Insulation Calculator

Work out how much insulation you need for a wall, ceiling or floor, and how many packs that is.

Formula and method

The insulated area comes from room dimensions or from a known coverage area. Multiplying it by the thickness and the number of layers gives the volume, and dividing the layered area by the package coverage gives the pack count. The waste allowance is applied before rounding up.

How to measure the inputs

Measure the room length, width and height, or enter a known coverage area.
Decide the thickness in millimetres and convert to the unit you will enter.
Read the coverage per pack from the packaging before ordering.

Worked example

Room length 13.12 ft; Room width 16.4 ft; Room height 8.2 ft; Insulation thickness 7.87 in; Coverage per package 43.06 ft²; Number of layers 2; Waste 10 % → Insulation packs: 11

Practical tips

Keep a small gap between insulation and the roof covering to allow ventilation.
Do not compress insulation to fit: compression reduces performance.
Wear gloves and a mask when handling mineral wool.

Assumptions

The insulated surface is flat and continuous, with no deductions for joists or studs.
The required thermal performance (U-value or R-value) has already been determined elsewhere.
Vapour control layers, tapes and fixings are not included.

Frequently asked questions

How thick should the insulation be?

Thickness depends on your climate zone and building regulations. Take the required U-value or R-value from local rules for your construction type, then enter the thickness that achieves it.

Should I add waste for insulation?

Yes. 5–10 % covers cuts around joists, studs, pipes and sockets. Rigid boards usually need more than flexible rolls.

Do I count the joists as insulated area?

This calculator treats the area as continuous. If you are cutting insulation between joists, order the coverage of the full area and accept the offcuts.

Can I use it for a loft and walls together?

Run the calculator separately for each area and add the pack counts, because the coverage per pack and thickness usually differ.

Related calculators

Estimate only

Results are material quantity estimates only. They are not structural design, and they are not a substitute for local building codes, engineer approval or manufacturer instructions. Structural sizing — bar diameters, spacing, foundation depth and load-bearing capacity — must be confirmed by a qualified engineer for your site conditions.

Browse by category

Insulation