Insulation
Insulation Coverage Calculator
Works out how much area your material will cover, which is the number on the bag rather than a volume sum.
Result
Required R-values are set by climate zone and by local energy code, and they differ for attics, walls and floors. The figures offered here are common code minimums rather than recommendations, and a minimum is a floor, not a target. Air sealing comes first: insulation slows heat conduction but does nothing about air leaking through gaps, and an uninsulated but sealed attic often outperforms an insulated leaky one.
Get a Free Business AuditHow the calculation works
Coverage is area per bag at a given installed thickness, and it falls as the thickness rises because the same material is spread over less area. Working from the manufacturer's chart rather than calculating volume is the right approach for loose fill, because the chart reflects installed density. For batts, coverage is simply the square footage per bag at that batt's thickness and width, and the only adjustment is waste from cutting.
What moves the number
- Installed thickness, since coverage per bag falls as depth increases.
- Whether the figure is initial or settled thickness on loose fill.
- Framing area, which reduces net insulated area in a cavity application.
- Waste from cutting around obstructions, which is real on batt work.
Where this one goes wrong
Measure the area you are actually insulating rather than taking it from the house footprint, which commonly includes a garage, a porch or a section with a different ceiling arrangement. Over-ordering wastes money and under-ordering stops a job partway. For loose fill, be clear whether the chart figure is initial or settled thickness, because installing to the settled number means ordering for the deeper initial blow. And remember coverage charts assume the material is installed at the intended density: fewer bags spread to the right depth is not the same product performance.
Before you quote
- Measure the actual area to be insulated, not the building footprint.
- Note whether coverage is quoted at initial or settled thickness.
- Add a cutting waste allowance for batt work.
Where the rules come from
Questions
About this calculator
How do I work out insulation coverage?
From the manufacturer's coverage chart, which gives area per bag at a stated installed thickness. Coverage falls as the thickness rises, since the same material spreads over less area. For loose fill this is more reliable than a volume calculation because it reflects installed density.
Should I use the house footprint for attic area?
No. The footprint usually includes a garage, a porch or areas with a different ceiling, so it overstates what you are insulating. Measuring the actual attic floor area is the only way to order correctly, and over-ordering is as wasteful as running short is disruptive.
Why does coverage drop at greater thickness?
Because a bag holds a fixed amount of material, and spreading it deeper covers proportionally less floor. Doubling the installed thickness roughly halves the area a bag covers, which is why bag counts rise faster than people expect when the target R goes up.
The formula
thickness = (target R - existing R) / R-value per inch
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How to use this calculator
- Length (ft) (ft). Enter your own figure; the page starts at 40 ft.
- Width (ft) (ft). Enter your own figure; the page starts at 28 ft.
- Target R-value. Enter your own figure; the page starts at R-38, attic.
- Material. Enter your own figure; the page starts at Blown cellulose (R-3.7 per inch).
- Existing R-value in place. Enter your own figure; the page starts at 0.
- Cost per square foot at that depth ($). Enter your own figure; the page starts at $1.35.
The result updates as you type, so you can try a figure, see what it does to thickness needed, and change it back. Nothing you enter is sent anywhere or saved.
A worked example
These are the figures the calculator opens with. Every number in the results column below is produced by the same calculation the page runs, so you can follow it through before putting your own figures in.
What goes in
| Input | Value |
|---|---|
| Length (ft) | 40 ft |
| Width (ft) | 28 ft |
| Target R-value | R-38, attic |
| Material | Blown cellulose (R-3.7 per inch) |
| Existing R-value in place | 0 |
| Cost per square foot at that depth | $1.35 |
What comes out
| Result | Value |
|---|---|
| Thickness needed main result | 10.3 |
| Area | 1,120.0 |
| R-value still to add | 38.0 |
| Estimated material cost | $1,512 |
| Bags if blown | 38 |
You need 10.3 inches to add R-38.0 across 1,120.0 square feet, roughly $1,512 in material.
How sensitive the answer is
Holding everything else steady and changing only cost per square foot at that depth, this is what happens to thickness needed. It is worth knowing which of your inputs the answer actually hinges on, and which barely move it.
| Cost per square foot at that depth ($) | Thickness needed |
|---|---|
| $0.68 | 10.3 |
| $1.01 | 10.3 |
| $1.35 example above | 10.3 |
| $2.03 | 10.3 |
| $2.7 | 10.3 |
Every other input is held at the example value while this one changes, which is not how a real job behaves. Treat it as a guide to which number matters most, not as five separate quotes.
What each figure means
- Length (ft) (ft)
- Measured in ft, starting at 40 ft.
- Width (ft) (ft)
- Measured in ft, starting at 28 ft.
- Target R-value
- Choose from R-13, walls, mild, R-19, walls or floors, R-30, attic, mild climate, R-38, attic, R-49, attic, cold climate, R-60, attic, very cold.
- Material
- Choose from Blown fibreglass (R-2.5 per inch), Blown cellulose (R-3.7 per inch), Fibreglass batt (R-3.2 per inch), Rigid foam board (R-5 per inch), Open cell spray foam (R-3.7 per inch), Closed cell spray foam (R-6.5 per inch).
- Existing R-value in place
- A number, between 0 and 80, starting at 0.
- Cost per square foot at that depth ($)
- A money amount, starting at $1.35.
What the results tell you
| Result | What it is |
|---|---|
| Thickness needed | The headline result. A decimal figure. |
| Area | A decimal figure. |
| R-value still to add | A decimal figure. |
| Estimated material cost | A money figure. |
| Bags if blown | A number. |
Keep going
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