Insulation Calculator
Your estimate
Thickness needed10 7/8 in10.86 in · 27.6 cm
Packs / bags30
- Area
- 1,200 ft²133.33 yd² · 111.48 m²
- Volume
- 40.21 yd³1,085.7 ft³ · 30.74 m³
How thick the insulation must be for an R-value — and how many bags or packs cover the area.
Updated 2026-10-20 · Based on: coverage figures from paint and drywall makers (USG).
How to use the Insulation Calculator
- Enter the attic or wall area.
- Enter the target R-value (R-38 to R-60 for US attics).
- Choose the insulation type.
- Type the coverage per bag or pack at that R-value from the label.
Formula
Thickness = R-value ÷ R per inch. Blown cellulose ≈ R-3.5 per inch, fiberglass batts ≈ R-3.2, blown fiberglass ≈ R-2.5, closed-cell spray foam ≈ R-6.5.
Metric R (m²K/W) = US R × 0.1761.
Worked example
With the values preset in the calculator above:
- Length
- 30 ft
- Width
- 40 ft
- Target R-value
- 38 R (US)
- Insulation type
- Blown cellulose
- Coverage per bag / pack
- 40 ft²
- Thickness needed
- 10 7/8 in (10.86 in)
- Area
- 1,200 ft² (133.33 yd²)
- Volume
- 40.21 yd³ (1,085.7 ft³)
- Packs / bags
- 30
Frequently asked questions
How thick is R-38 blown cellulose?
About 10 7/8 in.
What R-value do I need in the attic?
The US DOE recommends R-30 to R-60 depending on climate zone; R-49 to R-60 in cold northern zones.
Can I add new insulation on top of old?
Yes, if the old insulation is dry and not moldy. Unfaced batts or loose fill go on top.
More calculators
Estimates only — check quantities with your supplier and local building code before you order.