Insulation Calculator

About Insulation Calculator

Insulation Calculator works out two things at once: how much insulation to buy, and what R-value the finished assembly will actually reach. Enter the area, pick your framing, material and target R-value, and read the packages and the effective R straight away. It works completely offline and nothing you enter ever leaves your phone.

HOW IT WORKS
The app follows the standard building-science method:

1. It measures the gross area from the length and height you enter.

2. Framing takes a share of that area, and insulation cannot go where wood already is. Walls use the standard parallel-path framing fractions — 25 percent at 16 inches on center and 22 percent at 24 inches on center, which already include plates, headers and corners. Attic and floor joists use the plain joist geometry, one 1.5 inch member per bay. What is left is the cavity area you actually fill.

3. A waste allowance you choose is added on top for cuts and offcuts.

4. The thickness you need is the target R-value divided by the material's R-value per inch. Batts and rolls are capped at the cavity depth; loose fill blown over ceiling joists is not.

5. A package holds a fixed volume of insulation, so its coverage changes with depth: rated coverage times rated thickness, divided by your thickness. Divide the order area by that and round up to whole packages.

6. Finally it averages the two heat paths — through the insulated cavity and straight through the framing at R-1.25 per inch — to give the effective whole-assembly R-value, plus the metric RSI.

Worked example: a 40 by 9 foot wall is 360 square feet gross. At 16 inches on center that leaves 270 square feet of cavity, or 297 square feet with 10 percent waste. Fiberglass at R-3.5 per inch needs 5.43 inches for R-19, so an 88.1 square foot bundle rated at 3.5 inches covers 56.8 square feet at that depth — 6 bundles. The cavity is R-19, but the wall as a whole comes out at R-15.6, or RSI 2.74.

WHAT YOU CAN DO
- Enter any area and read the cavity area with framing already deducted
- Switch between 16 and 24 inch on-center framing and see the difference instantly
- Choose fiberglass, mineral wool, cellulose, XPS, EPS or closed-cell spray foam
- Compare batts, rolls and blown-in bags at the depth your target R-value needs
- Add continuous exterior board insulation and watch the assembly R-value climb
- Load the recommended R-value for your climate zone for attic, wall or floor
- Open Details for a full step-by-step breakdown, the assembly R-value maths and the climate-zone table
- Learn why a wall never reaches the number printed on the batt

BUILT FOR CLARITY
A warm mineral-gold design puts the big number first, with the supporting figures right beside it. Light and dark themes, a tiny download, no clutter and no account required.

AN ESTIMATE, NOT AN ENERGY MODEL
R-values per inch are the midpoints of published Department of Energy ranges, and the climate-zone recommendations are the 2021 IECC Table R402.1.3 prescriptive minimums that DOE and ENERGY STAR publish. Real jobs vary with brand, installed density, settling, compression, air sealing and moisture control. Coverage figures differ between manufacturers, so always read the chart on the bag or bundle you buy, and confirm the R-values required where you build with your local building department. This app is for general information and education only and is not professional construction, energy or code-compliance advice.

PRIVACY
The app collects no personal data and works fully offline. Ads are served by Google AdMob, which may use an advertising identifier as described in Google's policies.

Download Insulation Calculator and get a quick, sensible picture of what your next job needs — and what it will actually perform at.