Floor underlay Calculator: measuring notes and method
Use the net floor being covered
Measure beneath the floor finish and exclude areas where underlay is not installed. Use roll dimensions for the actual underlay product rather than the flooring pack dimensions.
Check whether seams overlap
A separate moisture-control film may have a different overlap from the underlay itself. Calculate the two materials independently when their layouts differ.
Formula
Effective roll area = (roll width − side overlap) × roll length. Rolls = area with waste ÷ effective coverage, rounded up.
Worked examples in metric and imperial units
Illustrative values match Reset example and stay fixed when you edit the calculator. Both columns describe the same calculation; converted inputs are rounded.
Metric worked example
- Total surface area
- 24 m²
- Roll width
- 1 m
- Roll length
- 20 m
- Side lap
- 100 mm
- Waste allowance
- 10 %
Rolls to buy: 2
Imperial worked example
- Total surface area
- 258.3339 ft²
- Roll width
- 3.2808 ft
- Roll length
- 65.6168 ft
- Side lap
- 3.937 in
- Waste allowance
- 10 %
Rolls to buy: 2
Follow the calculation
Lengths in this working are in metres.
- Effective width = 1 − 0.1 = 0.9 m.
- Effective area per roll = 0.9 × 20 = 18 m².
- Divide 26.4 m² including allowance by effective roll area and round up.
Reviewed 19 October 2026. How calculations are checked · Report a correction