Enter your attic square footage, what is up there now, and what you are shooting for. This gives you inches of new material, bag count, and material cost using the coverage numbers off the bag you are actually buying, not a hardcoded guess about your product. Guess the bag count low and you either eat a second supply run or leave the attic short of the R-value that is on the paperwork.
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The values below are starting points. Replace every one of them with your own — the answer is only as good as what you put in.
Floor area you are covering, measured flat. Leave out vaulted sections and anything over a garage or porch.
Set to 0 for a bare attic. Otherwise measure depth at the drywall and multiply by the old material's R-per-inch, then round down.
R-49 is a starting point, not a code citation. Targets move with climate zone and with what the homeowner is buying. Check your local code and replace this with the number you are quoting.
Read it off your bag: divide any R-value on the coverage chart by the settled thickness next to it. 3.5 is a starting point for loose-fill cellulose, not a fact about your product.
Pull two rows off your bag chart, multiply square feet per bag by that row's settled inches for each, then average the two. 300 is a starting point; your bag is the truth.
What your supplier charges today, delivered. Replace this with your real number.
Covers hopper leftovers and the spots you blow past depth. 5 is a starting point; set it to 0 for raw material math.
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Order this many. Partial bags round up, and this is the number to compare against your empty-bag count when the job is done.
Settled depth of new material going on top of whatever is already there. Check it with a ruler pushed to the drywall, not the joist. Shown to two decimals; the bag count is computed at full precision, so re-multiplying the rounded depth by hand can land a hair off the totals below.
Bags only. Stack labor, machine, baffles, and air sealing on top before you quote it.
Cents rather than dollars, because a dollars-per-square-foot figure will not reconcile at two decimals. Multiply this by your square footage and divide by 100 and you land within a few cents of the material cost above — the gap is this field's own rounding. Sanity-check it against your last job on the same product before the number leaves your truck.
Target minus existing. If this is 0, the attic already meets your target and needs nothing.
Square-foot-inches of material the attic needs. This is the number your bag's coverage divides into.
The raw math with no overage, partial bags and all. Compare it to bags to buy to see what your waste percentage and the round-up are costing you.
Raw bags multiplied by one plus your overage percentage, before the round-up. Bags to buy is this number ceilinged, because the supply house does not sell partial bags.
Cross-check this against the row on your bag chart nearest the R-value gap you are closing, not your target R-value. On a top-off those are two different rows. If they are far apart, one of your two bag inputs is wrong.
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Every bag of blow-in has a coverage chart printed on it, listing square feet per bag at each R-value next to a settled thickness. Those rows look like separate facts. They are not. Multiply any row's square feet by that row's inches and you land on the same figure every time: square feet covered per bag at one inch of settled depth. Say a row reads 43.0 square feet at 8.6 inches settled. Multiply and you get 369.8, call it 370. Say the R-49 row on that same bag reads 26.3 square feet at 14.1 inches. That gives 371. A gap that small is the chart rounding, not a different bag. Run it on the rows of the bag in front of you and you will see the same thing. That single figure is what this calculator asks for, because it works at any target R-value on any product without you hunting for a matching row. Pull two rows off your bag, multiply each, average them, and enter that.
The expensive mistake in blow-in is hitting the depth with too few bags. Coverage charts assume a settled density, not just a thickness. Run the material fluffy out of the hose and a ruler will read full depth while the attic holds a lot less material than the estimate called for. It looks finished. It settles over the first winter, loses depth, and never performs at the R-value on the paperwork. The bag count is the number that has to be right. Depth is the field check that tells you the coverage is even and that you did not leave the eaves thin. Count your empty bags before you pack up and compare them to the estimate. Come in well under and you did not save material, you shorted the assembly, and the callback is yours.
