How Much Acoustic Underlay Do I Need? (Calculator Guide)
Written by our acoustic insulation specialist — 15+ years experience supplying soundproofing to UK homeowners, developers and contractors. About our experts.
One of the most common questions before purchasing acoustic underlay: how much do I need? This guide explains exactly how to calculate your requirements, with worked examples and waste factors built in.
The Basic Calculation
Acoustic underlay is sold and measured by square metres (m²). The calculation is straightforward:
Floor area (m²) = Length (m) × Width (m) Order quantity = Floor area × 1.10 (add 10% for waste)
For rooms with irregular shapes, break them into rectangles, calculate each section, and add them together.
Worked Examples
Example 1: Living Room (4m × 5m)
- Floor area: 4 × 5 = 20 m²
- Plus 10% waste: 20 × 1.10 = 22 m²
Example 2: Open Plan Kitchen-Diner (6m × 7m)
- Floor area: 6 × 7 = 42 m²
- Plus 10% waste: 42 × 1.10 = 46.2 m² (round up to 47 m²)
Example 3: Whole Flat (multiple rooms)
- Bedroom 1: 3.5 × 4 = 14 m²
- Bedroom 2: 3 × 3.5 = 10.5 m²
- Lounge: 4.5 × 5 = 22.5 m²
- Hallway: 1.2 × 5 = 6 m²
- Total: 53 m² + 10% = 58.3 m² (order 59 m²)
When to Add More Than 10%
Standard 10% waste allowance is sufficient for rectangular rooms. Increase the allowance in these situations:
- Bay windows: Add 15% — bay angles create significant waste cuts
- Diagonal floor laying: Add 15–20% — angular cuts waste more material
- Complex room shapes (L-shape, etc.): Add 15%
- Rooms under 3m² each: Add 20% — proportionally more waste from cuts
Product-Specific Guidance
6mm Recycled Rubber Acoustic Underlay
Available per square metre. Our #1 seller with 74 dB ΔLw. Order your calculated area plus waste. Cuts cleanly with a heavy-duty knife.
3mm Recycled Rubber Acoustic Underlay
Available per square metre. 54 dB ΔLw. Easier to handle than 6mm due to lower weight per roll. Ideal for large areas.
Cork & Rubber Composite Underlay
Available in 3mm, 4mm, and 5mm. Order for your calculated area. Cork cuts cleanly with a sharp knife — no specialist tools required.
12mm Acoustic Flooring System
Panel-based system. Check individual product page for panel dimensions and coverage per pack. Calculate number of panels required based on floor area, adding 10% for cuts.
SilentMat 15mm
Roll format. Order by square metre. At 15mm thickness, offcuts can be repurposed for step nosings or small areas.
Ordering Tips
- Always round up — underlay cannot be returned if cut.
- Order for the whole project — batch consistency matters. Different production batches can have minor dimensional variations.
- Keep offcuts — useful for repairs, thresholds, and future projects.
Need help calculating? Browse our acoustic underlay range and use the quantity calculator on each product page, or contact us with your room dimensions.