Measure each area
Area = length × width or height. Subtract the area you will not insulate, such as doors and windows, on that same area. Openings cannot exceed the gross area. Include each surface once.
Tool Switcher: Insulation
Project toolsTools
One product. One material order.
Insulation Calculator
Combine wall, ceiling, or floor areas, subtract openings, and estimate whole packages of batt or roll insulation using your product’s coverage.
- Openings excluded per area
- US and metric units
- Copy and print your order
Batts and rolls
Insulation Calculator
Subtract openings and combine areas into one material order.
Methodology
Net area first, whole packages last
Combine net coverage
Add all net areas for one selected product. Planned coverage = exact net area × (1 + allowance ÷ 100). Allowance starts at zero; enter your own cut and offcut allowance.
Read the packaging
Packages = ceiling(planned coverage ÷ coverage per package). Use the total coverage in one purchasable pack of batts or rolls. Package size, dimensions, and R-value vary; the 40 ft² starting value is an arithmetic example.
Price the order
Purchased coverage = whole packages × package coverage. Material cost = packages × your package price. A blank price produces no cost estimate. Labor, delivery, tax, and installation accessories are excluded.
Worked example
A 12 × 10 ft area with 20 ft² of openings
120 − 20 = 100 ft² exact coverage. A chosen 10% allowance gives 110 ft². At an illustrative 40 ft² per package, buy 3 packages, providing 120 ft². At an example $30 per package, materials cost $90. These are sample purchasing inputs, not a product recommendation.
Frequently asked questions
A second look at the estimate.
How many insulation packages do I need?
Subtract non-insulated openings from each measured surface, add the net areas, apply your allowance once, and divide by the coverage in one package. Round up at the end. Check that the product fits your cavities; area math does not plan individual batt cuts.
Does this work for insulation rolls?
Yes. Choose Rolls and enter total coverage in one purchase unit. If a pack contains several rolls, use that pack’s combined coverage and price. The result counts purchased packages, not individual pieces inside them.
Does R-value change the number of packages?
Only indirectly through the coverage of the product you select. The optional US product R-value is recorded on the ticket; it does not change the area formula, recommend a local R-value, convert to RSI, or calculate energy savings.
Can I calculate blown-in insulation bags?
No. Blown-in coverage depends on the manufacturer’s coverage chart, installed thickness, and target R-value. This batt and roll area calculation does not determine those bag quantities.
Does switching units preserve package coverage?
Yes. Dimensions, excluded opening area, and coverage per package all convert together. The package price and US product R-value remain unchanged.
What if openings cover the entire area?
The net area is zero and no packages are needed for that area. Openings greater than its gross area are an error. Do not enter zero dimensions to remove an area; use its Remove button.
Sources and limitations
Area-based purchasing of one selected batt or roll product. Package coverage must match the exact product, dimensions, R-value, and purchasable packaging. Starting dimensions and 40 ft² per package are arithmetic examples, not a manufacturer preset. No universal allowance or R-value is recommended; verify cavity fit and installation requirements separately.
This is a material purchasing estimate. It does not select a required R-value, design moisture control or ventilation, confirm fire clearances or code compliance, or estimate utility savings or tax incentives. Use your selected product’s label and installation instructions.
- Johns Manville: Unfaced Batts & Rolls
Manufacturer product format, dimensions and availability; does not establish a universal package coverage.
- Johns Manville: Unfaced, Kraft-Faced and Foil-Faced Fiberglass Data Sheet
Product properties and installation limitations; not a local R-value or code recommendation.
Last reviewed: October 5, 2026