Existing R-value is the input people guess at, and guessing high is what puts you short on bags. Measure depth in several spots with a ruler pushed down to the drywall, not to the top of the joist. Then multiply that depth by the R-per-inch of the old material, not the product you are about to blow. Old loose-fill fiberglass and old cellulose do not rate the same, and neither one still rates what it did the day it went in. Material that is matted, dusty, or has been walked on performs under its chart, so round your existing R-value down. If the attic has bare spots, thin runs near the eaves, or a stretch someone stored boxes on, estimate those areas separately. Averaging them away hides the exact place the heat is leaving.
Every figure below is made up to show the arithmetic, not pulled from a real job or a real invoice. Run yours. A 1,400 square foot attic with 5 inches of old blown fiberglass, measured at the drywall. At roughly 2.5 R per inch that is R-12.5, and because it is matted and dirty you call it R-12. Target is R-49, so the gap is 37. The cellulose you are blowing runs 3.5 R per inch, which puts 10.57 inches of new material on top, call it 10.6, and about 15.6 inches total in the attic when you walk out. Multiply 1,400 square feet by that depth and the attic needs 14,800 square-foot-inches of coverage (the tool multiplies at full precision, not the rounded 10.57 on screen). Say your bag covers 300 of those: 49.33 bags of raw math, and 5 percent for hopper leftovers and overshoot takes it to 51.8, which the supply house sells you as 52. At $15 a bag that is $780 in material, 55.71 cents a square foot, before labor, machine, baffles, and air sealing.
This is a material calculator. It counts bags and multiplies by what you pay for one. It does not price labor, blower rental or delivery, baffles at the eaves, air sealing the top plates and penetrations, dam-and-cover work around can lights and chimneys, decking you have to pull, or the catwalk you rebuild so the furnace stays serviceable. Those lines can easily add up to more than the insulation. Price them separately and stack them on top. Two things the bag count also cannot tell you: whether the eave depth physically allows the finished thickness without choking the soffit vents, and whether the ceiling is sealed well enough that the R-value you install is the R-value the house gets. Air seal first. Insulation over a leaky ceiling underperforms every chart there is.
About 49 bags at the 5 percent overage this page starts with. That is a bare 1,000 square foot attic going to R-49 with a cellulose that rates 3.5 R per inch and covers 300 square-foot-inches per bag: 14 inches of new material, 46.67 bags of raw math, 49.00 once the overage is added, and 49 bags to buy. Set the overage field to zero and it is 47. Change the R-value gap or the bag and the answer moves, so run your own square footage, existing R-value, and the coverage printed on the bag you are actually buying.
It varies by product. Loose-fill cellulose commonly lands near 3.5 R per inch settled, and loose-fill fiberglass runs lower. The only number that matters is the one on the bag in your truck, which is why this calculator makes you enter it instead of assuming. Divide any R-value on that bag's coverage chart by the settled thickness listed next to it and you have the real R-per-inch for that specific product.
Divide 49 by your product's R-per-inch. At 3.5 R per inch that is 14 inches of settled material. At 2.5 it is 19.6 inches. If the attic already has insulation, subtract its R-value first: going from R-12 to R-49 at 3.5 R per inch needs 10.57 inches of new material, not 14. Then confirm the eaves can take the finished depth without blocking the soffit vents.
Yes, in almost every attic, and it is the normal way this work gets done. Blow-in goes over old blown-in or over old batts without stripping anything, as long as what is up there is dry and not contaminated. Do not add a product with a facing or vapor retarder on top, because the facing traps moisture in the assembly. Air seal the ceiling penetrations before you cover them; you will not reach them afterward.
Usually because the chart row you read is for a different R-value than the one you are installing, or because you mixed one product's R-per-inch with another product's coverage. Both numbers have to come off the same bag. The other common cause is entering existing R-value too high, which shrinks the gap and undercounts bags. This calculator also adds a waste percentage on top, which you can set to zero.
No. It prices material only: bags, and what you pay per bag. Labor, blower rental, baffles, air sealing, and dam work around can lights and chimneys get priced separately, and they can easily add up to more than the insulation itself. Treat the material number as the floor of your estimate and build your labor and equipment lines on top of it.
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