Insulation Calculator
Estimate everything you need to insulate a wall, attic, or floor from one page: gross and net area, batts or rolls required, blown-in bags by depth and R-value, a recommended R-value for your application, required depth, waste, and total cost. Wall mode deducts windows and doors automatically, so you order the right amount the first time.
Example Projects
Attic Batts
40 × 20 ft • R-49
Wall Batts
40 lf × 8 ft • R-15
Blown-In Attic
40 × 30 ft • Cellulose R-38
Crawl Space Floor
30 × 24 ft • R-25
Configure Insulation View Results
Insulation Estimate
View Calculation Breakdown
| Gross area | 0 ft² |
| Net insulation area | 0 ft² |
| Net area with waste | 0 ft² |
How this was calculated:
Everything behind the Insulation Calculator
Formulas, reference charts, and detailed answers — expand any section you need.
Recommended R-Value by Application
R-value measures resistance to heat flow. These are general planning ranges; local energy codes set specific minimums by climate zone and should always be confirmed before buying material.
| Application | Typical R-Value Range |
|---|---|
| Attic | R-30 to R-60 |
| Exterior Walls | R-13 to R-21 |
| Floors Over Unheated Space | R-25 to R-30 |
| Basement Walls | R-10 to R-19 |
Blown-In Bags Needed by Attic Area
Blown-in coverage per bag drops as you install to a higher R-value (a thicker layer). Divide your area by the bag coverage listed for your target depth:
| Attic Area | Bags Needed (R-38, typical coverage) |
|---|---|
| 800 sq ft | About 25 to 28 bags |
| 1,200 sq ft | About 38 to 42 bags |
| 1,600 sq ft | About 50 to 55 bags |
Actual bag count varies by manufacturer and product — always check the bag's coverage chart.
Approximate R-Value Per Inch by Material
Required depth to reach a target R-value equals the target divided by the material's R-per-inch. Higher R-per-inch means a thinner layer for the same performance.
| Material | Approx. R-Value Per Inch |
|---|---|
| Fiberglass Batts | ~3.2 per inch |
| Mineral Wool Batts | ~4.2 per inch |
| Blown-In Fiberglass | ~2.5 per inch |
| Blown-In Cellulose | ~3.5 per inch |
| Closed-Cell Spray Foam | ~6.5 per inch |
Insulation Waste Guide
| Situation | Recommended Waste |
|---|---|
| Simple open areas | 5% |
| Framing obstructions, wiring, plumbing | 10% |
| Cut-up or irregular spaces | 10 - 15% |
Insulation Calculator: How Much Insulation Do I Need?
Start by measuring the total square footage of the area you're insulating, whether it's an attic floor, exterior wall, or crawl space. For walls, subtract window and door area to get net area. Then divide net area by the coverage of a single batt, roll, or bag of your chosen insulation type to find the quantity needed.
Example: An 800 sq ft attic using batts that cover 40 sq ft per bundle requires 20 bundles, before adding waste for framing obstructions.
How Many Bags of Blown-In Insulation Do I Need?
Blown-in insulation coverage depends on both area and desired depth, since bag coverage decreases as you install to a higher R-value (a thicker layer). Manufacturers publish a coverage chart per bag at various depths and R-values. Divide your attic square footage by the bag coverage listed for your target depth to get the number of bags required. This calculator lets you enter the target R-value and coverage per bag, then rounds up and adds waste so you don't run short mid-blow.
What R-Value Do I Need for My Attic?
R-value measures insulation's resistance to heat flow, and required levels vary by climate zone and application. Attics typically need the highest R-value in a home, commonly R-30 to R-60 depending on region, since heat loss through an uninsulated attic is one of the largest sources of energy waste in most houses. The calculator suggests a target R-value for each application and shows the depth of your chosen material needed to reach it.
Fiberglass Batts vs. Blown-In Insulation
Batts are pre-cut panels that work well in open, unobstructed spaces like standard stud walls and joist bays. Blown-in insulation (fiberglass or cellulose) is loose fill, applied with special equipment, and better suited for attics with irregular framing or existing insulation, since it fills gaps and covers obstructions more completely than batts. Switch the calculator's application and insulation type to compare how many batts versus bags a job would take.
Insulation Cost Calculator
Insulation cost depends on the material, the area, and the R-value you're targeting. Batts and rolls are priced per bundle or roll; blown-in is priced per bag; spray foam is priced by the board foot (one square foot at one inch thick). Turn on "Estimate Cost & Budget" and enter your cost per unit and any delivery charge to get a total material budget in your choice of 150+ currencies.
Common Insulation Project Examples
- Attic Batts: An 800 sq ft attic (40 ft × 20 ft) with batts covering 40 sq ft per bundle needs 20 bundles before waste, or about 22 bundles with a 10% allowance.
- Exterior Wall: A 40 ft × 8 ft wall (320 sq ft gross) less two 3 ft × 4 ft windows and one 3 ft × 6.67 ft door (about 45 sq ft) leaves roughly 275 sq ft net — about 8 batt bundles at 40 sq ft each with 10% waste.
- Blown-In Attic: A 1,200 sq ft attic at R-38 typically needs about 38 to 42 bags of cellulose, depending on the bag's coverage chart.