Calculate exactly how much insulation you need for your project — walls, attics, crawl spaces, and more. Free, instant, and accurate.
An insulation calculator is a free online tool that helps you determine exactly how much insulation material you need for any home improvement project. Whether you are insulating an attic, walls, crawl space, or garage, this calculator takes your room dimensions and desired R-value and instantly tells you the insulation square footage, number of bags or rolls needed, and the estimated cost.
Instead of guessing at the store or buying too much (or too little), our free insulation calculator gives you a precise shopping list. It works for all common insulation types including blown in insulation, spray foam insulation, fiberglass insulation, cellulose insulation, batt insulation, roll insulation, and foam board insulation.
Calculating how much insulation you need is straightforward. Here is the step-by-step process our insulation calculator uses:
Measure the length and width of the space you want to insulate. Multiply them together to get the total square footage.
The R-value measures how well insulation resists heat flow. Higher R-values mean better insulating performance. The right R-value depends on your climate zone and where you are insulating. Our insulation R-value calculator recommends: R-13 for interior walls, R-19 for floors and crawl spaces, R-30 to R-60 for attics depending on your climate.
Different insulation materials have different R-values per inch of thickness, which determines how much material you need:
| Insulation Type | R-Value per Inch | Best For | Cost per Sq Ft |
|---|---|---|---|
| Blown-In Cellulose | R-3.2 to R-3.8 | Attics, enclosed walls | $0.80 – $1.50 |
| Blown-In Fiberglass | R-2.2 to R-2.7 | Attics, open spaces | $0.70 – $1.30 |
| Fiberglass Batts | R-3.1 to R-3.4 | Walls, floors, ceilings | $0.50 – $1.50 |
| Spray Foam (Open Cell) | R-3.5 to R-3.7 | Walls, attics, rooflines | $1.00 – $2.00 |
| Spray Foam (Closed Cell) | R-5.5 to R-6.5 | Crawl spaces, basements | $1.50 – $3.50 |
| Roll Insulation | R-3.0 to R-3.3 | Large open areas | $0.40 – $1.20 |
| Foam Board (Rigid) | R-3.8 to R-6.5 | Basement walls, exterior | $0.75 – $2.00 |
Our insulation calculator divides your total square footage by the coverage per bag or roll (based on your chosen R-value) to determine exactly how many bags, rolls, or board feet you need. It then multiplies by the average cost per unit to give you an insulation cost estimate.
To use our wall insulation calculator, measure each wall's height and width. Subtract the area of any windows and doors (a standard window is about 15 sq ft, a door about 21 sq ft). The remaining area is what you need to insulate. Most wall insulation uses R-13 or R-15 fiberglass batts or blown in insulation for enclosed cavities.
For blown in insulation, the calculation depends on the desired depth. Our blown in insulation calculator determines the required thickness (in inches) for your target R-value, then calculates the number of bags based on the manufacturer's coverage chart. A typical bag of cellulose insulation covers about 40 sq ft at R-13 or about 14 sq ft at R-49.
Our spray foam insulation calculator works in board feet. One board foot equals 1 sq ft at 1 inch thick. To calculate: multiply your square footage by the required thickness. For example, 500 sq ft at 3 inches thick = 1,500 board feet. The spray foam insulation cost calculator then estimates your total based on whether you choose open-cell or closed-cell foam.
An insulation calculator is essential in these common scenarios:
The right insulation R-value depends on where you live and what part of the house you are insulating. The U.S. Department of Energy divides the country into climate zones 1 through 7, each with recommended R-values. Here is a simplified guide:
| Location | Climate Zones 1-3 (South) | Zones 4-5 (Central) | Zones 6-7 (North) |
|---|---|---|---|
| Attic / Ceiling | R-30 to R-49 | R-38 to R-60 | R-49 to R-60 |
| Exterior Walls | R-13 to R-15 | R-13 to R-21 | R-13 to R-21 |
| Floors / Crawl Space | R-13 | R-19 to R-25 | R-25 to R-30 |
| Basement Walls | R-5 to R-11 | R-11 to R-15 | R-15 to R-19 |
Use our insulation R-value calculator above — just select the location and your climate zone, and it automatically recommends the right R-value for you. Understanding how to calculate R-value of insulation is important: it is the measure of thermal resistance per inch of material thickness. To reach a total R-value, divide the target by the R-value per inch of your chosen material to find the required thickness. For example, to reach R-49 with cellulose (R-3.5/inch), you need about 14 inches of material.
Need other building material calculations? Try our free tools: