Free Insulation Calculator
Calculate the insulation you need for an attic, wall or floor. Estimate square footage, packages, blown-in bags, spray foam board feet and insulation thickness based on your project.
Insulation bags, R-value, thickness, and cost calculator
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How Much Insulation Do I Need?
Insulation quantity is not one number for every house. You need the insulated surface area, the product type, the additional R-value you want, and the coverage printed on that product. A 1,000 sq ft attic, a 1,200 sq ft attic, and a 12×12 room with 8 ft walls are different jobs because attics use floor area and walls use wall area.
Batts and rolls are sold by labeled square feet per package. Blown-in cellulose and fiberglass are sold by bags whose coverage changes with R-value. Spray foam is estimated in board feet. Rigid foam is counted as whole boards.
How to Use This Insulation Calculator
- Choose fiberglass batt, mineral wool, blown-in cellulose or fiberglass, open- or closed-cell spray foam, or rigid board.
- Select attic, walls, ceiling, or floor / crawl space, then enter known square footage or dimensions.
- For walls, use perimeter × height. A 12×12 floor plan is not 144 sq ft of wall.
- Deduct doors and windows when those openings will not be insulated.
- Enter manufacturer coverage, bag-chart coverage, foam thickness, or board size. Edit waste once.
- Read packages, bags, board feet, or boards, then copy, share, or print the result.
Insulation Calculation Formulas
Area and waste
- Rectangle area = length × width
- Room wall area = 2 × (length + width) × wall height
- Wall run area = wall length × height
- Net area = max(0, gross area − openings)
- Ordering area = net area × (1 + waste % / 100)
Batts and rolls
Packages = ceil(ordering area ÷ labeled coverage per package)
Do not assume every R-13, R-19, or R-30 package covers the same square feet.
Blown-in insulation
Additional R = max(0, target R − existing R)
Chart bags = ceil(ordering area ÷ sq ft per bag at that R-value)
Volume (cu ft) = area (sq ft) × depth (in) / 12
Density-based bags = ceil(volume × installed density ÷ bag weight). That path is approximate, not a certified bag chart.
Spray foam board feet
Board feet = area (sq ft) × thickness (in)
A yield allowance is applied once: estimated board feet = board feet × (1 + waste % / 100).
R-value and thickness
Approximate inches = additional R ÷ product R-value per inch
R-value measures resistance to heat flow. Higher R means more resistance. Framing, air sealing, and compression change installed performance.
Insulation Calculator Examples
| Example | Net area | Primary result | Notes |
|---|---|---|---|
| 1,000 sq ft attic batts | 1000 sq ft | 28 packages required | 28 packages |
| 1,200 sq ft attic batts | 1200 sq ft | 33 packages required | 33 packages |
| 12×12 room walls | 384 sq ft | 11 packages required | 11 packages |
| Blown-in over existing R-19 | 1000 sq ft | 23 bags required | 23 bags |
| Spray foam board feet | 1000 sq ft | 3,300 board feet | 3300 bd ft |
- 1,000 sq ft attic batts, 10% waste, 40 sq ft/package: 1,100 sq ft to order, ceil(1100 ÷ 40) = 28 packages.
- 1,200 sq ft attic batts, 10% waste, 40 sq ft/package: 1,320 sq ft, 33 packages.
- 12×12 room, 8 ft walls: 384 sq ft gross. At 10% waste and 40 sq ft/package: 422.4 sq ft, 11 packages.
- 1,000 sq ft blown-in with an example 45 sq ft/bag chart value and 0% waste: ceil(1000 ÷ 45) = 23 bags. Replace 45 with your bag chart.
- 1,000 sq ft spray foam at 3 inches and 10% yield: 3,000 nominal board feet, 3,300 estimated board feet.
What R-Value Insulation Do I Need?
R-value is the insulation’s resistance to heat flow. Existing R-value is what is already installed. Additional R-value is max(0, target − existing). Attics, walls, floors, and crawl spaces use different targets. A state can contain more than one IECC climate zone.
The table below is ENERGY STAR recommended insulation for retrofitting existing wood-framed buildings. ENERGY STAR notes that the guidance is based on the 2021 IECC Residential Provisions Table R402.1.3. It is not a legally binding local building-code lookup, and new-construction assemblies can differ.
| Climate zone | Uninsulated attic | Attic with 3–4 in | Floor |
|---|---|---|---|
| 1 | R-30 | R-25 | R-13 |
| 2 | R-49 | R-38 | R-13 |
| 3 | R-49 | R-38 | R-19 |
| 4A and 4B | R-60 | R-49 | R-19 |
| 4C, 5, and 6 | R-60 | R-49 | R-30 |
| 7 and 8 | R-60 | R-49 | R-38 |
Wall retrofit notes from the same ENERGY STAR page: when siding is removed on an uninsulated wood-frame wall, blow the cavity; Zone 3 adds R-5 sheathing, and Zones 4–8 add R-5 to R-10 sheathing. Insulated 2×4 walls in Zones 4–8 add R-10 sheathing. Basement and crawlspace walls use R-5 / R-13 in Zone 3, R-10 / R-13 in 4A–4B, and R-15 / R-19 in 4C and 5–8. Framing and assembly details can change what your code official requires.
How Many Bags or Rolls of Insulation Do I Need?
Always round physical packages up. If ordering area is 440 sq ft and a roll covers 49 sq ft, you need 9 packages, not 8.98. Blown-in bags are not interchangeable with batt packages. A blown-in bag chart at R-49 covers far less area per bag than a batt package labeled 40 sq ft.
