Bags is only half the answer
Every insulation calculator online gives you a bag count. The chart on
the bag gives you two numbers, and the manufacturer is explicit that both
have to be met: "Failure by the installer to provide both the
required bags and at least the minimum thickness will result in lower
insulation R-value." Blow the right bags too thin, or fluff too few
bags to the right depth, and you do not have the R-value on the label.
bags = area ÷ 1,000 × bags per 1,000 ft²
depth = the chart's minimum settled thickness
1,000 ft² of attic
R-30 12.4 bags at 10.5 in settled
R-49 21.5 bags at 16.5 in settled
Both, or neither.
The one place the constants are required by law
The FTC R-value Rule, 16 CFR Part 460, makes every bag of loose-fill sold
in the US carry a chart: bags per 1,000 square feet, maximum net coverage
per bag, minimum weight per square foot and minimum settled thickness,
for every R-value. Nothing else on this site has that guarantee. It is
also why a calculator has no excuse for guessing here — the number exists,
it is printed, and it is specific to the product you are buying.
Published coverage, open-blow attic
Owens Corning ProCat, the FTC-required disclosure. This is one product's chart — every loose-fill bag carries its own, and the bag in your hand is the authority.
A wall is not an attic
An attic is an open blow: material laid loose to a settled depth. A wall
is dense-pack — the same product driven in at two to three times the
density behind netting or into a closed cavity, so that it cannot settle
and leave a void at the top. That is why the wall rows publish an
installed density and much lower coverage per bag, and why a 2x4 wall at
R-15 takes more bags per square foot than an R-30 attic does.
The eaves take extra
Blown out to the roof line the material tapers into a wedge instead of
lying flat, so it takes more per foot — and it depends on the pitch,
because a shallow roof leaves less room to fill. The chart publishes the
extra bags per 100 feet of roof line at R-30, R-49 and R-60 across three
pitches, and the tool adds them when you give it a roof line length.
What R-value you need is not a product question
It is set by the energy code your jurisdiction has adopted and by your
climate zone, and it differs between an attic, a wall and a floor over
unheated space. This page will not pick one for you. What it does is turn
the R-value you have been given into a bag count and a depth you can
check with a ruler.
The rest of the envelope
Insulation goes in between framing the
framing calculator
counts, at the spacing that decides how much of the wall is cavity and
how much is stud. Above it, the
rafter calculator gives the
slope that sets how far you can blow out to the eaves and the
gutter calculator handles the
water coming off it. Inside, the
drywall calculator covers the
board that holds it all in, and the
carpet calculator the floor
below.
Common questions
How many bags of insulation do I need?
Read it off the bag's own coverage chart, which is what this page does. At R-30, a thousand square feet of attic takes 13 bags — the chart publishes 12.4 per thousand square feet, or one bag per 80.6 square feet. Going to R-49 takes it to 22. Those figures come from the manufacturer, not from a rule of thumb, because the law requires them to publish it.
Why does the depth matter if I have the right number of bags?
Because the manufacturer says both govern, in those words: "Failure by the installer to provide both the required bags and at least the minimum thickness will result in lower insulation R-value." R-30 means 12.4 bags per thousand square feet AND 10.5 inches settled. Blow the bags in too thin over too large an area, or fluff a smaller number of bags up to the right depth, and you do not have R-30 — you have something less, and no way to tell from looking.
Where do these coverage numbers come from?
The FTC R-value Rule, 16 CFR Part 460. It requires every bag of loose-fill insulation sold in the US to carry a chart giving bags per 1,000 square feet, maximum net coverage per bag, minimum weight per square foot and minimum settled thickness, for every R-value. It is one of the very few places in construction where the constants are legally required to exist and be printed. This page uses one manufacturer's published chart; the bag in your hand is the authority.
What R-value do I actually need?
A code question, and this calculator will not answer it. It follows your climate zone and the assembly, it is set by the energy code your jurisdiction has adopted, and it differs between an attic, a wall and a floor over an unheated space. R-30 to R-49 in an attic covers most of the US and none of that is a substitute for looking up your zone. What this page does is turn the R-value you have been told into bags and a depth.
Is a wall the same as an attic?
No, and the chart treats them separately. An attic is an open blow — material laid loose to a settled depth. A wall is dense-pack: the same product driven in at a much higher density behind netting or into a closed cavity, so it cannot settle. That is why the wall rows publish an installed density and a much lower coverage per bag. A 2x4 wall at R-15 takes 17 bags a thousand square feet against 12.4 for an R-30 attic, for less than half the R-value.
Do I need extra bags at the eaves?
Yes, and the chart publishes it. Blowing out to the roof line means the material tapers into a wedge rather than lying flat, so it takes more per foot of eave — and it depends on the pitch, because a shallower roof gives you less room. The chart gives extra bags per 100 feet of roof line at R-30, R-49 and R-60 for three pitches, and the tool adds them when you enter a roof line length.
What about baffles, lights and walkways?
All of them reduce the area you are actually covering, and none of them are in the arithmetic. Baffles keep the soffit vents clear at the eaves, older recessed lights need clearance unless they are IC-rated, and a boarded walkway is area you are not blowing over. Measure the ceiling you are actually insulating, not the footprint of the house.
Batts or blown-in?
For an existing attic, blown-in almost always. It fills round obstructions and over irregular joist spacing in a way batts cannot, and gaps are what destroy real-world performance — a batt installation with compressed edges and unfilled corners underperforms its label badly. Batts make sense in open framing you can access cleanly, and in walls being built rather than retrofitted. This calculator covers the loose-fill case because that is where the published chart lives.
Sources
The rare case where the constants are legally required to exist: the FTC R-value Rule, 16 CFR Part 460, makes the manufacturer disclose bags per 1,000 sq ft, maximum coverage per bag and minimum settled thickness for every R-value, and print it on the bag. This is one product's chart, named as theirs — a different loose-fill will have a different chart and the bag is the authority. Both halves of the disclosure are stored because the manufacturer is explicit that both govern: hitting the bag count without hitting the thickness, or the thickness without the bags, gives a lower R-value than the label. Almost every insulation calculator online reports only one of the two.
Unit conversions are exact by definition and need no source.
How we choose and cite these figures
