Insulation calculator

Count batts, bags and foam boards for walls, attics, floors and exterior insulation.

tT
Letters on the drawing match the letters next to the inputs.
sq ft
in
in
More settings
in
in
pcs
%
Bags of insulation5 packs

Shopping list

  • Bags of insulation5 packs
  • Batts or boards55 pcs
  • Volume to buy155.4 ft³

How we got there

Layers of boards
1 pcs
Actual thickness
3.5 in
Coverage per bag
107 sq ft
Insulation volume
145.8 ft³

Results are estimates. Always check the coverage printed on the product and your local building code.

How it is calculated

The insulation thickness is divided by the batt or board thickness to get the number of layers. The area is multiplied by the layers and extra, then divided by the area of one batt or board. Batts are rounded up to whole bags. Batt sizes and pieces per bag can be changed to match the label.

FormulaLayers = thickness ÷ batt thickness, rounded upBatts = area × layers × (1 + extra) ÷ batt areaBags = batts ÷ batts per bag

Example

500 sq ft of 2×4 wall with R-13 batts (3.5 × 15 × 93 in) needs 55 batts with 5 % extra, which is 5 bags of 11 batts (about 107 sq ft per bag).

Tips from practice

  • Use 15 in batts for studs at 16 in on center and 23 in batts for 24 in spacing.
  • Do not compress batts: squeezed insulation loses R-value.
  • Check the R-value your climate zone requires in the energy code.

Questions and answers

How many square feet are in a bag of R-13?

A typical bag of R-13 15 × 93 in batts covers about 106 sq ft. Check the label and enter your bag size.

What thickness is R-19 or R-30?

R-19 fiberglass is about 6.25 in, R-30 about 10 in. R-13 and R-15 fit 2×4 walls, R-19 to R-21 fit 2×6 walls.

How many foam boards?

Foam boards are usually 4 × 8 ft, 32 sq ft each. Choose EPS, XPS or polyiso and the calculator counts sheets.

Found a mistake in a formula? Write to [email protected]