Attic Insulation Calculator — Area, Thickness and Cost
Enter attic length and width, target R-value and material R per inch. Estimate coverage area with waste, thickness in inches and material cost in USD.
Last updated · View algorithm, constants and default inputs (JSON)
About this calculator
Use project specifications for target R-value, material performance and local code. The thickness is calculated from your entered R-per-inch; the area and price are estimates with your waste allowance.
Worked example
calculated with the default values below
Area length 40 ft · Area width 24 ft · Target R-value 38 R · R-value per inch of material 3.2 R/in · Waste allowance 10 % · Price per square foot 1.2 USD/ft² Area to insulate 960.0 sq ft · Insulation thickness needed 11.9 in · Area including waste 1,056.0 sq ft · Estimated material cost 1,267.20 USD
General background references
Disclaimer
These calculators provide planning estimates only. Building codes, wiring regulations and safety requirements differ by country and change over time, and nothing here replaces a licensed professional or a permit. For structural, electrical or gas work, follow local code and have the installation inspected.
Implementation source
This record describes the implementation used by this page. It is not independent validation. External references, when available, support only the stated formula or definition; estimates and custom rules retain their own assumptions.
Formula and variables
t = R ÷ r ; A = L × W × (1 + w ÷ 100) ; C = A × p
- R
- target R-value
- r
- R-value per inch
- L
- area length (ft)
- W
- area width (ft)
- w
- waste allowance (%)
- p
- price per sq ft
t is insulation thickness in inches; A includes waste allowance; C is total material cost.
Frequently asked questions
How thick should my insulation be for a target R-38?
Thickness equals the target R-value divided by the material's R per inch, so R-38 with fiberglass at R-3.2 per inch needs about 11.9 inches of batt. Doubling the R-value doubles the thickness, while the waste allowance enlarges area rather than depth. A 40 by 24 foot attic of 960 sq ft at 10% waste and $1.20 per sq ft costs roughly $1,267.