Insulation calculator

How to use the insulation calculator

Estimate how many insulation packages you need for a wall, attic, floor, or other area. Enter dimensions or a known area, then choose the target R-value, material, package coverage, waste, and optional price.

1. Choose Feet / inches or Metres / cm.
2. Enter Length and Width, or fill in Or a known area to override the dimensions.
3. Enter the target R-value and choose the insulation material.
4. Enter Coverage per package and Waste %.
5. Optionally enter Price per package (optional), then review packages, purchase area, thickness, and cost.

What to enter

Feet / inches or Metres / cm

Choose imperial or metric units. Imperial uses feet, inches, and sq ft. Metric uses metres, centimetres, and m².

Length

Enter the area length in feet for imperial or metres for metric. The default is 30 ft or 30 m, depending on the selected unit system.

plus inches

Add remaining inches in imperial mode or centimetres in metric mode. The default is 0.

Width

Enter the area width in feet for imperial or metres for metric. The default is 8 ft or 8 m, depending on the selected unit system.

plus inches

Add remaining inches in imperial mode or centimetres in metric mode. The default is 0.

Or a known area

Enter the area in sq ft or m². When provided, it overrides Length × Width.

R-value

Enter the positive target R-value. The default is R-13.

Material

Choose fiberglass, mineral wool, or rigid foam. This affects the suggested thickness only. The default is fiberglass.

Coverage per package

Enter the coverage printed on the package in sq ft or m². The default is 40 sq ft or 40 m², depending on the unit system.

Waste %

Add material for cuts, obstructions, and unusual spacing. The default is 5%. The allowed range is 0% to 50%.

Price per package (optional)

Enter a non-negative price to estimate material cost. Leave it blank to omit the cost result.

Example: 240 sq ft at R-13

Using the defaults in imperial mode: 30 ft × 8 ft = 240 sq ft. With 5% waste, 240 sq ft × 1.05 = 252 sq ft to purchase. At 40 sq ft per package, 252 sq ft ÷ 40 sq ft = 6.3 packages, which rounds up to 7 packages. For fiberglass, the suggested thickness is R-13 ÷ 3.2 R per inch = 4.0625 in, rounded to 4.06 in.

How the insulation estimate is calculated

The tool calculates area from Length × Width after converting feet and inches or metres and centimetres. If Or a known area is entered, that value replaces the dimension calculation. Purchase area = area to insulate × (1 + Waste % ÷ 100). Exact packages = purchase area ÷ Coverage per package, and the package count rounds up to a whole package. Suggested thickness = target R-value ÷ the material's default R-value per inch: fiberglass 3.2, mineral wool 4.0, or rigid foam 5.0. Results are rounded to two decimals where displayed. If a price is entered, material cost = whole packages × price per package, rounded to two decimals.

Reading the results

Packages to buy is the whole-package quantity after the waste allowance and rounding up. Area to insulate is the entered or calculated area. Area to purchase includes the selected waste percentage. Suggested thickness is a guide based on the selected material's code constant and is shown in inches in imperial mode or centimetres in metric mode. If you enter a price, Material cost shows the estimated total for the packages to buy.

Limitations and common mistakes

• The calculator estimates batts and rolls from area and package coverage. It does not calculate loose-fill or blown insulation bags from depth.
• Suggested thickness is based only on the target R-value and selected material's default R-value per inch. It does not select a product or verify the available framing cavity.
• The waste percentage must be between 0% and 50%, and package quantities are rounded up to whole packages.
• Check the target R-value for your location and project separately; the tool's disclaimer says target R-values depend on climate zone and current county energy code.

Frequently asked questions