Paint & interior · updated 2026
Flooring Underlayment Calculator
Underlayment is sold in rolls of fixed square footage, and the only real math is area ÷ roll size plus overlap waste. This underlayment calculator takes your floor area and roll size and returns roll counts with 5% for seam overlaps, with 2026 costs. It covers foam underlayment for laminate and engineered wood — LVP with an attached pad usually needs none at all.
Show the math
Rolls = ⌈400.0 × 1.05 ÷ 200.0⌉ = 3 (5% for overlaps)
Cost = 400.0 × 1.05 × $0.25–$0.50/sq ft
What the numbers mean
The calculator converts your measurements into a practical purchase quantity using a documented coverage or spacing assumption.
Any whole-unit quantity is rounded up so the result does not leave you short during the project.
Use the cost range as a planning baseline and confirm the final quantity, availability, and delivery charge with your local supplier.
2026 material prices
| Material | 2026 average price | Note |
|---|---|---|
| Foam underlayment, per sq ft | $0.25 – $0.50 / sq ft | For laminate and engineered wood |
| Cork underlayment, per sq ft | $0.70 – $1.20 / sq ft | Best sound deadening |
| 6-mil poly vapor barrier, per sq ft | $0.10 – $0.20 / sq ft | Over concrete slabs |
Wave 1 Batch 3 content price guide, Sep 2026. 2026 U.S. national averages. Prices vary by region and retailer — confirm locally before buying.
Pro tips
- Over concrete, a vapor barrier is mandatory — slab moisture ruins laminate and engineered wood from below.
- Tape the seams with the manufacturer's tape; overlapping without taping lets moisture migrate through.
- Don't double up underlayment to "fix" an uneven floor — it voids warranties and makes floors feel bouncy.
- LVP with an attached pad usually needs no extra underlayment — check the warranty before buying any.
- Roll it out the day before install so it relaxes flat; curled edges telegraph through thin flooring.
- A 400 sq ft room needs 3 two-hundred-sq-ft rolls — about $105–$210 in foam underlayment.
Common questions
Frequently asked questions
How much underlayment for 400 sq ft?
⌈400 × 1.05 ÷ 200⌉ = 3 rolls.
How about 600 sq ft with 100-sq-ft rolls?
⌈630 ÷ 100⌉ = 7 rolls.
Why 5% extra?
Overlaps at seams and cuts around doorways — underlayment waste is small but real.
Do I need underlayment over concrete?
Yes — plus a vapor barrier; concrete wicks moisture year-round.
Does LVP need underlayment?
Usually no, if it has an attached pad — extra layers can void the warranty.
What does it cost?
About $0.25–$0.50/sq ft — a 400 sq ft room runs $105–$210.
Related calculators
Planning estimates only. Prices are 2026 U.S. national averages and vary by region — confirm with your local supplier before buying.