Flooring underlay calculator
Flooring underlay
calculator
Calculate full underlay rolls for up to five rooms using the actual roll width and length. See the strip layout and estimate cutting waste, taped seam length and joint tape rolls.
Enter measurements ↓
Simple shopping list
What you'll need
Buy complete rolls and tape every adjoining strip. The allowance covers trimming around walls, thresholds and awkward corners.
- Underlay rolls
Selected roll size
- Calculate above
- Joint tape
For strip seams
- Calculate above
- Utility knife
For clean perimeter cuts
- 1 tool
The maths
How we work out your underlay
Underlay is installed in full strips, so area alone can underestimate the order. We divide each room width by the selected roll width, round up to whole strips, then total the strip lengths and add your waste allowance before dividing by the roll length.
strips = ⌈ room width ÷ roll width ⌉
rolls = ⌈ strip length × waste ÷ roll length ⌉
joint tape = (strips − 1) × room length + 5%
- 5%
- tidy square rooms
- 10%
- recommended waste
- 15%
- awkward layouts
- +5%
- joint tape allowance
- 1–5
- rooms per calculation
- ft ↔ m
- converted automatically
- 01
Measure every room
Enter the full floor length and width for each room that will use the same underlay product.
- 02
Choose the roll size
Use the width and length printed on the product so the strip layout matches what you can actually buy.
- 03
Split the floor into strips
Each room width is divided by the roll width and rounded up because a partial final strip still consumes material.
- 04
Allow for cuts
The selected 5–15% allowance covers thresholds, damaged edges, alcoves and trimming along walls.
- 05
Round up to complete rolls
The required strip length is divided by the roll length and rounded up to a practical shopping quantity.
Good to know
Underlay questions, answered
Most floating laminate and engineered floors do. Some vinyl products have underlay attached, so check the flooring instructions before adding another layer. Glue-down flooring normally does not use a loose underlay.
Ten percent is a practical default. Five percent can work in simple rectangular rooms, while narrow halls, alcoves, columns and several doorways make fifteen percent safer.
Yes, adjoining strips are normally joined with the tape specified by the underlay manufacturer. This stops the strips moving during installation and can maintain the vapour-control layer.
Usually no. Most underlays are laid edge to edge and taped, while products with an integrated flap follow their own overlap instructions. Always use the detail printed on the roll.
Follow the flooring manufacturer. A common approach is to run the underlay perpendicular to the floorboards, but product instructions and integrated seams take priority.
Do not double the underlay unless the flooring manufacturer explicitly permits it. Too much movement under click joints can damage them and may invalidate the warranty.
Include them in the room dimensions when they will be covered. For a separate closet or irregular section, add it as another small room so its strip requirement is not missed.
Underlay is sold in complete rolls and laid in full-width strips. The final strip and final roll can both leave an offcut, so a strip-based order is more realistic than area divided by roll coverage.