How this site is built
Review the calculation methodology to see which figures are exact geometry, which depend on material assumptions, and which figures have recorded sources. The source registry records every cited figure and its review status.
How many bags of blown-in insulation do I need?
Multiply attic length by width, include any ordering allowance, then divide by the coverage per bag shown for your target R-value on the exact product’s coverage chart. Round up to whole bags. Calculate irregular attic sections separately and combine their area.
The starting 40 ft² per bag is illustrative; it is not tied to a particular R-value or product. Set the Coverage per unit field to your selected product’s stated ft² per bag. For a hypothetical 1,000 ft² attic at 40 ft² per bag with no allowance, the quantity is 25 bags.
The calculator does not select an R-value, installation depth or installed density. Follow the product chart’s bag quantity and thickness requirements together; adding a guessed allowance cannot replace them.
What to read from an insulation bag coverage chart
| Chart entry | How to use it |
|---|---|
| Target R-value | Choose the row specified for your assembly. |
| Maximum coverage per bag | Enter this row’s ft² per bag as Coverage per unit. |
| Minimum bags per 1,000 ft² | Cross-check your attic area against this minimum. |
| Minimum installed or settled thickness | Follow the same row’s installation requirements. |
Product charts are specific to the material and bag size. Do not transfer a fiberglass coverage number to cellulose or another product.
Blown-in insulation bag questions
Can this calculate R-38 or R-49 insulation bags?
Yes, when you enter the ft²-per-bag coverage from the matching R-value row of your chosen product. The calculator uses your coverage input; it does not assume a universal bags-per-R-value conversion.
Is the same coverage valid for fiberglass and cellulose?
No. Use the chart for the exact product and bag size. Required installed thickness, density and bag quantity differ.
What if the attic already has insulation?
Have the required added thermal performance assessed before choosing a coverage-chart row. Existing depth alone does not establish a reliable R-value or justify subtracting an arbitrary number of bags.
Does this calculate insulation batts?
No. Use the batt insulation calculator for wall cavities and individual batt dimensions. This page estimates loose-fill bags from product coverage.
Formula used for blown-in insulation calculator
The measured quantity stays separate from allowance and whole-unit purchase rounding.
- Measured quantity
bags = ceiling(adjusted area ÷ bag coverage at target depth) - Allowance
adjusted quantity = measured quantity × (1 + allowance ÷ 100) - Order rounding
purchase quantity = adjusted quantity rounded up to a purchasable unit
Variables and units
For the default “Blown-in insulation” mode. Other modes label their own fields in the calculator.
length- Length: ft, in, yd, m, cm.
width- Width: ft, in, yd, m, cm.
coverage- Coverage per unit: ft² per bag; optional.
allowance- Waste or ordering allowance: %; optional.
unitPrice- Your price per ordering unit: USD; optional.
delivery- Delivery charge: USD; optional.
tax- Sales tax: %; optional.
Worked example: blown-in insulation order
The default measurements demonstrate the complete blown-in insulation calculator formula and purchase rounding.
| Field | Value |
|---|
| Result | Value |
|---|---|
| Measured quantity | 240 ft² |
| With allowance | 264 ft² |
| Rounded purchase quantity | 7 bags |
These figures are generated by the same calculation the calculator above uses, so they update automatically if the method changes.
Where these numbers come from
This calculator uses geometry and values shown in the form. Editable product dimensions, coverage figures, prices and allowances are inputs, not universal recommendations; confirm them against the product label, drawing or supplier quote.
Content release:
Published by Material Totals. Review the methodology and source assumptions.
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