Underlayment Calculator
How many rolls of flooring underlayment
Adding underlayment beneath new flooring? Enter the floor area to find how many rolls to buy.
How much underlay do I need?
Choosing underlayment for vinyl plank, laminate or wood starts with whether moisture barrier flooring protection is required at all.
Underlayment matches the floor area almost exactly — it rolls out edge to edge with only a small overlap or butt seam — so it needs far less waste than the flooring above it. This calculator uses 5%, against 10 to 12% for the planks themselves.
| Room | Area | With 5% | Rolls |
|---|---|---|---|
| 10 × 12 ft | 120 ft² | 126 ft² | 2 |
| 12 × 15 ft | 180 ft² | 189 ft² | 2 |
| 15 × 20 ft | 300 ft² | 315 ft² | 4 |
| 20 × 25 ft | 500 ft² | 525 ft² | 6 |
| 25 × 40 ft | 1,000 ft² | 1,050 ft² | 11 |
Roll sizes vary more than you would expect — 100 ft² is common but 200 and 630 ft² rolls exist, and premium acoustic products often come in smaller ones. Check the label and divide your area by the actual figure if it differs.
Check whether you need it at all
The first question is not how much but whether. A great deal of modern laminate and vinyl plank has underlayment pre-attached to the underside of each plank, and adding a separate layer beneath it is not a bonus — it is a warranty problem. Two layers of cushioning let the floor flex too much, which stresses and eventually separates the click joints.
| Flooring | Underlayment | Notes |
|---|---|---|
| Floating laminate | Foam or felt | Unless pre-attached |
| Floating LVP / SPC | Thin, dense foam only | 1 to 1.5 mm; thick foam is wrong here |
| Floating engineered wood | Foam or felt | |
| Glue-down anything | None | Adhesive bonds to the subfloor |
| Nail-down solid hardwood | 15 lb felt or rosin paper | Slip sheet, not cushion |
| Over concrete slab | Moisture barrier required | Built-in or separate 6 mil poly |
| Tile | Backer board or membrane | Different product entirely |
The rigid-core exception is worth spelling out. SPC and other rigid planks want a thin, dense underlayment, typically 1 to 1.5 mm. The plush 3 mm foam that suits laminate is actively harmful under rigid core: the plank does not flex, so it rocks on the compressible layer and the joints work loose.
What underlayment actually does
- Moisture barrier. The one genuinely non-negotiable function, over any concrete slab. Concrete releases water vapour for years, and trapped moisture under a floating floor grows mould and cups wood.
- Sound. Reduces both the hollow click you hear in the room and the impact noise the neighbour below hears.
- Minor levelling. It smooths very small imperfections. It does not fix a subfloor — nothing on this page substitutes for levelling compound.
- Comfort and warmth. A small but real difference underfoot, particularly over concrete.
If you are in a condo or apartment, check the building's rules before buying. Many require a minimum IIC and STC rating — usually 50 or higher — for hard flooring, and the certificate matters at inspection. Acoustic underlayment costs several times what basic foam does, so find this out before you order.
Laying it properly
- Run it perpendicular to the flooring so the seams never line up with the plank joints.
- Butt seams, do not overlap, unless the product specifically has an overlap flap. A double thickness at every seam creates a visible ridge.
- Tape every seam with the product's own tape, especially on a moisture barrier, where an untaped seam is simply a hole in the barrier.
- Run it up the wall a couple of inches on a moisture barrier, then trim after the flooring is in.
- Lay only what you can cover that day. Walking on bare underlayment tears and compresses it.
Once it is down, the laminate, vinyl plank and hardwood calculators cover the floor itself.
How this underlayment calculator works out its numbers
Rolls are floor area × (1 + waste) ÷ coverage per roll, rounded up, using 100 ft² per roll and a 5% default waste allowance.
- 5% is deliberately low. Underlayment rolls out edge to edge with butted seams, so waste is a small seam allowance and the odd end cut, not the 10 to 12% that plank flooring loses to cutting and culling.
- Roll coverage varies widely: 100 ft² is typical, but 200 and 630 ft² rolls exist and acoustic products are often smaller. The label is the authority.
- No check for pre-attached underlayment. If your flooring already has it, you need none, and adding some voids most warranties. That is a decision the calculator cannot make for you.
- Not included: seam tape, separate poly vapour barrier, and levelling compound.
- IIC and STC guidance reflects common condo requirements rather than a code minimum. Building rules vary and take precedence.
Reviewed September 2026.
Figures reviewed . Every worked example on this page is checked against the calculator above.
Underlayment Calculator: frequently asked questions
How many square feet in a roll of underlayment?
100 ft² is the most common size and what this calculator uses, but 200 and 630 ft² rolls are also sold, and acoustic products often come in smaller ones. Check the label and divide your area by the actual figure.
How much underlayment do I need?
Your floor area plus about 5%, which is much less than the flooring itself needs. Underlayment rolls out edge to edge with only a small seam allowance, so there is very little cutting waste. A 15 × 20 ft room takes 4 rolls at 100 ft² each.
Do I always need underlayment?
No. Much laminate and vinyl plank has it pre-attached, and adding a second layer voids most warranties because the floor flexes too much and the click joints separate. Glue-down floors need none at all.
Do I need a moisture barrier under flooring on concrete?
Yes, always. Concrete releases water vapour for years, and trapped moisture under a floating floor grows mould and cups wood. Use an underlayment with a built-in barrier or lay separate 6 mil poly, and tape every seam.
What underlayment for SPC or rigid core vinyl?
A thin, dense one, typically 1 to 1.5 mm. The plush 3 mm foam that suits laminate is wrong under rigid core: the plank will not flex, so it rocks on the compressible layer and the joints work loose over time.
Should underlayment seams overlap?
Butt them, unless your product has a purpose-made overlap flap. A double thickness at every seam creates a visible ridge through the floor. Tape the seams, which on a moisture barrier is what makes it a barrier at all.
What IIC rating do I need for a condo?
Many buildings require 50 or higher for hard flooring, and some ask for the certificate at inspection. Check your building's rules before ordering, since acoustic underlayment costs several times what basic foam does.
Does underlayment level an uneven subfloor?
Only very minor imperfections. It is not a substitute for levelling compound, and a real hump or dip will still telegraph through and stress the joints. Check flatness with a long straightedge before you buy anything.