Softonic

Insulation Calculator

Step 1 / 3 Bags needed
Next

Insulating a wall, attic or floor starts with one question: how much do you actually need to buy? Enter the length and width of the area, the coverage each bag or roll provides at your target R-value, and a waste factor for trimming and gaps. The calculator returns the total square footage, the number of bags or rolls to order, and an optional cost estimate if you add a price per bag. It works for blown-in cellulose, fiberglass batts and mineral wool alike.

How to estimate insulation

  1. 1

    Measure the area

    Enter the length and width of the surface you want to insulate, in feet. The calculator multiplies them into total square footage.

  2. 2

    Enter coverage and waste

    Read the coverage per bag or roll from the product bag at your target R-value, then add a waste factor (usually 5 - 15%) for trimming and gaps.

  3. 3

    Read bags and cost

    The result shows the waste-adjusted area, the number of bags or rolls to buy rounded up, and the total cost if you entered a price per bag.

The formula

The math behind an insulation estimate is short:

  • Area = length × width
  • Adjusted area = area × (1 + waste % ÷ 100)
  • Bags needed = ceil(adjusted area ÷ coverage per bag)
  • Total cost = bags needed × price per bag

Coverage per bag is the square footage one bag (or roll) covers at the depth required for your target R-value, always round the bag count up, because you cannot buy a fraction of a bag.

Worked example: a 20 × 15 ft attic

  • Area: 20 × 15 = 300 sq ft
  • Waste at 10%: 300 × 1.10 = 330 sq ft
  • Coverage per bag at R-38: 40 sq ft
  • Bags: 330 ÷ 40 = 8.25 → round up to 9 bags
  • At $32 per bag: 9 × 32 = $288

Typical coverage by R-value

Target R-value Application Coverage per bag (sq ft)
R-13 2×4 walls 80 - 110
R-19 Floors, 2×6 walls 55 - 75
R-30 Attic (warm) 45 - 60
R-38 Attic (cold) 35 - 45
R-49 Attic (very cold) 28 - 35

Coverage shrinks as the target R-value rises, because deeper coverage means fewer square feet per bag.

Common pitfalls

  • Reading the wrong coverage line. Bags list coverage for several R-values; pick the row for your target, not the easiest one.
  • Forgetting waste. Rim joists, around wiring, and odd-shaped cavities all eat material. Skipping the waste factor leaves you one bag short on the last cavity.
  • Rounding down the bag count. Always round up. A half bag short means an uninsulated gap.
  • Ignoring settling. Blown-in cellulose settles over time; buy to the installed depth on the chart, not the loose-fill depth.

Frequently Asked Questions

Multiply the area by your waste factor, divide by the coverage per bag at your target R-value, and round up. For a 300 sq ft attic at R-38 with 10% waste and 40 sq ft per bag, that is 9 bags.

For a simple rectangular attic, 5 - 10% is plenty. For walls with lots of studs, wiring and outlets, or oddly shaped rooms, use 10 - 15% to cover the extra trimming and gaps.

Yes. Enter the coverage per package from the product label, square feet per roll for batts, or square feet per bag at the installed depth for blown-in. The bag-or-roll count and cost work the same way for either.

No. The calculation runs entirely in your browser as you type. Nothing you enter, dimensions, coverage or price, is uploaded to a server, saved or shared.

Related Tools

BMI Calculator

Calculate body mass index from height and weight. Shows WHO category, healthy-weight range and limitations of BMI.

Social Security Calculator

Estimate a US Social Security retirement benefit from your PIA and claiming age. Compare monthly and annual amounts at 62, FRA 67 and 70.

Age Calculator

Calculate exact age in years, months and days from a birth date, plus total days, hours and the next birthday countdown.

Dog Age Calculator

Convert your dog's age to human years using modern veterinary research. Breed-size aware: small, medium and large dogs age differently.

Fahrenheit to Celsius Converter

Convert Fahrenheit temperatures to Celsius with the exact formula, Kelvin output and common reference points for weather, cooking and health.

Time Zone Converter

Convert a time between any two time zones, with daylight-saving shifts applied automatically.

Tool available in other languages