Insulation Calculator
Measure each wall, attic, or floor section as its own area to get the total square footage insulation is sold against. The R-value charts below turn that number into a bag count.
Total area: 100.0 sq ft
How it works
Enter each surface as its own area. A room's walls, an attic floor, and a crawlspace ceiling take different R-values, so measure them separately with the Add area button instead of averaging them into one rectangle.
Measure walls floor to ceiling and wall to wall, and do not deduct the studs. Batt bags state net cavity coverage, so framing at 16 or 24 inch spacing is already priced into the number on the bag.
Take the total to the R-value chart below, divide by that R-value's coverage per bag, and round up. Add 5–10% for cuts around outlets, pipes, and bays narrower than the batt.
Reference chart
Fiberglass batt R-value, thickness, and typical bag coverage
| R-value | Thickness | Typical use | Coverage per bag |
|---|---|---|---|
| R-13 | 3.5 in | 2x4 walls | 40 sq ft |
| R-15 | 3.5 in | 2x4 walls, dense | 40 sq ft |
| R-19 | 6.25 in | 2x6 walls, floors | 49 sq ft |
| R-21 | 5.5 in | 2x6 walls, dense | 40 sq ft |
| R-30 | 8.25 in | Attic, floors | 31 sq ft |
| R-38 | 10.25 in | Attic | 21 sq ft |
| R-49 | 13 in | Attic, cold zones | 21 sq ft |
What R-value do you need? Start with the climate zone
R-value measures resistance to heat flow, and the target is set by where the house is and which surface is being insulated — not by preference or budget. The US Department of Energy splits the country into eight climate zones and publishes a recommended range for each one.
Attics carry the highest targets because heat rises and the attic floor is the largest uninterrupted surface in most houses. Walls are capped by cavity depth instead: a 2x4 stud bay physically cannot hold more than about R-15 of fiberglass, which is why colder zones add rigid foam sheathing on the outside rather than forcing more into the cavity.
The zone column below is abbreviated to the states people search for most. Check your own county on the DOE map before ordering, because several states span two zones.
DOE recommended R-values by climate zone
| Climate zone | Attic | 2x4 wall | 2x6 wall | Floor |
|---|---|---|---|---|
| 1 — south FL, HI | R30–R49 | R13–R15 | R19–R21 | R13 |
| 2 — TX, LA, GA | R49–R60 | R13–R15 | R19–R21 | R13 |
| 3 — CA, AZ, NC | R49–R60 | R13–R15 | R19–R21 | R19–R25 |
| 4 — VA, MO, OR | R60 | R13–R15 | R19–R21 | R25–R30 |
| 5 — IL, NE, MA | R60 | R13–R15 | R19–R21 | R25–R30 |
| 6 — MN, VT, MT | R60 | R13–R15 | R19–R21 | R25–R30 |
| 7–8 — AK, far north | R60 | R13–R15 | R19–R21 | R25–R30 |
Batts, blown-in, and rigid board do not cover the same area
The square footage is the same whichever product goes in, but the number of packages it takes is not. Batts are sold by net cavity coverage, loose fill by the bag at a stated depth, and rigid foam by the sheet — three different denominators for the same wall.
Batts are the default for open stud and joist bays because they need no equipment. Loose fill wins on an existing attic floor, where it flows over wiring and around obstructions that would otherwise force dozens of hand cuts. Rigid board is the choice against masonry and on the outside of sheathing, where there is no cavity at all.
Loose-fill coverage is always quoted at a target R-value. The same bag covers roughly twice the area at R-19 as it does at R-38, so read the coverage line for the R-value you are actually installing rather than the headline number on the front.
Package coverage by insulation form
| Form | Sold as | Typical coverage | Best for |
|---|---|---|---|
| Fiberglass batt | Bag of pre-cut pieces | 21–49 sq ft per bag | Open stud and joist bays |
| Mineral wool batt | Bag of pre-cut pieces | About 60 sq ft at R-15 | Fire and sound separation |
| Blown fiberglass | Bag of loose fill | 40–60 sq ft at R-30 | Existing attic floors |
| Blown cellulose | Bag of loose fill | 30–40 sq ft at R-30 | Attic floors, dense-pack walls |
| Rigid foam board | 4 x 8 ft sheet | 32 sq ft per sheet | Basement walls, sheathing |
What to subtract, and what to leave in
Do not deduct framing. A bag of R-13 states 40 square feet of net cavity coverage, and that figure already assumes studs at 16 inches on center. Measuring the cavities one by one and ordering against that smaller number is the standard way to come up short, because the coverage on the bag was built for gross wall area.
Do deduct large openings. A picture window and a patio door together can account for 60 square feet of a wall that never gets insulated. Small windows are usually left in and absorbed by the waste allowance rather than measured out one at a time.
Then add 5 to 10 percent on top. Cuts around outlet boxes, pipes, and bays narrower than the batt all produce offcuts too small to reuse. Attics run at the high end of that range, since rafter bays are rarely uniform and everything near the eaves gets trimmed.
Frequently asked questions
How much insulation do I need for a 1,000 sq ft attic?
At R-38 a bag of batts covers about 21 sq ft, so 1,000 ÷ 21 ≈ 48 bags. Add 10% for cuts around rafters and eaves and order 53. At R-30 the same attic takes about 33 bags, because coverage rises to roughly 31 sq ft per bag.
How many square feet does a bag of insulation cover?
It depends entirely on R-value. Fiberglass batts run about 40 sq ft per bag at R-13, 49 sq ft at R-19, 31 sq ft at R-30, and 21 sq ft at R-38. Every bag prints its own net coverage — use that figure over any estimate.
What R-value do I need for my attic?
R-49 to R-60 covers most of the United States. R-30 to R-49 is enough only in the hottest zones, and zones 4 through 8 all target R-60. Check your climate zone in the chart above before ordering.
Should I subtract windows and doors from my insulation square footage?
Subtract large openings such as patio doors and picture windows. Leave small windows in, since they fall inside the 5 to 10 percent waste allowance you should be adding anyway. Never subtract the studs — batt coverage already assumes them.
Can I add new insulation over old insulation in an attic?
Yes, and it is the usual way to reach current R-values. Use unfaced batts or loose fill over the existing layer, because a second faced batt traps moisture between the two vapor barriers. Lay new batts across the joists, perpendicular to the old ones.
Is blown-in or batt insulation better for an attic?
Blown-in generally wins on an existing attic floor because it flows around wiring, ducts, and framing that would otherwise need dozens of hand cuts. Batts are easier where the bays are open and uniform, and they need no blower rental.
Link to this calculator
Contractors, teachers, and forum members are welcome to share this page. Copy the snippet below to credit the source.
<a href="https://diymaterialcalculators.com/insulation/">Insulation Calculator</a> — free online calculator