How many rolls of insulation do I need?

Take the surface area, add a waste allowance for offcuts, multiply by the number of layers needed to hit your target R-value, then divide by the coverage of one pack and round up. The calculator does all of this — for example, a 40 m² loft needing two layers of a 5.5 m² roll works out at ceil(40 × 1.1 × 2 ÷ 5.5) = 16 rolls.

What is an R-value and what is RSI?

R-value measures resistance to heat flow — higher is warmer. Metric R-value (RSI) is in m²·K/W; the US/imperial R-value is in ft²·°F·h/Btu. They differ only by units: 1 RSI is about 5.678 US R. The calculator shows both so you can match whatever the product label or building code uses.

Why does the tool round layers up?

A partial layer will not meet a performance spec, so if your target R-value sits between one and two layers, you need the next whole layer. The same logic applies to packs — you cannot buy two-thirds of a roll, so any fraction rounds up to a full pack.

How much waste should I allow?

Between 5% and 15% is typical. Lofts with lots of joists, pipes and awkward edges waste more; a clean rectangular wall wastes less. The default is 10%, which suits most DIY jobs. Increase it for cut-heavy work or steep roofs.

Does a higher R-value always cost more?

Reaching a higher target usually means a thicker product or an extra layer, which raises both material area and cost. Use the layers figure and the cost-per-area output to compare a single thick product against two thinner layers and see which is cheaper for your target.

Yes. Every figure is calculated in your browser and nothing is uploaded. Your inputs are saved only in your own browser so the form is still filled in when you come back.

Insulation Calculator

An insulation calculator that turns a surface area, a target R-value and a product’s pack coverage into the three numbers you actually need before a trip to the merchant: how many packs or rolls to buy, how many layers that means, and the total cost. It is built for DIY loft, wall, floor and roof jobs, as well as for trades pricing a quick quote — anyone who knows the area and the spec but not how many bags that turns into.

How it works

Insulation is specified by its R-value: the resistance a layer offers to heat passing through it. The build target (set by a building code, a retrofit grant or your own comfort goal) is also an R-value, so the first job is to work out how many layers of your chosen product stack up to meet it. Because a partial layer does not meet a spec, the tool divides the target by the per-layer R-value and rounds up to the next whole layer.

It then sizes the material. The net surface area is length times width (or whatever you enter directly), grown by a waste allowance for offcuts around joists, pipes and edges. Multiply that by the number of layers and you have the total area of material to lay. Divide by the coverage printed on each pack, round up, and you have the pack count. Multiply by the unit price for the cost. Every intermediate figure is shown in the “Show the working” panel so the estimate is auditable, not a black box.

The calculator speaks both unit systems. Switch between metric (RSI, in m²·K/W, with areas in m²) and imperial (US R-value, with areas in ft²) and every field converts so the physical setup is preserved. The relationship is 1 RSI ≈ 5.678 US R, and 1 m² ≈ 10.764 ft².

Formula note

The core steps are: layers = ceil(targetR ÷ productR), then material area = area × (1 + waste%) × layers, then packs = ceil(material area ÷ pack coverage), and finally cost = packs × price per pack. Rounding up at both the layer and pack stages is deliberate: you can buy neither a partial layer nor a partial roll.

Worked example

A loft measuring 8 m by 5 m gives a net area of 40 m². The target is 6.1 RSI (a common UK loft figure, roughly US R-35), and the chosen mineral-wool roll delivers about 2.2 RSI per 100 mm layer and covers 5.5 m² per roll at £22 each.

Layers: ceil(6.1 ÷ 2.2) = 3 layers, achieving 6.6 RSI (just over target).
Material area: 40 × 1.10 × 3 = 132 m² including 10% waste.
Rolls: ceil(132 ÷ 5.5) = 24 rolls.
Cost: 24 × £22 = £528, about £13.20 per m² of loft.

Switch to imperial and the same job reads as roughly 431 ft² at a target near R-35, with the roll coverage shown in ft² — the pack count and cost stay identical because only the units changed, not the physical build. Always confirm the coverage and declared lambda value on the manufacturer’s label before ordering.