Insulation calculator: how many insulation batts do I need?
64 batts
320 square feet of surface · for 40 × 8 ft
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3/8 or 10' 6".
64 batts
320 square feet of surface
A 320 square foot wall takes 64 batts of 15 by 48 inches, because a 15 by 48 inch batt covers exactly 5 square feet. The R-value you need is set by your climate zone, not by this count.
A worked example
A 13 by 9 ft wall in 15 × 93 in batts, with a 5% waste allowance needs 13 batts. 117 square feet at 9.69 square feet per batt is 12.1, and the 5% allowance takes it to 13.
What R-value of insulation do I need?
The R-value you need is set by your climate zone, not by the size of the wall. In the 2021 International Energy Conservation Code, the model energy code the US Department of Energy tracks, the minimum ceiling insulation required by code (IECC) runs from R-30 in the warmest zone 1, to R-49 in zones 2 and 3, and up to R-60 in zones 4 through 8. Wood-frame walls run from R-13 in zones 1 and 2, to R-20 in zone 3, and R-30 in zones 4 and up, where R-20 plus R-5 of continuous insulation is an accepted alternative. Find your zone on the Department of Energy map, read the minimum ceiling and wall R-values for it, then buy the batt labeled with that R-value in the size this calculator counts. For an existing house, ENERGY STAR's retrofit table, built on the same 2021 code table, gives how much to add to the attic floor: in an uninsulated attic R-30 in zone 1, R-49 in zones 2 and 3 and R-60 in zones 4 through 8; over 3 to 4 inches already there, R-25 in zone 1, R-38 in zones 2 and 3 and R-49 in zones 4 through 8.
What else it is called
Insulation batts are also called blankets, and the same material comes in rolls you cut yourself. Bags are labeled by square feet of coverage as well as by batt count, so divide this count by the batts per bag on the label.
Common sizes
| Area | 15 × 48 in | 15 × 93 in | 23 × 48 in | 23 × 93 in | 16 × 48 in | 24 × 48 in |
|---|---|---|---|---|---|---|
| 8 × 8 ft — 64 sq ft | 13 | 7 | 9 | 5 | 12 | 8 |
| 10 × 10 ft — 100 sq ft | 20 | 11 | 14 | 7 | 19 | 13 |
| 10 × 12 ft — 120 sq ft | 24 | 13 | 16 | 9 | 23 | 15 |
| 12 × 12 ft — 144 sq ft | 29 | 15 | 19 | 10 | 27 | 18 |
| 12 × 16 ft — 192 sq ft | 39 | 20 | 26 | 13 | 36 | 24 |
| 16 × 20 ft — 320 sq ft | 64 | 34 | 42 | 22 | 60 | 40 |
| 20 × 20 ft — 400 sq ft | 80 | 42 | 53 | 27 | 75 | 50 |
Pieces, rounded up, with no waste allowance added.
Where the numbers come from
- Batt coverage is the size you select, multiplied out: a 15 by 48 inch batt covers 5 square feet. You tell the page which batt you are buying, so no figure here is ours to source.
- Minimum R-values required by code (IECC), by climate zone: the 2021 International Energy Conservation Code, Table R402.1.3, as published by the US Department of Energy Building America Solution Center (PNNL). These are minimums, not targets: it sets ceilings from R-30 to R-60 and wood-frame walls from R-13 to R-30 depending on the zone. Read October 1, 2026. Source
- Attic R-values to add in an existing house: ENERGY STAR, Recommended Home Insulation R-Values, table for retrofitting existing wood-framed buildings, based on the 2021 IECC Table R402.1.3. Uninsulated attic: R30 in zone 1, R49 in zones 2 and 3, R60 in zones 4 to 8; with 3 to 4 inches already: R25, R38 and R49. Read October 12, 2026. Source
This counts batts for a flat area. It does not subtract windows and doors, and it does not add for the odd bays at the corners: leaving both in is what most people want, because a scrap from one bay rarely fits the next one.
By Javier Miralles · Last updated . How this site is made.
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