minus the overlap loss), rounded up. ⚠️ DO NOT DOUBLE IT UP. This is the mistake worth publishing a whole calculator to prevent. Two layers of underlayment feels like more cushioning and better sound, and it is neither - it makes the floor SPONGY, and a floating floor that flexes underfoot puts the load onto the locking joints of the planks rather than the subfloor. Those joints then work loose, gap and eventually break. It also voids most flooring warranties outright. If you want more cushion, buy a thicker single underlayment rated for it. ONE LAYER ONLY ALSO APPLIES TO PADDED FLOORING. Many laminate and vinyl planks come with underlayment already attached, and adding another layer under them is the same error. Check the plank before buying anything at all - it is a common way to waste a whole purchase. THE MOISTURE BARRIER IS A SEPARATE QUESTION FROM THE CUSHION. Over concrete, or over any below-grade or on-grade slab, you need a vapour barrier - either a combined underlayment that includes one or a separate 6-mil sheet. Over a wooden subfloor with a ventilated space beneath, you usually do not, and adding one can trap moisture in the wrong place. This is decided by what is UNDER the floor, not by what the floor is. BUY THE ROLL SIZE THAT SUITS THE ROOM. Underlayment comes in very different rolls - 100 square feet is common for foam, but 200, 630 and larger exist, and a big roll in a small room is mostly offcut. The rounding is coarse: a 180 square foot room needing 198 with waste takes 3 rolls of 100 nominal, and leaves a lot spare. RUN IT THE SAME DIRECTION AS THE PLANKS, seams staggered, and tape every seam if the product calls for it. Do not lay it all out at once - roll out one or two runs ahead of the flooring so nothing gets walked on, dragged or torn while you work.

Common questions

How much underlayment do I need?

For a 15 by 12 room - 180 square feet - with 10% waste you need 198 square feet laid. But a roll labelled 100 square feet really covers about 95 once you allow 5% for overlaps and trimming, so you need 3 rolls. That is 300 square feet nominal for a 180 square foot room, which sounds excessive until you realise the rounding is in whole rolls.

Why doesn't a roll cover what it says?

Because the number on the packaging is the nominal area of the material itself, and you lose some of it laying it. Seams either overlap or carry a taped flap that sits over the neighbouring run, and every run gets trimmed at the wall. About 5% is a realistic allowance for a plain rectangular room; a room with alcoves, a chimney breast or several doorways will lose more. The calculator reports what a roll really covers alongside what it claims.

Can I use two layers for extra cushioning?

No, and this is the single most important thing on this page. Two layers of underlayment feel like better cushioning and better sound insulation, and deliver neither. What they actually create is a spongy floor - and a floating floor that flexes underfoot transfers the load into the locking joints of the planks instead of down into the subfloor. Those joints work loose, open into gaps and eventually break. It also voids most flooring warranties outright. If you want more cushion, buy a thicker single underlayment that is rated for it.

What if my flooring already has underlayment attached?

Then you do not add any, and you should check before buying. A great many laminate and luxury vinyl planks come with a pad already bonded to the back, and putting a separate underlayment beneath them is exactly the doubling-up problem - same sponginess, same joint failure, same voided warranty. It is a common way to waste an entire purchase, because the packaging for the underlayment and the packaging for the planks each say nothing about the other.

Do I need a moisture barrier?

That is decided by what is UNDER the floor, not by what the floor is. Over concrete - any below-grade or on-grade slab - you need a vapour barrier, either a combined underlayment that includes one or a separate 6-mil sheet. Over a wooden subfloor with a ventilated space beneath, you usually do not, and adding one can trap moisture on the wrong side where it will do more harm than good. If you are unsure about a slab, tape a square of plastic sheet down for 24 hours and look for condensation underneath.

Which way should it run?

The same direction as the planks, with the seams staggered rather than lining up, and taped if the product calls for it. Do not roll the whole room out before you start - lay one or two runs ahead of the flooring as you go. Underlayment that has been walked on, knelt on and dragged across for an afternoon arrives at the plank stage torn, bunched and compressed, and every one of those faults telegraphs into the finished floor.

Why is there so much spare?

Because underlayment rounding is coarse and rolls are large. A 180 square foot room needs 3 rolls of a nominal 100, so about a third of the last roll is left. Buying a smaller roll size for a small room genuinely helps, and the sizes vary far more than people expect - 100 square feet is common for foam, but 200 and 630 square foot rolls exist. Match the roll to the room rather than buying whatever is stacked at the end of the aisle.

Does thicker underlayment mean a better floor?

Not past what the flooring is rated for, and this is where the doubling-up instinct comes from. Underlayment has a job - to smooth out minor subfloor variation, to reduce sound, and sometimes to block moisture - and beyond the thickness the plank manufacturer specifies, more of it makes the floor less stable rather than more comfortable. The plank's locking system was designed against a particular amount of give. Check what the flooring specifies and buy that, once.

Take it further with AI

Copy this into ChatGPT or Claude with your own numbers filled in. It hands over the figures this calculator worked out, so the answer is built on real arithmetic instead of a guess.

I used the Underlayment Calculator at https://www.bfcbrilliance.com/tools/underlayment-calculator.

What I entered:
- Room length (ft): ___
- Room width (ft): ___
- Nominal coverage per roll (sq ft): ___
- Lost to overlap and trimming (%): ___
- Waste (%): ___

What it calculated:
- Rolls to buy: ___
- Floor area: ___
- Needed with waste: ___
- What one roll REALLY covers: ___
- Lost per roll: ___

Use those figures as given — they are already worked out, so please don't recalculate or estimate your own. Help me turn them into a plan: what to buy or do, in what order, roughly what it should cost, and the mistakes people most often make with this job.

Where the job touches structure, gas, or electrics, tell me to check local building code and whether a permit or a licensed trade is required.

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