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Attic insulation calculator

Attics are sold in bags and inches. Manufacturers print “bags per 1,000 sq ft at X inches / R-value.” This page uses your attic area and that chart so you are not guessing from a viral rule of thumb. Enter attic sq ft, bags-per-area (or depth + coverage), and waste. It does not inspect ventilation, baffles, or air sealing — those decide whether the inches you buy actually perform.

sq ft
From bag chart at target depth/R
5%
0510152025

+5% so you don't come up short

ORDER ESTIMATEATTIC
Bags to buy36
Attic area1200 sq ft
Waste / buffer5%
How it calculates
bags = ceil((attic sq ft ÷ 1000) × bags_per_1000 × (1 + 5%)). Use the chart row for your target depth/R.

Estimate only — not a contractor bid. Confirm bag yields and local codes before you pour or order.

When to use

Topping up an existing attic; comparing two blown products’ bag charts; planning a weekend blow with a rental machine.

When not to

Cathedral ceilings with no attic void. Spray foam kits. Wall cavities → insulation calculator. Moisture or pest remediation first — do not bury problems under cellulose.

Assumptions

You enter bags per 1,000 sq ft (or equivalent) from the bag chart for your target R / depth. Waste/settling buffer default 5–10%. No automatic climate-zone R table. Existing insulation inches are a subtraction you apply before ordering.

Worked examples

Example 1

1,200 sq ft attic, chart says 28 bags / 1,000 sq ft at target depth, waste 5%
→ 1.2 × 28 × 1.05 ≈ 36 bags

Confirm the chart row matches the R you want, not the marketing front.

Example 2

800 sq ft, topping up — enter the additional bags/1,000 for remaining inches, not the full rebuild row
→ depends on the remaining inches row

Mixing fiber types can be fine; burying knob-and-tube is not.

How it calculates

Attic area × bag chart rate with a small waste buffer.

  1. Enter attic footprint sq ft.
  2. Enter bags per 1,000 sq ft from the chart row for your target depth/R.
  3. Set waste; read bags.
attic area (sq ft)
bags = (area ÷ 1000) × bags_per_1000 × (1 + waste%)
round up whole bags

Common traps

  1. Chart row must match depth/R — wrong row = wrong truck.
  2. Baffles and soffit clearance eat edge coverage; measure the real insulated footprint.
  3. Settling — cellulose charts sometimes assume settled depth; read the footnote.
  4. Knee walls and storage platforms are not “the same 1,200 sq ft.”

Next tool

Walls / floors → insulation calculator.

Roof pitch before roofing work → roof pitch calculator.

FAQ

(sq ft ÷ 1000) × bags_per_1000 from the bag chart, plus a little waste.

Related

Estimate only — not a contractor bid or engineered takeoff. Confirm bag yields and local codes before you pour or order.