Insulation calculator
Area to cover and coverage per pack: packs needed.
Your inputs
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Example loaded. You can change any value.
There is no figure yet: nothing is calculated until you ask for it.
- Enter your measurements.
- Check the units and the assumptions.
- Press Calculate, or load the example.
Rather see a worked case? Press “Load example” and change any value afterwards.
Next step
Insulation is closed with board, over the very same square metres.
Where each number comes from Conversion constants are exact; densities and coverage rates come from cited sources; everything else is an assumption you can change. Methodology · Sources and updates
How to use it
- Measure the area you are insulating.
- Check the coverage per pack on the packaging.
- Adjust the waste for the stud or joist spacing.
- The result rounds up to whole packs.
How it is calculated
The quantity maths is simple; what matters is that insulation has two quantities that must not be confused:
Area decides how many packs you buy. Thickness decides how much it insulates. They are independent: buying more packs of the same thickness does not improve performance, it just covers more wall.
Thermal resistance (the R-value) is what measures performance, and it depends on thickness and material. This calculator does not work it out: check the R-value your climate zone and building regulations require.
Worked example
Insulating 323 sq ft of partition with packs covering 62 sq ft each and 10 % waste:
- With waste: 323 x 1.10 = 355 sq ft
- Packs: 355 / 62 = 5.7 → 6 packs
- Coverage bought: 372 sq ft; left over: 17 sq ft
Good to know
- Cut slightly wide. Batts should friction-fit between studs. A loose fit leaves thermal bridges at the edges.
- One gap ruins the assembly. 5 % of the area left uninsulated can cut overall performance by much more than 5 %.
- Protect yourself. Mineral wool itches: gloves, long sleeves, goggles and a mask.
- Vapour barrier. Often required, and it goes on the warm side. Not part of this calculation, but part of the shopping list.
Frequently asked questions
How many packs of insulation do I need?
Divide the area including waste by the coverage per pack and round up. For 323 sq ft with 62 sq ft packs and 10 % waste, that is 6 packs.
How much waste should I allow for insulation?
10 % is reasonable: batts are cut to the stud spacing and there are always narrow strips left over that are hard to use.
Do more packs mean better insulation?
No. Area decides how many packs you buy; thickness decides how well it insulates. To improve performance you increase thickness or change material.
Can I add insulation over existing insulation?
It is common in lofts, laying the new layer perpendicular to the old one to cover the joints. Work out the area to cover exactly as here.