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Construction & Home
Insulation Calculator — Batt Rolls & Blown-In Bags by R-Value
How much insulation to buy for an attic, wall or floor by US DOE R-value zone
Insulation is sold by how much it resists heat flow — its R-value — not by how thick it looks. Buy too little and your attic bleeds heat all winter and bakes the house all summer; buy too much of the wrong product and you waste money and bury your ceiling joists. This calculator turns a simple length × width measurement into the number of batt rolls or blown-in bags you actually need to hit a target R-value, then rounds up to whole packages — because you can't buy half a bag.
The core math has two steps. First, the coverage area: area (sq ft) = length × width of the space you're insulating. A 25 ft × 40 ft attic floor is 25 × 40 = 1,000 sq ft. Second, the quantity, which depends on the product:
• Batt / roll insulation: each roll or bundle lists the square feet it covers at a given R-value (e.g. an R-30 attic batt bundle covers ~88 sq ft). Rolls = ceil(area ÷ coverage per bundle).
• Blown-in (loose-fill) insulation: each bag lists how many bags cover 1,000 sq ft at a target R-value, plus the settled depth. Higher R needs more bags and more depth. Bags = ceil(area ÷ coverage per bag).
How much R-value do you need? The U.S. Department of Energy splits the country into climate zones and recommends roughly R-30 to R-60 for attics depending on where you live — R-30 to R-49 in the warm South, R-49 to R-60 across the cold North. A widely used all-around target is R-49 for an attic, which this calculator uses as the default. Walls typically run R-13 to R-21, and floors R-25 to R-30.
Loose-fill also has a depth that goes with each R-value: fiberglass blown-in needs roughly 0.5 inch of settled thickness per R-1, so an R-49 attic wants about 16–18 inches of fill. The depth markers (cardboard rulers) you staple to the joists let you verify you actually reached the rating.
The biggest estimating mistakes are forgetting the waste factor (5–10% for trimming around joists, wiring and obstructions), ignoring framing that the batts have to fit between, and mixing up the coverage number for one R-value with the bag count for another. This tool keeps R-value, coverage, bag/roll count and depth lined up so you walk into the store with one honest number — and a few extra to be safe.
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Calculator
Fill in the fields and click "Calculate" for instant results.
📰 Formula
• Coverage area (sq ft) = length × width • Batt rolls = ceil( area × (1 + waste) ÷ coverage per bundle ) • Blown-in bags = ceil( area × (1 + waste) ÷ coverage per bag at target R ) • Settled depth (loose-fill fiberglass) ≈ R-value × 0.5 in per R-1 Material constants used: • Default attic target = R-49 (DOE; zones range R-30 to R-60) • Batt coverage example: R-30 attic bundle ≈ 88 sq ft; R-13 wall roll ≈ 40 sq ft • Blown-in coverage: bags per 1,000 sq ft rises with R-value (R-30 ≈ 23 bags, R-49 ≈ 38 bags per 1,000 sq ft, fiberglass) • Loose-fill fiberglass depth ≈ 0.5 in per R-1 (R-49 ≈ 16–18 in)
📰 Formula
• Coverage area (sq ft) = length × width • Batt rolls = ceil( area × (1 + waste) ÷ coverage per bundle ) • Blown-in bags = ceil( area × (1 + waste) ÷ coverage per bag at target R ) • Settled depth (loose-fill fiberglass) ≈ R-value × 0.5 in per R-1 Material constants used: • Default attic target = R-49 (DOE; zones range R-30 to R-60) • Batt coverage example: R-30 attic bundle ≈ 88 sq ft; R-13 wall roll ≈ 40 sq ft • Blown-in coverage: bags per 1,000 sq ft rises with R-value (R-30 ≈ 23 bags, R-49 ≈ 38 bags per 1,000 sq ft, fiberglass) • Loose-fill fiberglass depth ≈ 0.5 in per R-1 (R-49 ≈ 16–18 in)
🧪 Worked examples
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Example 3
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Example 4
⚠️ Common mistakes
- Buying for thickness instead of R-value — a 'looks full' attic can still be under-rated.
- Forgetting the 5–10% waste for trimming around joists, wiring and obstructions.
- Using one R-value's coverage number with another R-value's bag count.
- Skipping the settled-depth check, so loose-fill ends up shallower than the rated R-value.
- Insulating only part of the attic floor — gaps and uncovered edges kill the average R-value.
💡 Tips
- Look up your DOE climate zone first; most US attics target R-49, ranging R-30 (South) to R-60 (far North).
- Always add about 10% waste for cuts around framing, vents, wiring and the attic hatch.
- Staple depth markers (R-value rulers) to the joists so you can verify the settled fill depth.
- For loose-fill, read the bag's coverage chart for YOUR target R — bags per 1,000 sq ft changes with R-value.
- Don't block soffit vents — use baffles at the eaves so attic airflow stays open.
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❓ Frequently asked questions
How much insulation do I need for my attic?
Measure the attic floor (length × width) for the square footage, pick a target R-value for your climate (often R-49), then divide by the coverage the product lists at that R-value and round up. A 1,000 sq ft attic blown to R-49 takes roughly 38 bags of fiberglass loose-fill.
What R-value do I need for my attic?
The US Department of Energy recommends about R-30 to R-60 for attics depending on climate zone — R-30 to R-49 in the warm South and R-49 to R-60 across the colder North. R-49 is a solid all-around target and the default this calculator uses.
How many bags of blown-in insulation do I need?
Each bag lists how many bags cover 1,000 sq ft at a given R-value. Divide your area (plus waste) by the bag's coverage at your target R and round up. At R-49, fiberglass loose-fill runs roughly 38 bags per 1,000 sq ft.
How deep should attic insulation be for R-49?
For blown-in fiberglass, figure about 0.5 inch of settled thickness per R-1, so R-49 is roughly 16 to 18 inches deep. Cellulose is a bit denser and reaches the same R-value in slightly less depth — always follow the bag's coverage chart.
What is R-value in insulation?
R-value measures how well a material resists conductive heat flow — higher R means more resistance and better insulation. It's the number building codes and the DOE specify, which is why you buy insulation by R-value, not by inches of thickness.
How do I calculate how many batt rolls I need?
Find your area in square feet, then divide by the square feet each roll or bundle covers (printed on the package for that R-value) and round up. For a 400 sq ft wall with R-13 rolls covering 40 sq ft each, that's ceil(400 ÷ 40) = 10 rolls before waste.
Should I add a waste factor for insulation?
Yes. Add about 5–10% extra for trimming batts around joists, wiring, plumbing, vents and the attic hatch. This calculator defaults to a 10% waste factor, which you can adjust before it rounds up to whole bags or rolls.
Is blown-in or batt insulation better for an attic?
Blown-in (loose-fill) fills gaps and irregular spaces well and is fast over an open attic floor, while batts are tidy between evenly spaced joists. Many attics use both — batts on the floor topped with loose-fill to reach the target R-value.
Can I add new insulation over old attic insulation?
Usually yes — as long as the old insulation is dry and not moldy, you can add blown-in or unfaced batts on top to raise the total R-value. Don't use faced batts over existing insulation, since the second vapor barrier can trap moisture.