Insulation Calculator

Background.

A typical attic job begins with a ruler pushed down into the old insulation and a question at the hardware store: how many bags will take this ceiling from R-19 to R-49? Two numbers settle it, and only one of them is about heat. The R-value decides how thick the finished layer has to be. The coverage printed on the package decides how many square feet each bag, roll or bundle will fill at that thickness. With the area, the coverage and a waste allowance in hand, the package count follows.

R-values stacked in layers simply add. R-19 already on the attic floor plus R-30 of new loose-fill gives R-49, which is why the calculator subtracts the existing level from the target and reports the difference as its own figure. That difference is the one to carry to the product label. For blown insulation you read the coverage from the R-30 row of the bag chart, not the R-49 row, because the new material only has to supply the gap.

The same physics explains why the first inches are worth more than the last. Conducted heat through a layer is proportional to 1/R, so going from R-19 to R-49 cuts the heat flowing through that layer by about 61%, while a further step from R-49 to R-60 trims only about 18% of what is left. Neither figure is a promise about your bills. Air leaking through gaps around light fittings, pipes and the attic hatch carries heat straight past the insulation, which is why air sealing comes first.

Seven assemblies are listed: attic floor, framed wall, floor, crawlspace, basement wall, roof deck and rim joist. The choice labels the result. The arithmetic is identical for all seven, since a square foot of coverage is a square foot wherever it goes. Which R-value each assembly should reach, and which product suits it, depends on your climate zone, local code and the manufacturer's instructions.

What is insulation calculator?

R-value is thermal resistance: how strongly a layer resists heat flowing through it. In US units it is measured in ft²·°F·h/Btu, so one square foot of an R-49 layer with a 1 °F difference across it passes 1/49 of a Btu each hour. The metric equivalent, RSI, is in m²·K/W, and one US R unit equals about 0.1761 RSI, which makes R-49 roughly RSI 8.63. Under the Federal Trade Commission's R-value rule (16 CFR Part 460), the R-values sellers quote must come from standard laboratory tests such as ASTM C518, run at a mean temperature of 75 °F. Labels must carry the sentence 'R means resistance to heat flow. The higher the R-value, the greater the insulating power.' An installed layer reaches its labelled value only at its labelled thickness, dry, uncompressed and with no air moving through it. A finished ceiling also gains a little resistance from the drywall and the thin films of still air on each surface; the estimate here deals only with the insulation layers you enter.

How to use this calculator.

1. 01Measure the surface that will be insulated. Type the total into Total Surface Area, or enter Length and Width in feet; when both are filled in and greater than zero they are multiplied and the area box is ignored. Clear one of them to go back to a typed area, which is easier for L-shaped attics or several walls added together.
2. 02Put anything that will not be insulated into Excluded Area: a boarded storage platform, a chimney chase, a large window in a basement wall. It must be smaller than the gross area.
3. 03Pick the assembly, then enter the existing and target R-values. If the old insulation has no label, measure its depth and multiply by a typical R per inch for the material (the FAQ below gives ranges). Enter 0 for a bare surface.
4. 04Enter the coverage per package exactly as the label states it for the R-value you are adding. Batts and rolls print square feet per package; for loose-fill, divide 1,000 by the bags-per-1,000-square-feet figure on the chart, so 25 bags per 1,000 sq ft means 40 sq ft per bag. The loose-fill chart also lists an initial installed thickness and a minimum settled thickness for each R-value; marking the installed depth on a few joists before you start shows when each area is done.
5. 05Set a waste allowance between 0 and 30% and enter the shelf price of one package. Review the package count and cost, and buy whole packages.

The formula.

The estimate runs in five steps. Gross area A is either the area you typed or length × width. Net area is A − X, where X is the excluded area. Waste-adjusted area is (A − X) × (1 + w), with w the waste percentage divided by 100. Packages = ⌈(A − X) × (1 + w) ÷ C⌉, where C is the coverage of one package and the brackets mean rounding up to the next whole number, because a store will not sell you half a bag. Material cost is that whole number multiplied by the price per package, so the cost covers what you actually carry out of the store. Separately, R to add = max(0, target − existing). It is reported for reference and does not enter the package count. The R-value reaches the count only through C, the coverage you read off the label, and that is where most errors creep in: loose-fill coverage per bag falls steeply as the R-value rises, because each bag is spread thicker. The model assumes the R-value of a layer stays the same everywhere across the surface. It ignores framing members, which conduct heat more readily than the insulation between them and pull a framed wall's whole-assembly value below its cavity rating, and it ignores settling, compression and air movement. Waste is applied to net area, before rounding, so a small change in waste can tip the count over a whole-package boundary.

A worked example.

Frequently asked questions.

References& sources.

1. [1]U.S. Department of Energy (n.d.). 'Insulation.' Energy Saver.
2. [2]Federal Trade Commission (2025). '16 CFR Part 460: Labeling and Advertising of Home Insulation.' eCFR.
3. [3]ASTM International (2022). 'ASTM C518-21: Standard Test Method for Steady-State Thermal Transmission Properties by Means of the Heat Flow Meter Apparatus.' ASTM.
4. [4]National Institute of Standards and Technology (2008). 'Guide for the Use of the International System of Units (SI), NIST Special Publication 811.' U.S. Department of Commerce.
5. [5]Bureau International des Poids et Mesures (2019). 'The International System of Units (SI Brochure), 9th edition.' BIPM.