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Insulation Calculator

Area divided by package coverage, plus an R-value thickness guide.

Also known as insulation rolls · batts quantity · insulation coverage

Quick answer

With the default inputs: Area to insulate 1000 ft² · Coverage per bag 88.1 ft² · Framing spacing 16 in etc.Insulation required 13 bags

Use your own dimensions ↓

Inputs

ft²
ft²
in
%

Result

Insulation required
13bags
Area with waste1100ft²
Coverage per bag88.1ft²
Framing spacing16in
Batt width15in
Part bag43ft²
R-13 reference3.5in
R-19 reference6.25in
R-30 reference9.5in
How this is worked out

Bags = area x (1 + waste) ÷ package coverage, rounded up. The coverage figure has to come off the package, not a generic number: R-13 in 15 in width and in 23 in width differ more than twofold in coverage, and piece counts vary by manufacturer. Batt width must match the framing: 15 in for 16 in centres (an inch goes to each stud) and 23 in for 24 in centres. Wrong width means gaps everywhere and you may as well not have insulated. The R-value thicknesses are references only - about 3.5 in for R-13, 6.25 in for R-19 and 9.5 in for R-30, with actual thickness depending on the material. 10% waste is normal; irregular framing or lots of pipes and boxes takes it to 15%.

Next up: Skirting Board Calculator · Plasterboard Quantity Calculator

Formula

Bags = area × (1+waste) ÷ coverage per baground up
Batt width = framing spacing − 1 in15 for 16 in, 23 for 24 in
R-value ≈ thickness ÷ conductivityfibreglass about 3.7 R per inch
1000 ft² with 88.1 ft² per bag (R-13 at 15 in wide) and 10% waste: 1100 ft², so 13 bags. Switch to 23 in wide batts for 24 in centres and the coverage per bag changes - read the new package figure.

How to work it out

  1. Work out the area to insulate: net wall area, and ceiling length x width with no deduction for joists.
  2. Read the coverage figure printed on the package - not a generic value, since the same R-value in a different width differs more than twofold.
  3. Match the width to the framing: 15 in for 16 in centres, 23 in for 24 in centres.
  4. Bags = area x (1 + waste) ÷ coverage per bag, rounded up.
  5. Confirm the R-value by climate zone: walls R-13 to 21, ceilings R-30 to 60, floors R-19 to 30.

Where it goes wrong

Material and cost

Fibreglass batts run about USD 0.50-1.20 per ft² by R-value and width, with mineral wool 30-50% more. Blown-in material is 0.80-1.50 per ft² but needs specialist equipment. Installed pricing is 1.50-3.50 per ft² for walls and 2.00-4.50 for ceilings; doing it yourself saves more than half.

On site

Basis

FAQ

What R-value do I need?
It follows climate zone: walls commonly R-13 to R-21, ceilings R-30 to R-60 and floors R-19 to R-30 in North America. Cold climates want R-49 or better overhead because heat rises and ceiling insulation gives the best return. Wall values are capped by stud depth - 2x4 takes R-13 to R-15 and 2x6 takes R-19 to R-21 - so going higher means exterior or continuous insulation.
What happens if the batts do not fit tightly?
Performance falls off a cliff rather than degrading gradually: a 5% gap can lose 20-30% of the effective R-value because air convects through it. The usual culprits are batts not pressed against the stud face, boxes, pipe penetrations and the narrow strips between window frames and studs. Fill those with offcuts or foam. Compressing batts also hurts - stuffed in too tight, the R-value drops.
Do I need a vapour barrier?
Cold climates do: the barrier goes on the warm side of the insulation to stop interior moisture entering the cavity and condensing. In warm humid climates such as the south-eastern US it is more nuanced - put it on the wrong side and you trap moisture in the wall, which is worse than none. It has to be continuous, with seams lapped and taped and proper gaskets at outlets and penetrations. Get this wrong and the mould shows up years later.

Cite this page

Use this format in reports, method statements or coursework. The link is permanent.

Building Calculator. Insulation Calculator. 2026-10-25. https://jianzhujisuan.com/en/insulation-batts

Results are estimating aids only and are not a design basis. Construction and final accounts must follow the drawings, the applicable standards and an approved method statement. Anything affecting structural safety, shoring or falsework must be checked by a suitably qualified engineer.