Insulation Calculator
By Taro Schenker | Updated February 11, 2026
Quick Answer: To calculate insulation, divide your target R-value by the R-per-inch rating to get required depth, then divide total area by coverage per unit. R-per-inch values: fiberglass batt 3.2, mineral wool 3.3, blown cellulose 3.5, blown fiberglass 2.5, rigid XPS 5.0, rigid EPS 3.8, rigid polyiso 6.0. For batts, each unit covers about 10 sq ft (15" x 8 ft). Blown-in bags cover about 2.5 cu ft (cellulose) or 4 cu ft (fiberglass). Rigid foam sheets are 4 x 8 ft (32 sq ft). Add 5-10% for waste. A 1,000 sq ft attic at R-38 with fiberglass batts needs about 100 batts at 11.9 inches deep.
Calculate how much insulation you need for your walls, attic, or floors. Choose between batt, blown-in, or rigid foam insulation and get the exact quantity, required depth, total weight, and estimated cost for your project.
How to Calculate Insulation
The formula for calculating insulation depends on the type you choose, but the core steps are the same:
Area (sq ft) = Length (ft) x Width (ft)
Units Needed = Area / Coverage per Unit
- Measure the length and width of the area to insulate in feet.
- Determine the R-value required for your climate zone and application (walls, attic, or floor).
- Choose your insulation type — each has a different R-value per inch, which determines the thickness needed.
- Divide the total area by the coverage per unit (batt, bag, or sheet) to get the quantity needed.
- Add 5-10% for waste from cutting and fitting around obstacles.
R-Value Guide
R-value measures thermal resistance — the higher the number, the better the insulation performs. Here is how much thickness each type requires to reach common R-values:
| R-Value | Fiberglass Batt | Blown Cellulose | Rigid XPS | Recommended Use |
|---|---|---|---|---|
| R-13 | 4.1" | 3.7" | 2.6" | 2x4 interior walls |
| R-19 | 5.9" | 5.4" | 3.8" | 2x6 exterior walls |
| R-30 | 9.4" | 8.6" | 6.0" | Floors, cathedral ceilings |
| R-38 | 11.9" | 10.9" | 7.6" | Attics (mild climate) |
| R-49 | 15.3" | 14.0" | 9.8" | Attics (cold climate) |
Insulation Types Compared
- Fiberglass Batt: The most common type for new construction. Pre-cut to fit 16" or 24" on-center stud bays. R-3.2 per inch. Affordable and easy to install in open walls, but can leave gaps if not fitted carefully.
- Mineral Wool Batt: Denser and more fire resistant than fiberglass. R-3.3 per inch. Provides better soundproofing and holds its shape when cut. Slightly more expensive but easier to work with.
- Blown Cellulose: Made from recycled paper treated with fire retardant. R-3.5 per inch. Excellent for retrofitting attics and filling irregular cavities. Requires a blowing machine for installation.
- Blown Fiberglass: Loose-fill fiberglass blown into place. R-2.5 per inch. Lighter than cellulose and does not absorb moisture. Good for attic top-ups where weight is a concern.
- Rigid XPS (Extruded Polystyrene): Blue or pink foam boards. R-5.0 per inch. Excellent moisture resistance. Ideal for basement walls, exterior sheathing, and under-slab applications.
- Rigid EPS (Expanded Polystyrene): White foam boards. R-3.8 per inch. Most affordable rigid foam option. Common in ICF (insulated concrete form) construction and EIFS systems.
- Rigid Polyiso: Foil-faced foam boards. R-6.0 per inch — the highest R-value per inch of any common insulation. Best for roof assemblies and continuous exterior insulation. Performance decreases in cold temperatures.
Formula Summary
Depth Formula: Calculate the required insulation depth by dividing the target R-value by the insulation material's R-value per inch.
$$D_{\text{in}} = \frac{R_{\text{target}}}{R_{\text{per inch}}}$$
Batt Count: $$\text{Batts} = \lceil \frac{A_{\text{sq ft}} \times (1 + W\%)}{W_{\text{batt}} \times L_{\text{batt}}} \rceil$$ where batt width is 15/12 ft (16" OC) or 23/12 ft (24" OC), and batt length is 8 ft.
Blown-In Bags: $$\text{Bags} = \lceil \frac{A_{\text{sq ft}} \times D_{\text{in}} / 12 \times (1 + W\%)}{V_{\text{bag}}} \rceil$$ where $V_{\text{bag}}$ = 2.5 cu ft (cellulose) or 4 cu ft (fiberglass).
Rigid Foam Sheets: $$\text{Sheets} = \lceil \frac{A_{\text{sq ft}} \times (1 + W\%)}{32} \rceil$$ (4 x 8 ft sheets = 32 sq ft each)
| Insulation Type | R-per-inch | Weight (lbs/sq ft) |
|---|---|---|
| Fiberglass Batt | 3.2 | 0.5 |
| Mineral Wool | 3.3 | 0.8 |
| Blown Cellulose | 3.5 | 1.2 |
| Blown Fiberglass | 2.5 | 0.7 |
| Rigid XPS | 5.0 | 0.3 |
| Rigid EPS | 3.8 | 0.2 |
| Rigid Polyiso | 6.0 | 0.3 |
Standard Assumptions: Waste factor of 5-10% accounts for cutting around outlets, pipes, and irregular framing. Batt length is 8 ft. Stud spacing is 16" or 24" on center. Blown cellulose settles approximately 5% after installation.
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- Plywood Calculator — estimate plywood sheets for subfloors and sheathing
- Shingle Calculator — figure out how many shingles you need for your roof
Estimates are for planning purposes only. Consult a qualified contractor for critical projects. Actual material requirements may vary based on site conditions, waste, and installation methods.
Pricing last verified: February 2026. Material prices vary by region, season, and supplier. Prices shown are national averages for planning purposes.