Home Improvement
Underlay Calculator
Enter room area or length × width, waste and roll coverage to see rolls to buy.
How much underlay do I need?
Underlay is ordered by roll coverage, not by board count: measure the same floor area you are covering with laminate, vinyl or engineered wood, multiply length × width, add a small waste allowance for cuts around doorways, and divide by the square metres printed on the roll. Match underlay type to the floor product — acoustic, moisture-resistant and underfloor-heating grades are not interchangeable. Jointing tape, door bars and damp-proof membranes are separate line items. For the floor finish itself see the laminate flooring tool, and for general area maths the flooring calculator.
Worked example
A 4.5 × 3.2 m room is 14.4 m². With 5% waste ≈ 15.1 m². At 10 m² per roll that is 2 rolls; at £28 each ≈ £56.
Limits and assumptions
Does not include tape, thresholds or overlapping vapour barriers. Irregular L-shaped rooms should be split into rectangles and summed.
Frequently asked questions
Should underlay area match flooring area?
Yes — use the same net floor area, then add waste for cuts. Do not double-count under skirting thickness.
Is 5% waste enough?
Simple rectangles often sit near 5%. Many doorways, diagonals or first-time laying may need 8–10%.
Does roll direction matter?
Some underlays have a printed lay direction or foil face. Quantity maths stays the same; follow the roll instructions.
Can I mix roll sizes?
Work in total m² needed, then buy any mix of roll coverages that sum to at least that figure.