How Much Do I Need?

Insulation Calculator

How much insulation do I need?

Enter the area you are insulating and the coverage printed on the pack. The calculator works out how many packs you need for one or more layers. If you add a target R-value, it also shows how deep the insulation must be.

Units
Area to insulate

For an attic or loft, the floor. For walls, the wall length.

For walls, use the wall height.

For L-shaped or irregular spaces, split them into rectangles and add each one.

Insulation

Printed on the pack. Thicker products cover less area per pack.

Attics are often insulated in two layers, the second laid across the first.

For cutting around joists, pipes and awkward corners.

Depth for an R-value (optional)

Local codes and energy programmes set the R-value to aim for.

On the product. Fiberglass and mineral wool batts are often about 3–4.

Cost (optional)

Add the price to see an estimated total.

Results update as you type.

Your estimate

Insulation to buy

16 packs

40 ft² packs for 600 ft², including 5% waste.

Estimated amount to buy

Packs
16

Calculated quantity (no waste)

Area
600 ft²
Area × 1 layer
600 ft²
Packs
15
How this was calculated
  1. Area = 30 ft × 20 ft = 600 ft²
  2. Packs = 600 ft² × 1 layer ÷ 40 ft² per pack = 15
  3. With 5% waste = 15 × 1.05 = 15.75, rounded up to 16
  • Pack coverage is for the product at its stated thickness. Compressing insulation to fit a shallow space reduces its R-value.
  • Keep insulation clear of recessed lights, flues and chimneys unless the product and fixtures are rated for contact, and keep eaves ventilation open.

How the insulation calculation works

  1. Area = length × width. Add rectangles for L-shaped attics or several walls.
  2. Packs = area × number of layers ÷ coverage per pack, plus the waste allowance, rounded up.
  3. Depth (optional) = target R-value ÷ R-value per inch of the product.

US R-values are converted to metric RSI (m²·K/W) by multiplying by 0.1761. For example, R-38 is about RSI 6.7.

Batts, rolls and the coverage figure

Batts and rolls are sized to fit between standard joist and stud spacings, and the coverage on the pack is the area of wall or ceiling they fill, so you don't subtract the framing. Thicker, higher-R products cover less area per pack. Loose-fill (blown) insulation is sold differently: its bag chart gives the bags per 1,000 ft² needed for each R-value, and settled depth matters more than bag count.

Rounding rules

  • Packs are rounded up to whole packs after the waste allowance.
  • The depth for an R-value is shown to one decimal place.