Batt vs Blown-In vs Spray Foam Insulation
| Type | Sold as | This calculator uses |
|---|---|---|
| Fiberglass or mineral wool batt / roll | Packages | Labeled sq ft per package |
| Blown-in cellulose or fiberglass | Bags | Bag chart or labeled density estimate |
| Open- or closed-cell spray foam | Board feet | Area × inches, yield once |
| Rigid foam board | Boards / packs | Whole boards from board size and layers |
Insulation Thickness and Depth
Nominal thickness is R-value divided by R per inch for that product. Illustrative starting points on this page are about R-3.2/in for fiberglass batts, R-4.0/in for mineral wool, R-3.5/in for cellulose, R-2.7/in for blown fiberglass, R-3.7/in for open-cell foam, and R-6.5/in for closed-cell foam. Those defaults are editable examples, not certified manufacturer ratings. Settled blown-in depth can be less than installed depth; use the bag chart when it lists both.
Common Insulation Calculation Mistakes
- Ignoring the coverage printed on the package or bag and using a generic R-13 or R-49 factor.
- Treating a 12×12 room as 144 sq ft of wall insulation. Wall area needs height.
- Skipping door and window deductions, or deducting openings from attic floor area by mistake.
- Assuming every blown-in bag covers the same square feet at every R-value.
- Counting spray foam as square feet only, without multiplying by inches to get board feet.
- Targeting a total R-value without subtracting existing insulation.
- Double-counting settling and waste, or treating a density estimate as a certified bag chart.
- Using a rough material cost as a fixed contractor quote or as guaranteed energy savings.
Frequently Asked Questions
How much insulation do I need for a 1,000 sq ft attic?
At 10% waste and 40 sq ft of labeled batt coverage per package, order 1,100 sq ft and buy 28 packages. Blown-in bag counts depend on the bag chart at your additional R-value, not on that batt coverage.
How do I calculate insulation square footage?
Attics and floors: length × width. Walls: 2 × (length + width) × wall height, or wall length × height. Subtract doors and windows. Do not use floor area as wall area.
How many bags of blown-in insulation do I need?
Read square feet per bag at the R-value you are adding from the manufacturer chart, then bags = ceil(ordering area ÷ sq ft per bag). Coverage changes with R-value, density, and brand. A density estimate is only an approximation.
How many rolls of fiberglass insulation do I need?
Use the square feet printed on the roll or batt package. Packages = ceil(ordering area ÷ labeled coverage). R-13, R-19, and R-30 products do not share one coverage number.
What R-value should I use for my attic?
ENERGY STAR retrofit guidance for existing wood-framed attics ranges from R-30 in Zone 1 (uninsulated) to R-60 in Zones 4–8. If you already have 3–4 inches, the add-on target is lower. Confirm your IECC climate zone and local code.
How thick is R-38 insulation?
Thickness = R-value ÷ product R per inch. At an illustrative R-3.2 per inch for fiberglass, R-38 is about 11.9 in. Blown-in and foam products use different R-per-inch values. Use the product data sheet.
How thick is R-49 insulation?
At R-3.2 per inch, R-49 is about 15.3 in. At R-3.5 per inch, it is 14.0 in. Manufacturer batt and blown-in charts list installed thickness; do not treat a simplified R-per-inch split as a certified depth.
How much insulation do I need for a 12×12 room?
Walls at 8 ft high are 2 × (12 + 12) × 8 = 384 sq ft, not 144 sq ft. With 10% waste and 40 sq ft per package that is 11 packages before door and window deductions.
Is spray foam better than fiberglass insulation?
They solve different jobs. Fiberglass batts are common in open cavities. Spray foam air-seals and is sold in board feet. Closed-cell has a higher R per inch than open-cell. Choose from cavity depth, moisture, budget, and a qualified installer—not from a single ranking.
How do I calculate spray foam board feet?
Board feet = area in square feet × thickness in inches. A 1,000 sq ft wall at 3 inches is 3,000 board feet. A 10% yield allowance once makes 3,300 estimated board feet.
Can I add insulation on top of existing insulation?
Often yes in attics, if the existing material is dry, evenly distributed, and compatible. Additional R-value = max(0, target − existing). Do not bury a vapor retarder in the wrong place, and do not compress batts.
How much extra insulation should I buy?
Waste is editable. Many batt jobs start around 5–10% for cutting. Spray foam uses a yield allowance. The calculator applies that percent once and then rounds packages, bags, or boards up.
What is the difference between R-13 and R-19 insulation?
R-19 resists heat flow more than R-13. In wood-frame walls, R-13 commonly fits a 2×4 cavity and R-19 a 2×6 cavity when the batt is uncompressed. Package coverage still comes from the label.
How much does attic insulation cost per square foot?
Enter your local package, bag, board, board-foot, and optional labor prices. This page does not hard-code a national average. The total is a planning estimate, not a contractor quote, tax credit, or energy-savings guarantee.
Does insulation bag coverage change with R-value?
Yes. Higher R-value blown-in usually means more depth, so each bag covers fewer square feet. Always use the row on the bag chart that matches the R-value you are installing.
Methodology, sources, and limits
- Area, waste, package, bag, board-foot, and board formulas above. Physical packages always round up. Net area cannot go below zero.
- ENERGY STAR recommended home insulation R-values for existing wood-framed buildings. Not a local code search.
- R-value per inch defaults are illustrative and editable. Manufacturer coverage charts and data sheets control purchasing.
- This page estimates material quantity in your browser. It is not an installation manual, energy-savings guarantee, tax-credit calculator, or contractor bid.