Insulation Calculator
Work out how many packs of insulation batts or rolls a ceiling, wall or floor needs from its area and the coverage per pack.
How many packs of insulation the area takes.
How the insulation calculator works
Insulation is sold in packs that cover a stated area — typically 4–10 m² (40–100 sq ft) depending on thickness and R-value. Packs needed = area × (1 + waste) ÷ coverage per pack, rounded up. A 5% allowance covers cuts around joists, downlights and services.
Measure the area to insulate, not the room: a ceiling is the floor area below it; walls are length × height less large openings.
Formula: packs = area × (1 + waste) ÷ coverage per pack
Worked examples
| Inputs | Packs to buy | Note |
|---|---|---|
| 8 × 5 m ceiling, 4.5 m² packs | 10 | 10 packs |
| 8 × 5 m ceiling, 8 m² packs | 6 | 6 packs |
| 26 × 16 ft ceiling, 48 sq ft packs | 10 | 10 packs |
FAQFrequently asked questions
What R-value do I need?
It depends on climate zone and whether it is a ceiling, wall or floor; ceilings need the highest. Building codes and energy-rating schemes set minimums for your region.
Do batts need to fit between joists?
Yes — batts are made in widths to suit common joist spacings (430 and 580 mm; 15 and 23 in). Measure the joist gap before buying.
How much waste should I allow?
5% for a clear ceiling; 10% where there are many downlights, pipes or awkward corners.
Can I add a second layer?
Yes, laid across the first to cover the joists, and R-values add. Run the calculator again for the second layer and pin the first result.
Where these figures come from
- The Engineering ToolBox — material densities, coverage and unit data — bulk densities, concrete at 2,400 kg/m³, board and sheet sizes
- Brick Development Association — brick sizes and quantities — 215 × 102.5 × 65 mm, 60 bricks per m²
- UK Government — Approved Documents (Building Regulations) — Building Regulations guidance
Last checked: September 2026. Coverage rates and material sizes are typical manufacturer figures — check the product you buy.