Calculated result
7packages
Batts or rolls to order
Formula
EXT-INSUL-001Length × width × (1 + waste) ÷ package coverage
240 ft²
Net coverage area
264 ft²
Area including waste
Exterior calculator
Insulation calculator
Batt or roll count from net coverage area, product coverage and a visible cutting allowance.
By Feroz Sheikh · Formula EXT-INSUL-001 v1.0.0 · Tested formula
Formula
EXT-INSUL-001v1.0.0Project inputs
Enter your measurements. Switching units converts the same project, including coverage and unit rates. Package yields keep the unit printed beside each field.
Refine measurements and rates at any time. Your current values are included in the material-list PDF.
Planning estimate, not engineering advice. Confirm site measurements and product specifications. Calculation limits.
This calculator counts batts or rolls from net area and the product coverage you enter. Formula EXT-INSUL-001 v1.0.0. Blown-in fill is a separate bags-per-thousand tool. Drop the packages into an estimate. It is not an R-value design.
- Measure the actual cavity or attic area, not the building footprint alone.
- Enter net coverage from the selected insulation package.
- Increase allowance around openings, wiring and irregular bays.
Size chart
Batt insulation chart (40 ft² coverage, 10% waste)
Coverage comes from the package. Blown-in fill is a separate bags-per-thousand tool.
| Area | Primary | Check |
|---|---|---|
| 12 × 10 ft | 4 packages | 120 ft² |
| 20 × 12 ft | 7 packages | 240 ft² |
| 24 × 16 ft | 11 packages | 384 ft² |
| 30 × 20 ft | 17 packages | 600 ft² |
Metric example — 6 × 3.6 m wall. A 6 × 3.6 m net area at 3.7 m² per batt with 10% waste returns 7 packages.
- DOE. Batt coverage comes from the package. R-value is a product choice, not this count. U.S. DOE — types of insulation
Add your project costs
Enter supplier prices and any labour, equipment or delivery costs. Pick a currency — badges, inputs, and totals follow.
Costs use the square or package rate you type. Construclytic does not fetch siding or insulation prices. Trim, wrap and labour stay companion lines. For GCC projects, select QAR, AED, SAR, KWD, OMR or BHD and enter local supplier rates.
How this calculator works
A transparent insulation calculator takeoff.
Feroz SheikhWritten by Feroz Sheikh · founder
This calculator divides a rectangular coverage area by the selected batt or roll coverage and rounds to whole packages after a visible cutting allowance.
Batt insulation is net cavity area ÷ the selected batt coverage, then bags or bundles. This does not pick R-value or a climate zone. Blown-in is a different calculator with a different bag yield.
Worked example. Example: a 20 ft × 12 ft attic is 240 ft². With 10% waste and 40 ft² batts, the purchase quantity rounds to 7 batts or rolls.
See the example inputs and calculated result
This separate example uses the calculator's starting values in imperial units. It stays unchanged when you edit the tool above. Example rates, yields and dimensions are assumptions, not recommendations for your project.
| Input | Example value |
|---|---|
| Length | 20 ft |
| Width or height | 12 ft |
| Coverage per batt or roll | 40 ft² |
| Waste factor | 10 % |
Example result: Batts or rolls to order — 7 packages.
Length × width × (1 + waste) ÷ package coverage
Use the labelled units above when reproducing this example. To work in metric, switch the calculator to Metric; applicable measurements are converted, while product-specific units remain labelled.
- 1. Measure the actual cavity or attic area, not the building footprint alone.
- 2. Enter net coverage from the selected insulation package.
- 3. Increase allowance around openings, wiring and irregular bays.
Field notes that change the number
- Stud and joist spacing changes the batt count. 16 in O.C. is not 24 in O.C.
- Rim joists and band boards are extra lines.
- Vapor control is a separate product in many GCC and cold-climate walls.
Common mistakes
- Using floor area as wall cavity area.
- Mixing batt coverage with blown-in bags.
- Calling this an energy-code stamp.
Need the broader workflow? Read the exterior planning guide.
How-to: How to read a blown-in insulation bag chart · Foam board sheet count: openings, cuts and usable panels · How to plan a furnace BTU load
Questions
Questions about insulation calculator
Does this select an R-value?
No. Product R-value, cavity depth and climate requirements must be confirmed independently.
Can I use this for blown-in insulation?
Only if you enter the bag or bag-coverage stated by that product. This tool does not convert R-value into blown depth.
How many batts of insulation for a 20x12 wall?
A 20 × 12 ft net area at 40 ft² per batt with 10% waste returns 7 packages. Blown-in fill is a separate bags-per-thousand tool.
Does this pick an R-value?
No. Coverage comes from the package in your hand. R-value is a product choice, not this count.
What is the formula ID?
This insulation calculator publishes formula EXT-INSUL-001, version 1.0.0. The ID stays on the result and on the estimate line.