Insulation Calculator
Calculate how much insulation you need for walls or attics.
This tool is for informational and educational purposes only. It is not a substitute for professional financial, medical, legal, or engineering advice. See Terms of Service.
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Request a ToolHow to Use the Insulation Calculator
This calculator estimates how many insulation batts or rolls you need for a wall, attic, or any custom area. Here is how to get an accurate estimate:
- Choose your area type. Select Attic to enter simple length and width. Select Wall dimensions to enter room perimeter with height, and subtract doors and windows automatically. Select Custom area if you already know the square footage.
- Pick your R-value. R-value measures resistance to heat flow. Higher numbers mean better insulation. R-13 and R-15 are standard for 2x4 exterior walls. R-19 and R-21 fit 2x6 walls and floors. R-30 and R-38 are typical attic targets in most climate zones.
- Choose batt width. Select 15 inches if your studs or joists are spaced 16 inches on center (the most common residential spacing). Select 23 inches for 24-inch on-center framing.
- Set the waste factor. A 10% buffer accounts for cuts around obstacles, electrical boxes, and irregular spaces. Increase to 15% for attics with many obstructions or complex framing.
- Read your results. The primary result shows batts needed. The breakdown shows the equivalent roll count, R-value thickness, and coverage per batt so you can cross-check against store packaging.
Results update instantly. Use the Share button to send your inputs to a contractor or supplier, or Copy to paste into a shopping list.
About the Insulation Calculator
Insulation is sold in batts (pre-cut pieces) and rolls (continuous lengths you cut on site). Batts are the most common format for walls and floors. Rolls are often used in attics where you unroll the material across joists. A standard 15-inch batt for 8-foot walls is 93 inches long, covering approximately 9.69 square feet. A 23-inch batt covers about 14.86 square feet.
R-value determines the required product thickness. R-13 and R-15 both use standard 3.5-inch thick batts that fit a 2x4 stud cavity. R-19 uses 6.25-inch thick batts for floors or shallow cathedral ceilings. R-21 is a high-density 5.5-inch batt for 2x6 walls. Attic insulation targets R-30 (9.5 inches) to R-38 (12 inches) depending on your climate zone. The US Department of Energy provides climate zone maps to help you choose the right target for your location.
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Frequently Asked Questions
How many batts do I need for a 1,200 sq ft attic at R-30?
Using 15-inch wide batts (93 inches long) at a 10% waste factor, you would need approximately 137 batts for 1,200 square feet. Each batt covers about 9.69 square feet, so 1,200 divided by 9.69 equals 124 batts, rounded up to 137 with waste. Most contractors buy in bags of 8 to 16 batts, so you would typically purchase 9 to 18 bags depending on pack size. Rolls are often more practical for large attic areas since you can unroll them across joists without cutting every piece.
What R-value do I need for exterior walls?
For 2x4 exterior walls, R-13 or R-15 are the standard choices. R-13 is the code minimum in most US climate zones for 2x4 framing. R-15 is a higher-density version that fits the same 3.5-inch cavity and provides a modest improvement. For 2x6 exterior walls (common in cold climates), R-19 or R-21 are the typical choices. R-21 is a high-density batt that fits the 5.5-inch cavity with slightly better performance. Always check your local energy code, as requirements vary by climate zone and jurisdiction.
What is the difference between batts and rolls?
Batts are pre-cut insulation pieces, typically 93 inches long for 8-foot walls or 48 inches for short cuts. They are convenient for walls because you can friction-fit them between studs without measuring and cutting most pieces. Rolls are continuous lengths of insulation that you cut to size as you go. They produce less waste when covering large open areas like attics, where you unroll across joists. Rolls are also useful when insulating floors with irregular joist spacing. Both batts and rolls are available in the same R-values and widths, so the choice is mainly about application and personal preference.
Why subtract doors and windows from wall insulation calculations?
Doors and windows occupy space in the wall framing that does not get filled with batt insulation. A standard interior door rough opening is roughly 3 feet by 7 feet, about 21 square feet. A standard window rough opening is approximately 3 feet by 5 feet, about 15 square feet. Subtracting these openings from the gross wall area gives you a more accurate insulation quantity and prevents over-ordering. The calculator uses these average sizes as defaults. If your doors or windows are significantly larger or smaller, use the custom area option and compute the net area manually.