How to estimate insulation packs
Use the area you will actually insulate and the pack coverage of the selected product and thickness.
Back to calculator ↑Measure the net area
Measure width and length or wall height in metres. Multiply to get gross square metres, then deduct only openings or sections that will genuinely receive no insulation. For a broken-up roof or wall, split the plan into rectangles and add their net areas. Measure between framing members separately because panel width and fit matter even when the overall square metres look right.
If you already have a verified net area, enter it directly rather than deducting the same opening again. Allowable clearances around lights, flues and ventilation are installation matters that depend on the product and local requirements. A simple area figure cannot establish whether those details are safe.
Turn area into whole packs
Required material area equals net area × number of layers of the same chosen product × (1 + cutting allowance ÷ 100). Divide by the square metres per pack printed for that exact product and thickness, then round up. The layers field means repeating the same product. For a build-up made from different products, calculate each product separately using its own pack label.
Example: 20 m² of net area, one layer and 10% cutting allowance require 22 m² of material. At 5 m² per pack, 22 ÷ 5 = 4.4, so buy five packs. Those packs are labelled to cover 25 m². The extra 3 m² is arithmetic space beyond the allowed area, not a guarantee that every offcut will fit elsewhere.
Check thickness and suitability
Manufacturer pack tables show why coverage must come from the chosen SKU: a thicker version can contain fewer slabs and less area per pack. ROCKWOOL’s Cavity Slab 32 data, for example, lists different area per pack at 100 mm and 150 mm. A generic five-square-metre default is only a placeholder for planning.
The optional thickness fields only compare selected product thickness times layers with the entered available depth. They do not calculate thermal resistance, U-value, fire performance, condensation risk or energy savings. Do not compress a product to make it fit unless the manufacturer specifically allows the installation method.
Buying checks and common mistakes
Confirm product name, thickness, slab or roll dimensions, pieces per pack and square metres per pack on the actual label. Check the installation instructions for framing fit, supports, moisture control, ventilation and protective equipment. The calculator estimates quantity only; a site assessment remains necessary for a roof, wall or floor assembly.
Common mistakes include copying the coverage of a thinner SKU, forgetting a second layer, counting the same opening twice and treating whole-pack spare coverage as guaranteed usable material. Price input is optional and refers to a pack of the selected product; a blank price is unknown, not zero. Search links need independent product and listing checks.
Common questions
Can I enter two layers?
Yes, when both layers use the same product and pack coverage. Calculate different products separately.
Does fitting the cavity depth guarantee a valid installation?
No. The dimensional check does not assess ventilation, moisture, electrical clearances or assembly requirements.
Why enter the exact pack coverage?
Coverage changes with product, thickness and pieces per pack. Read the exact SKU label.
Sources and calculation guidance
Check these against your chosen product’s label and instructions.