Insulation Calculator
Packages of insulation from area and the coverage on the label.
From the label: R-13 rolls ~40, batt bundles 30 to 90, blown-in bags ~40 at R-38.
5% for open runs, 10% with many obstructions to cut around.
You need
16 packages
Uses the coverage printed on your product's label.
- Area to insulate
- 600 sq ft
- Area with waste
- 630 sq ft
Estimates only. Verify quantities before ordering.
How this is calculated
Packages = area x (1 + waste) / coverage per package, rounded up. Coverage comes from the product label at its rated R-value; blown-in bags state coverage per bag for each target R-value on a chart.
Measure the area you are insulating, enter the coverage printed on the package you plan to buy, and this calculator counts the packages. Every insulation product states its coverage per package at its rated R-value, and that number varies widely: a roll of R-13 covers about 40 square feet, batt bundles range from 30 to 90, and a bag of blown-in fiberglass covers around 40 square feet at attic R-values. That is why the label, not a generic table, is the input.
R-value targets depend on climate and location in the house. As a broad US guide, attics want R-38 to R-60, 2x4 walls take R-13 to R-15, 2x6 walls R-19 to R-21, and floors over unheated spaces R-25 to R-30. When topping up an existing attic, you only buy the difference: insulating from R-19 up to R-49 means adding R-30 worth of material.
Packages for common projects
Rows use typical label coverage: 40 sq ft per package (common for R-13 rolls and R-38 blown-in bags) with 5% waste. Always use your product's own label figure.
| Project | Packages | Area |
|---|---|---|
| 12 x 12 ft room ceiling | 4 packages | 144 sq ft |
| 20 x 20 ft attic section | 11 packages | 400 sq ft |
| 30 x 20 ft attic | 16 packages | 600 sq ft |
| 40 x 28 ft full attic | 30 packages | 1,120 sq ft |
| 50 x 30 ft full attic | 40 packages | 1,500 sq ft |
How to read the coverage on the package
Insulation is the rare material where the package states exactly what you need, and the figure to find is coverage in square feet rather than the R-value printed on the front. A bag of batts gives the square footage it covers at its rated thickness. A bag of blown-in gives something different: a small table pairing installed depths with the square footage one bag covers at each. Reading the wrong row of that table is the most common way an attic job finishes two bags short.
This is why the calculator asks for coverage instead of assuming a value. Batt coverage shifts with the cavity the product is made for, so a bag sized for 16 inch on center framing and one sized for 24 inch cover different areas at identical R-value. Blown-in varies more still, since the same bag spread thinner covers considerably more floor. Take the number from the package you actually intend to buy and the count above will match your receipt.
What you are actually buying
Batts and rolls are sold by the bag, priced per square foot of coverage, and need no equipment beyond a knife and a straightedge. They suit framing that is open and regular: a wall under construction, a floor over a crawlspace, joist bays you can reach. Check the width against your framing before buying a pallet, because a bag cut for 24 inch spacing wastes badly in a 16 inch wall.
Blown-in is sold by the bag as compressed material, and most home centers lend the blower free with a qualifying purchase, which changes the economics on a large attic considerably. It reaches around framing, wiring, and the awkward run near the eaves where batts leave gaps. Budget for a helper: one person feeds the hopper while the other works the hose, and the machine empties faster than one person can keep up with.
Buy unfaced material when you are topping up an attic that already has insulation, and faced only where you are insulating bare framing and need the vapor retarder. Suppliers will not usually take back opened bags, so it is worth confirming which you need before loading the cart rather than after.
Where the estimate can mislead you
The figure above covers the area you entered at the coverage you entered, and it assumes that area is clear. Attics rarely are. Ductwork, a chimney chase, stored boxes, and the shallow triangle near the eaves all reduce usable area or make the target depth impossible to reach, so the bag count can be right while the finished attic still underperforms.
Two adjustments are worth making before you order. Add for the attic hatch and any knee-wall door, which are frequently left bare and undo a surprising share of the gain. And keep insulation clear of soffit vents using baffles, which costs a strip of coverage at the perimeter but prevents a moisture problem that is far more expensive than the insulation. For R-value targets by climate and room, see our guide on how much insulation you need.
Common questions
What R-value do I need?
As a broad US guide: attics R-38 in the South up to R-60 in cold climates, 2x4 exterior walls R-13 to R-15, 2x6 walls R-19 to R-21, floors over crawl spaces R-25 to R-30. Your local code or utility program lists the exact target for your climate zone.
How many square feet does a bag of blown-in insulation cover?
It depends entirely on the target R-value, which is why every bag prints a coverage chart. A typical blowing-wool bag covers roughly 40 square feet at R-38 and 60 or more at R-30 or below. Read the chart for your target and divide your attic area by that figure, which is exactly what this calculator does.
Can I add new insulation over old?
Yes, if the old material is dry and not compacted. Lay new unfaced batts or blow loose fill directly over the top, running any new batts perpendicular to the joists. Never put a second vapor barrier (faced batts) over existing insulation, since it can trap moisture between layers.
Faced or unfaced insulation?
Faced batts (with the kraft-paper vapor retarder) go where the assembly has no existing vapor barrier, with the facing toward the living space. Unfaced is for topping up attics, sound walls, and layering over old insulation. Local practice varies in hot-humid climates, so check code if unsure.
How much insulation do I need for an attic?
Most attics want R-38 to R-60 depending on climate, which is roughly 12 to 20 inches of blown-in insulation. Measure the attic floor area, check the coverage table on the bag for your target depth, and enter that figure above. Insulate the full floor including over the top plates at the eaves, but keep insulation clear of soffit vents by using baffles, or you trade an insulation gain for a moisture problem.
Does insulation lose R-value over time?
Fiberglass batts and blown fiberglass hold their rating almost indefinitely if they stay dry and undisturbed. Cellulose settles roughly 10 to 20 percent in the first year, which is why bag coverage tables list a settled depth as well as an initial one; blow to the initial depth so the settled result is what you wanted. The real enemy is moisture: wet insulation of any type loses most of its value and does not fully recover when it dries.
Can I insulate over recessed lights?
Only if the fixtures are marked IC-rated, meaning rated for insulation contact. Older non-IC cans need a 3 inch clearance on all sides, which leaves a deliberate hole in your insulation and is a good reason to swap them for sealed IC-rated LED units before insulating. Either way, air seal around the fixture housing first, since recessed cans are among the largest air leaks in a typical ceiling.