Insulation Calculator

Please fill in the required fields with values greater than 0.
–area (sq ft)
–target R-value
–thickness needed (in)
–volume (cu ft)

Estimate only. R-value per inch varies by brand and density. Check the label and your local code for the R-value your area needs.

How to use this calculator

Enter the size of the area to insulate, such as an attic floor or a wall. Choose the target R-value and the type of insulation. Press Calculate to see the thickness you need and the total volume.

The formula

Thickness (in) = target R-value ÷ R-value per inch
Volume (cu ft) = area × thickness ÷ 12

Example: A 20 ft × 15 ft attic (300 sq ft) insulated to R-38 with blown cellulose (R-3.5 per inch) needs about 10.9 inches, or about 271 cubic feet.

Understanding your insulation estimate

The calculator gives two key numbers. Thickness tells you how deep the insulation must be to reach your target R-value. Volume tells you roughly how much material that takes. Together they help you compare products and plan how many bags or rolls to buy.

Why the insulation type matters

Each material resists heat differently per inch, so the same R-value needs very different depths. Take the 300 square foot attic from the example above. At R-38, blown cellulose needs about 10.9 inches. At R-49 with blown fiberglass, at about R-2.5 per inch, it needs about 19.6 inches. That is roughly 490 cubic feet of material. Make sure your attic has room for that depth without blocking vents.

Account for what you already have

Most attics already have some insulation. Measure its depth in a few spots and estimate its R-value with the same per-inch figures. For example, 4 inches of old cellulose gives about R-14. To reach R-38, you would add about R-24, which is about 6.9 inches of new cellulose. Enter the R-value you still need, not the full target, to get the added amount.

Tips before you start

  • Seal air leaks first. Gaps around pipes, wires, light fixtures and the attic hatch can let warm air bypass the insulation.
  • Keep soffit vents clear. Use baffles so insulation does not block airflow at the eaves.
  • Keep insulation away from chimneys and recessed lights that are not rated for contact.
  • Use depth markers stapled to joists so you can see when blown insulation is deep enough.

Common mistakes

  • Compressing batts to fit a shallow space. Squashed insulation loses R-value.
  • Covering wet or moldy insulation. Fix the roof or moisture problem first.
  • Buying by volume alone. Blown insulation settles, and bags list coverage by R-value, not cubic feet.
  • Leaving gaps around framing. Small voids lower the overall performance.

Buying and planning

Use the thickness from the calculator to check the coverage chart on the product you plan to buy. Bag charts show how many bags cover 1,000 square feet at each R-value. Compare a few types on price per square foot at your target R-value, not price per bag.

Wear a dust mask or respirator, goggles, gloves and long sleeves when you work with insulation. Check local building codes for the required R-value in your climate zone. Hire a pro for spray foam or if you find old wiring or vermiculite insulation.

Frequently asked questions

What R-value do I need for my attic?
Many cold areas need R-49 or more, and milder areas need R-30 to R-38. Check the local building code for your climate zone.
What does R-value mean?
R-value measures how well a material resists heat flow. A higher R-value means better insulation.
How thick is R-19 insulation?
About 6 inches for fiberglass batts.
Which insulation has the highest R-value per inch?
Closed-cell spray foam, at about R-6 to R-7 per inch.
Can I add new insulation over old?
Often yes, if the old insulation is dry and in good condition. Do not compress it.
How many bags of blown insulation do I need?
Check the coverage chart printed on the bag. It lists how many bags cover a set area, often 1,000 square feet, at each R-value. Divide your attic area by 1,000 and multiply by the chart’s bag count for your target. Round up and buy a bag or two extra.
Is blown-in or batt insulation better for an attic?
Blown-in insulation usually works better in attics with many obstructions, odd joist spacing or existing insulation, because it fills gaps easily. Batts are simpler to handle in open, evenly framed spaces and in walls. Both can reach the same R-value if they are installed at the right depth without gaps.
Can I install attic insulation myself?
Many homeowners add batts or blown insulation themselves. Some home stores loan a blowing machine with a minimum bag purchase. Plan for dust, heat and tight spaces, and walk only on joists. Spray foam, knob-and-tube wiring or suspected vermiculite are better left to a trained pro.