Contractor Calculator · Insulation
Insulation Calculator: Bags, Batts, and R-Value by Zone
Set the area, cavity depth, framing spacing, and target R-value. Bag counts, bundle counts, and the climate zone check update as you type.
Pricing the whole job? Estimate it from a photo in about six seconds.
Sets the batt width. Blown fill and foam ignore it.
Coverage spans the Owens Corning and Johns Manville attic charts. Charts and zone targets read September 11, 2026.
How the insulation calculator works
Insulation is not counted by volume. It is counted by how much area one bag or bundle covers at your R-value, and the maker prints that on the bag.
So the math is a division. Area divided by coverage at the target R-value, rounded up. Chase a higher R and coverage per bag drops, because the same material goes down deeper.
Cavity depth is the other half. The chart sets a settled thickness for each R-value, and if the depth is short the R-value is short, whatever the label says.
Worked example: R-30 across a 1,200 sq ft attic
At R-30 the Owens Corning open blow chart allows one bag to cover no more than 80.6 sq ft. Divide 1,200 by 80.6 and you get 14.89, which rounds up to 15 bags.
Run the same attic on the Johns Manville chart and R-30 covers 73 sq ft a bag, so 1,200 becomes 16.44, or 17 bags. Two honest charts, two bag counts.
Both charts also set a depth. R-30 needs 10.5 in of settled fill on the Owens Corning card and 10.8 in on the Johns Manville card. Blow it at 8 in and you sold R-30 and installed about R-22.
SimplyWise Cost Estimator turns a photo of the space into an itemized estimate in about 6 seconds.
Blown fill coverage per bag by R-value
Maximum net coverage in square feet per bag, and settled depth in inches, off the two maker charts.
| R-value | Owens Corning | Johns Manville | Settled depth |
|---|---|---|---|
| R-13 | 198.0 | 176 | 4.8 to 4.9 |
| R-19 | 128.7 | 119 | 7.0 |
| R-30 | 80.6 | 73 | 10.5 to 10.8 |
| R-38 | 60.9 | 57 | 13.3 to 13.4 |
| R-49 | 46.4 | 43 | 16.5 to 16.9 |
| R-60 | 36.9 | 34 | 19.8 to 20.2 |
Turn the count into insulation cost per square foot
SimplyWise Cost Estimator turns a photo into an itemized estimate in about 6 seconds. Pricing is $29.99/mo or $239.99/yr, about $20/mo on annual, with a 7-day free trial.
Batt bundle coverage by framing spacing
A 15 or 16 in batt fits 16 in on center and a 23 or 24 in batt fits 24 in, so the wider bundle covers more floor for the same piece count. Coverage is square feet per bundle, thickness in inches.
| R-value | 16 in on center | 24 in on center | Thickness |
|---|---|---|---|
| R-13 | 106.56 | 163.39 | 3.50 |
| R-19 | 87.18 | 133.68 | 6.50 |
| R-30 | 58.66 | 88.00 | 10.25 |
| R-38 | 42.66 | 64.00 | 12.50 |
| R-49 | 32.00 | 48.00 | 13.50 |
No kraft faced batt reaches R-60, which is why deep attics go to loose fill or a second layer over R-49.
What your climate zone asks for
The 2021 energy code raised attic levels hard. Zones 4 through 8 moved from R-49 to R-60, and zones 2 and 3 moved from R-38 to R-49. Zone 1 sits at R-30.
In blown fill that is 10.5 to 10.8 in of settled depth for zone 1, 16.5 to 16.9 in for zones 2 and 3, and 19.8 to 20.2 in for zones 4 and up. A 2x10 bay holds about 9.25 in, so R-60 buries the joists.
Order the count you will install
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Measure net area, not gross
Subtract what stays open, such as a chase or a stair well. Charts are written against net square footage.
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Pick the R-value first, then the material
The target sets the coverage. Picking the bag first is how crews end up two bags short at four in the afternoon.
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Check the depth before you quote
Compare the settled thickness for your target against the cavity you have. If the cavity is shallower, either the assembly changes or the R-value does.
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Round up, every time
Bags and bundles are whole units. Round up, then add for the piece cut wrong.
Keep reading
Sources
Maker coverage charts
- Owens Corning, ProCat Professional Loosefill Insulation, Specifications for Open Blow Attics (Pub. No. 10021711-A, March 2026; verified 2026-09-11).
- Johns Manville, Climate Pro B7600 Fiberglass Blowing Wool, Attic Coverage (coverage per bag and installed thickness; verified 2026-09-11).
- Johns Manville, Fiberglass Manufacturers Fact Sheet (kraft faced batt coverage by item number; verified 2026-09-11).
- Johns Manville, JM Corbond III Closed Cell Spray Polyurethane Foam data sheet (R-7.0 per inch; verified 2026-09-11).
Code and climate zones
- ENERGY STAR, Recommended Home Insulation R-Values (attic targets by zone; verified 2026-09-11).
- Montgomery County MD, 2021 Residential Energy Code Changes (Table R402.1.3 ceiling values; verified 2026-09-11).
- US Department of Energy, 2021 energy code residential analysis (Table D.5; verified 2026-09-11).
- PNNL Building America, Climate Zone Map (zone boundaries; verified 2026-09-11).
Product
- SimplyWise Cost Estimator, App Store (4.8 rating; verified 2026-09-09).
Common questions about sizing insulation
Bags, bundles, and coverage
How many bags of blown insulation do I need for R-30?
Divide area by the coverage on the bag. At R-30 that is 80.6 sq ft on the Owens Corning chart and 73 on the Johns Manville chart, so 1,200 sq ft takes 15 to 17 bags.
How much does one bundle of batts cover?
Kraft faced R-30 covers 58.66 sq ft at 16 in on center and 88.00 sq ft at 24 in.
Why do two makers give different bag counts?
Each maker tests its own product and prints its own coverage. The R-value matches, the density does not. Buy off the chart on the bag you are using.
Depth, R-value, and code
What R-value does my climate zone need in the attic?
Zone 1 wants R-30, zones 2 and 3 want R-49, and zones 4 through 8 want R-60. Walls are set separately.
How deep is R-60 in blown fill?
About 19.8 to 20.2 in of settled thickness. That is deeper than any common joist, so the fill covers the framing rather than sitting between it.
Can SimplyWise work out insulation cost per square foot from a photo?
Yes. SimplyWise Cost Estimator turns a photo into an itemized estimate in about 6 seconds, with material and labor lines. Free to try, at $29.99/mo.
Turn the bag count into a signed job.
Snap a photo and get an itemized estimate in about 6 seconds. Free to try.