Insulation Calculator
Work out insulation batts or blown-in bags from wall or attic area, deduct the framing, and check the R-value your thickness achieves against a target.
Batts: packs = (gross area − framing area) ÷ the coverage printed on the bale, where the framing deduction is member width ÷ spacing plus your allowance for plates and corners. Blown-in: bags = area × depth × installed density ÷ bag weight, which reproduces manufacturers' coverage charts — a 27.3 lb bag of blown fibreglass at 0.49 lb/ft³ covers about 63 sq ft at 10.5 in, the published R-30 figure. R per inch values are the DOE midpoints; the R-value your assembly achieves also depends on thermal bridging, air sealing and settling. This is a materials estimate, not an energy audit.
What is the Insulation Calculator?
The ByteTools Insulation Calculator sizes an insulation order two ways. For batts and rolls it deducts the framing area from your wall or attic and divides by the coverage printed on the bale.
- Batt packs from net cavity area ÷ the coverage printed on the bale
- Blown-in bags from area × depth × installed density ÷ bag weight
- Framing deduction derived from your stud spacing and member width, plus an editable allowance
- R per inch for ten materials, from loose fibreglass to closed-cell spray foam
- Shows the R-value achieved, the metric RSI, and the depth needed for a target R
- All calculated in your browser — nothing uploaded, no signup
How to use the Insulation Calculator
- 1
Choose imperial or metric, then pick batts or blown-in loose fill.
- 2
Select the material — its R per inch is shown in the hint — and say whether it goes in walls, an attic floor or an open area.
- 3
Enter the area dimensions, and for framed assemblies the spacing, member width and an allowance for plates and corners.
- 4
For batts, enter the coverage per bale and the installed thickness; for loose fill, the target depth, installed density and bag weight.
- 5
Set a target R-value and read the packs or bags, the R-value achieved and the depth the target needs.
About the Insulation Calculator
The ByteTools Insulation Calculator sizes an insulation order two ways. For batts and rolls it deducts the framing area from your wall or attic and divides by the coverage printed on the bale. For blown-in loose fill it multiplies area by depth by the installed density and divides by the bag weight, which is exactly how manufacturers build their coverage charts.
It also tells you what R-value the thickness you have chosen actually reaches, and how deep you would need to go to hit a target. R per inch values are the midpoints published by the US Department of Energy, from R-2.5 for blown fibreglass through R-3.5 for cellulose up to R-6.5 for closed-cell spray foam.
Everything runs locally in your browser and nothing is uploaded. This is a materials estimate, not an energy audit — the R-value an assembly delivers in practice also depends on thermal bridging through the studs, on air sealing, and on how much loose fill settles over the first few years. Add depth rather than shaving it.
Frequently asked questions
How many bags of insulation do I need for my attic?
Multiply the attic area by the depth in feet and the installed density, then divide by the bag weight. A 1,000 sq ft attic at 10.5 in of blown fibreglass, 0.49 lb per cubic foot, in 27.3 lb bags takes about 16 bags — the same answer the manufacturer's R-30 coverage chart gives.
What R-value do I need in my attic?
The US Department of Energy recommends R-30 to R-49 in warm climates and R-49 to R-60 in cold ones, with R-13 to R-21 in walls. Your local code sets the actual minimum, and the climate zone you are in matters far more than any national average.
How thick does insulation need to be for R-49?
It depends on the material. At R-3.5 per inch, blown cellulose needs about 14 inches; at R-2.5 per inch, loose fibreglass needs nearly 20; and closed-cell spray foam at R-6.5 gets there in about 7.5. Pick your material above and the calculator gives the depth for whatever target you set.
How much area do the studs take out of a wall?
A 1.5 in stud at 16 in centres is 9.4% of the wall by itself, and once plates, headers, corners and blocking are counted the framing factor is usually taken as 15% or more. That framing is also a thermal bridge, which is why the whole-wall R-value is lower than the batt's label.
Does blown-in insulation settle?
Loose-fill cellulose settles noticeably and manufacturers publish both an installed and a settled thickness, which is why coverage charts are stated at an installed density. Blow to the installed depth on the chart, not to the depth you want after settling, and check the depth markers as you go.
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