Insulation calculator
Calculate insulation packs for ceilings, walls or floors.
Packs needed = (area + waste) ÷ coverage per pack. The calculator works out the quantity – it doesn't choose the thickness.
The right thickness depends on the construction, the product and the rules that apply to your project. Check the manufacturer's instructions and your local requirements.
Insulation calculator
Calculate how much insulation you need for ceiling, wall or floor based on dimensions, thickness and waste.
Surface length
Surface width (or height for walls)
Allowance for cutting and extra consumption
Check packaging — varies by product and thickness
Your insulation project
26.4 m² insulation (100 mm) → 8 packs for 24 m² ceiling
Area
24 m²
Thickness
100 mm
Incl. waste
26.4 m²
Packs
8
Materials
24 m² ceiling + waste
8 × 3.6 = 28.8 m² (surplus: 2.4 m²)
ℹThe result is an estimate based on dimensions, insulation thickness, pack coverage and waste.
ℹActual requirements may vary depending on construction, cutting and product choice.
Your result will be saved in a new project where you can review the materials list.
Example: 24 m² ceiling
If you insulate a 24 m² ceiling with 100 mm insulation and add 10 % waste, you need about 26.4 m² of insulation. If a pack covers 3.6 m², you need 8 packs.
Calculation for a 6 × 4 metre ceiling:
The surplus from rounding is a natural part of the calculation — it provides a small reserve for mistakes and cutting.
How to calculate insulation
Amount or thickness?
The calculator works out the material amount from area, thickness, pack size and waste. It doesn't decide the thickness your construction needs – you choose that from the project and the rules that apply. To read how thickness relates to construction and U-value, see the guide on insulating walls and ceilings.
Always add waste
Insulation batts must be cut at edges, corners, pipes and electrical boxes. 10 % extra is a good starting point. With many penetrations or irregular shapes 15 % may be more realistic.
Thickness matters
Insulation performance (U-value) depends directly on thickness. Ceilings usually get considerably more insulation than walls, and the thickness you need depends on the construction, the material and whether it's a new build or a retrofit. Always check the requirements for your project rather than going by one standard figure.
Pack coverage varies
Each pack of insulation covers a certain number of m² — typically between 2 and 6 m² depending on product and thickness. Thicker insulation yields fewer m² per pack. Always check the packaging before ordering.
Ceiling, wall and floor have different needs
Ceiling insulation is typically laid in 2 cross-laid layers to avoid thermal bridges. Walls are usually insulated in a single layer between studs. Floor insulation requires a vapour barrier and must fit the construction depth. The calculator handles all three types.
How to use this calculator
Four quick steps – you don't need to be a tradesperson to follow them.
- Measure the area you're insulating – ceiling, wall or floor.
- Choose the insulation thickness (e.g. 95 mm, 195 mm).
- Enter how many m² come per pack – it's on the label in the shop.
- Check the pack count and round up. Add about 5 % for waste around studs and pipes.
Common mistakes
Small things that often cost an extra trip to the hardware store.
- Underestimating the volume – packs are bigger than they look, but you must never compress insulation.
- Skipping moisture protection – whether you need a vapour barrier or vapour-control layer, and where it goes, depends on the construction and the product. Follow the manufacturer's instructions.
- Wrong thickness – check what your local building regulations require.
- Not packing the gaps around studs, rafters and pipes properly.
Example: a 60 m² loft
A 60 m² loft with packs that cover 3 m² each is 20 packs by area. Add 5 % waste and you need 63 m² – 21 packs. Coverage per pack is printed on the label and varies with product and thickness.
Frequently asked questions
Short answers to the questions we hear most often about this calculator.
How thick should the insulation be?
It depends on whether it's a loft, wall or floor – and whether it's a new build or a retrofit. Lofts usually get considerably more than walls. Always check the requirements for your project rather than a single standard figure.
Can the calculator decide the right insulation thickness?
No. It works out the material amount from the area, thickness and pack size you enter – it doesn't pick the thickness for you. The right thickness depends on the construction, the material and the rules that apply to your project.
Do I need a vapour barrier?
It depends on the construction. In many builds a vapour barrier or vapour-control layer sits on the warm side of the insulation, but whether you need one, which type, and how it's sealed depends on the construction, the materials and the manufacturer's instructions. Follow those rather than a rule of thumb.
Does this work for both glass wool and rock wool?
Yes – it works off area and thickness, so the principle is the same either way.
How much waste should I plan for?
5 % is usually plenty. If you're filling around lots of close studs or pipework, push it to 10 %.
Use the results in a real project
With MyPlanDIY you can save materials, create budgets, track tasks and bring everything together in one project.
No credit card required. Free sample project included.
See also
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