R-13 Insulation Calculator
R-13 is the standard fiberglass batt for 2×4 walls: 3.5 inches thick, so it fills the stud cavity exactly. A bag of 15 × 93 in batts covers 106.56 sq ft, so a 500 sq ft wall with 10% waste takes 6 bags. Pick the batt or roll size you are buying and enter the area for bags, batts and the cavity it fits.
Your project
Covers cuts, compression and odd bays. 10% is the usual default.
Your results
- Bags to buy
- 6
- 106.56 sq ft per bag · 550 sq ft with waste
- Batts
- 57
- 9.69 sq ft each
- Thickness
- 3.5 in
- R-3.71 per inch
- Fits without compressing
- 2×4 (3.5 in) or deeper
Estimates use typical US material and labor pricing. Confirm quantities against the coverage chart on the bag before you buy.
Gear for this job
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Prices as of Oct 1, 2026, 4:38 AM UTC. Product prices and availability are accurate as of the date/time indicated and are subject to change. Any price and availability information displayed on Amazon.com at the time of purchase will apply to the purchase of this product.
How this calculator works
- Coverage = area × (1 + waste %)
- Bags = ceil(coverage ÷ sq ft per bag); batts = ceil(coverage ÷ (width × length ÷ 144))
- R per inch = R-value ÷ thickness
R-13 fiberglass batts and rolls: sizes, thickness and bag coverage
| Product | Thickness | Pieces per bag | Sq ft per bag |
|---|---|---|---|
| 15" × 93" batts (16" o.c. walls) | 3.5 in | 11 | 106.56 |
| 15" × 105" batts (9 ft walls) | 3.5 in | 11 | 120.31 |
| 23" × 93" batts (24" o.c.) | 3.5 in | 11 | 163.39 |
| 15" × 32 ft roll | 3.5 in | 1 | 40 |
Johns Manville Fiberglass Manufacturers Fact Sheet (wood framing, kraft-faced and unfaced). Other brands are close but not identical — use the square feet printed on your bag.
Bags of R-13 for common areas (10% waste)
| Area | 15" × 93" batts | 15" × 105" batts |
|---|---|---|
| 100 sq ft | 2 bags | 1 bags |
| 250 sq ft | 3 bags | 3 bags |
| 500 sq ft | 6 bags | 5 bags |
| 1,000 sq ft | 11 bags | 10 bags |
| 1,500 sq ft | 16 bags | 14 bags |
Bags = area × 1.10 ÷ sq ft per bag, rounded up.
Fiberglass batt R-values and thickness
| R-value | Thickness | R per inch | Fits |
|---|---|---|---|
| R-11 | 3.5 in | 3.14 | 2×4 wall |
| R-13 | 3.5 in | 3.71 | 2×4 wall |
| R-15 | 3.5 in | 4.29 | 2×4 wall (high density) |
| R-19 | 6.5 in | 2.92 | 2×8 floor/ceiling; compresses in 2×6 |
| R-21 | 5.5 in | 3.82 | 2×6 wall (high density) |
| R-30 | 10.25 in | 2.93 | 2×12 or attic floor |
| R-38 | 12.5 in | 3.04 | Attic floor |
| R-49 | 13.5 in | 3.63 | Attic floor |
Johns Manville fact sheet thicknesses; lumber depths 2×4 = 3.5 in, 2×6 = 5.5 in, 2×8 = 7.25 in, 2×10 = 9.25 in, 2×12 = 11.25 in.
Where R-13 belongs
R-13 is made for 2×4 exterior walls and interior walls you want quieter. In the 2021 IECC it meets the wood-frame wall minimum only in climate zones 1 and 2; colder zones need a denser R-15, a 2×6 wall with R-21, or R-13 plus continuous foam sheathing. It is also the 2021 IECC floor minimum in zones 1 and 2.
Kraft-faced R-13 has a paper vapor retarder that goes toward the heated side in cold climates, stapled to the face or sides of the studs. Unfaced R-13 is for walls where the vapor control is handled another way, for interior walls, and for layering over existing insulation.
R-13 vs R-15 vs R-19
R-13 and R-15 are both 3.5 in thick — R-15 is simply denser fiberglass, so it adds R-2 in the same 2×4 wall for a higher price per bag. R-19 is 6.5 in thick: it does not fit a 2×4 at all and is compressed to about R-17–18 in a 5.5 in 2×6 cavity, which is why 2×6 walls usually get R-21 (5.5 in, high density) instead.
Frequently asked questions
How thick is R-13 insulation?
3.5 inches — the depth of a 2×4 stud — for fiberglass batts and rolls. That works out to about R-3.7 per inch.
How many square feet are in a bag of R-13?
Johns Manville lists 106.56 sq ft for a bag of eleven 15 × 93 in batts, 120.31 sq ft for 15 × 105 in and 163.39 sq ft for 23 × 93 in. A 15 in × 32 ft roll covers 40 sq ft. Check the label on your brand.
What is the difference between R-13 and R-15 insulation?
Both are 3.5 in thick for 2×4 walls; R-15 is a denser, high-performance batt that gives R-2 more in the same cavity and costs more per square foot.
R-13 vs R-19: which do I need?
R-13 for 2×4 walls. R-19 is 6.5 in thick, so it is for 2×8 floors and ceilings or deeper cavities; squeezed into a 2×6 wall it gives about R-17–18.
Is R-13 good enough for exterior walls?
It meets the 2021 IECC minimum for wood-frame walls only in climate zones 1 and 2. Elsewhere the code asks for more — R-20-class cavity insulation or R-13 plus continuous insulation sheathing, depending on the zone.
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Bags to buy
6