Your climate zone and what's already up there go in; the R-value gap comes out as settled inches, cubic feet and bags to blow — or batt layers and packages to buy — with the vapor-retarder verdict for your zone.
What Your Results Mean
Two takeoffs run side by side because an attic answers to its starting condition, not to a preference. The loose-fill panel converts the R-gap directly into material: depth comes from the gap divided by the material's settled R-per-inch, volume is the attic times that depth, and bags divide the volume by what one bag yields when blown. The batt panel picks standard batts — R-13, R-19, R-30, R-38, R-49 — that close the same gap, stacks two layers crosswise when the gap outruns a single batt, and counts packages at the printed coverage. Read whichever route your attic is actually built for.
The one input people get wrong is the existing R-value, and it's the one that moves everything. R-values add, so the honest attic inventory is depth times R-per-inch of what's up there — six inches of old fiberglass batt is R-19, a foot of blown fiberglass is R-28, and the difference between those two attics is eleven points of R that never needs buying. If the material looks like small silver-brown pebbles, stop: that's vermiculite, some of it carries asbestos, and it needs a test before anything else happens in that attic.
The bag count deserves its caveat read aloud: bag yields differ by product, and loose-fill bags print a coverage chart listing settled thickness and square feet per bag for each R-target. This calculator's yield field is editable precisely because the chart on your actual bag is the authority. Plan with the typicals; buy with the chart.
Reading the Lines
- Gap to fill: target minus existing. The whole order exists to close exactly this number — R-values stack, so the gap is real arithmetic, not a guess.
- Settled depth: loose-fill settles after blowing; R-per-inch and coverage charts are already settled values, so don't add a fudge on top.
- Volume: cubic feet of material, the number that fills a truck or a blower hopper. Metric mode reads it in cubic metres.
- Bags: volume divided by bag yield, rounded up. If the count lands within a bag or two of a pallet break, the extra bag is cheaper than the second trip.
- Batt plan: the smallest standard batts covering the gap — single layer if the gap is R-49 or less, two crosswise layers (seams broken, joists covered) above that.
- Vapor retarder: a zone verdict, not a universal rule — the assumptions section spells out the reasoning and the one trap to never set.
What the Calculator Does Not Do
It sizes the material order, not the project. It can't see your air leaks — sealing top plates, penetrations and the hatch comes before insulation, and it's miserable work after. It doesn't price the job: blower rental, bag prices and installer quotes move by region and season, and the bag-count row is the input for that arithmetic, not the answer. Wall and cathedral assemblies are different problems — wall cavities want their own R and their own moisture logic, and there's no vented attic behind them. And it can't inspect: soffit baffles, 3-inch clearances at chimneys and flues, and boxed non-IC can lights are code-and-fire matters your local inspector owns.
Pro Tip
Buy by settled depth and coverage chart, never by bag weight. Two bags of "cellulose" at the same price can yield different cubic feet, and the label's coverage table — square feet at a stated R — is the only number that makes bag-count arithmetic honest. The attic insulation guide walks the whole method by hand.
The Math Behind the Calculator
The loose-fill side is one division and two multiplications; the batt side is a small search:
area = length × width
gap = target_R − existing_R
depth = gap ÷ R_per_inch (settled)
volume = area × depth ÷ 12 (cu ft)
bags = ceil(volume ÷ bag_yield_cuft)
packages = ceil(area ÷ package_coverage) per layer
The targets come from the Department of Energy's climate-zone attic recommendations — the table the zone selector draws from:
The zone field fills the range floor; the target stays editable because codes and owners aim higher. The R-per-inch constants are the typical settled values, and the batt search picks from the standard product set:
Two batt layers are chosen when the gap exceeds R-49, and the second runs crosswise — perpendicular to the first — so the seams of one layer sit under solid material of the other and the joists themselves stop acting as thermal bridges. Package counts apply the coverage field to each layer; metric mode converts lengths and areas, while bag and package counts stay in pieces because the labels do.
Assumptions & Limitations
Every number above rests on these constants. If your attic or product differs, change the matching input — or read the linked guide for the full reasoning.
- Loose-fill R-per-inch: cellulose 3.2, rock wool 2.9, blown fiberglass 2.3 — typical settled values; the bag's printed coverage chart wins
- Bag yields: cellulose 10, rock wool 13, fiberglass 33 cu ft per typical bag — editable, because products genuinely differ
- Existing R estimated as depth × R-per-inch; compressed material earns less — measure the loft actually present
- DOE zone floors: 30 (Z1–3), 38 (Z4), 49 (Z5–8); the DOE range tops out at R-60 and the target field accepts any value
- Vapor retarder verdicts: zones 1–2 skip (dry outward), zone 3 defers to local code, zones 4–8 keep retarders on the warm side of walls — and never new poly over loose fill on an attic floor
- Batt plan caps at two layers; gaps beyond R-98 are loose-fill territory and the panel says so
- Metric mode converts lengths, areas, depths and volumes; bag and package counts are piece counts everywhere
- Air-sealing, soffit baffles, chimney/flue clearances, IC-rated fixture rules and local code enforcement belong to the installer and inspector
Worked Example: The 1,200 Square Foot Zone 5 Attic
The same attic walked through in the attic insulation guide, so every number below is checkable against it. Leave the defaults as they load: 30 × 40 ft attic, Zone 5, six inches of old fiberglass batt recorded as R-19 existing:
- Area: 30 × 40 = 1,200 sq ft.
- Gap: R-49 target − R-19 existing = R-30.
- Cellulose: 30 ÷ 3.2 = 9.4 in settled; volume 1,200 × 9.375 ÷ 12 = 938 cu ft; at 10 cu ft per bag, 94 bags.
- Batt route: one layer of R-30 (nominal 9¼ in) — 27 packages at 45 sq ft each.
Now stress it. Set existing to 0 — the empty-attic case: the gap becomes R-49, cellulose deepens to 15.3 in (1,531 cu ft, 154 bags), while the batt route still finds a single R-49 layer at 14 in — 27 packages, the same count, because packages follow area, not R. That's the arithmetic behind the advice that an uninsulated attic is the one place insulation money behaves like an investment: the first R is the cheapest heat anyone ever buys.
Change the material to blown fiberglass at the same R-30 gap: 13.0 in deep, 1,304 cu ft, 40 bags at the typical 33-cu-ft yield — fewer bags, more inches, the trade the chart makes visible. And push the target to R-60 on the empty attic: the gap of 60 keeps cellulose at 18.8 in, and the batt panel splits to R-30 + R-30 crosswise — two layers because no single standard batt reaches R-60, the second one breaking the seams of the first.
Run your own attic before believing any of it — the ruler beside the joist, not the realtor's "well insulated" from the listing. And when the numbers point at an order, read the coverage chart on the actual bag or package you're buying, because the label is the tested number and the typical is only the neighborhood.