Insulation Calculator

Enter your area and the coverage per pack or roll to get a waste-adjusted pack count for walls, ceilings, and floors. Metric and imperial.

Extra for cuts around framing, pipes, and openings.

For walls, enter wall height. For ceilings/floors, enter room width.

From the product label.

For selecting the right batt width.

Determines max achievable R-value.

How to Calculate How Much Insulation You Need

The calculation is straightforward: measure the area you need to cover, add a waste factor, and divide by the coverage per pack or roll. The tricky part is getting the area right — especially for walls, where you need to subtract windows and doors from the gross wall area before you start.

For a typical room with 40 linear feet of exterior wall at 9 ft height, the gross wall area is 360 ft². Subtract two windows at 15 ft² each and a door at 20 ft², giving 310 ft² of insulated area. Add 10% waste to get 341 ft². If each batt pack covers 59.7 ft², you need 6 packs (rounded up from 5.71).

R-Value Requirements by Application and Climate

Application Warm climate (zones 1–2) Mixed climate (zones 3–4) Cold climate (zones 5–8)
Attic / ceilingR-30 to R-38R-38 to R-49R-49 to R-60
Exterior wall (cavity)R-13 to R-15R-13 to R-21R-15 to R-21
Floor over unconditioned spaceR-13R-19 to R-25R-25 to R-30
Basement wallR-5 to R-10R-10 to R-15R-15 to R-20

Based on US DOE recommendations and IECC climate zones. Check your local energy code for the enforceable minimum in your area.

Common Batt Pack Coverage — Quick Reference

Product R-value Fits stud spacing Coverage per pack
Fiberglass batt, 2×4 wallR-13 / R-1516 in o.c.~40–60 ft²
Fiberglass batt, 2×6 wallR-19 / R-2116 in o.c.~40–50 ft²
Mineral wool battR-15 / R-2316 or 24 in o.c.~40–55 ft²
Fiberglass roll, attic floorR-3816 or 24 in o.c.~40–65 ft²
Fiberglass roll, attic floorR-4916 or 24 in o.c.~35–50 ft²

Coverage ranges vary by manufacturer and pack size. Always check the label — use the listed coverage per pack in the calculator above.

Batts vs Rolls

Batts come pre-cut to standard lengths and fit neatly into stud or joist bays. They're the right choice for walls and most floor applications where framing is regular. Rolls work better for large open areas like attic floors — you unroll them across the joists and cut to length, which reduces waste on wide open spans. Both types are measured and sold by coverage area, so the calculation method is the same.

Why waste percentage changes by job

A simple attic floor with no obstructions wastes very little material — 5% is usually enough. A wall with lots of windows, electrical boxes, and plumbing penetrations can waste 12–15% because every cut-out leaves an offcut that's too small to reuse. When in doubt, use 10% for a typical job. Having one extra pack is a much better outcome than stopping work mid-wall to make another trip to the store.

Frequently Asked Questions

Measure the total area to cover, add a waste allowance of 5–15%, then divide by the coverage per pack or roll on the product label. Round up to the nearest whole pack. For example: 120 m² area with 10% waste = 132 m² adjusted. If each pack covers 11 m², you need 12 packs. The calculator above handles this automatically — just enter your area, coverage, and waste percentage.
It depends on where you are and what you're insulating. In the US, the DOE recommends R-49 to R-60 in the attic for cold climates (zones 5–8), R-38 to R-49 for mixed climates (zones 3–4), and R-30 to R-38 for warm climates (zones 1–2). Exterior walls across most zones target R-13 to R-21 depending on stud depth. See the reference table above for a full breakdown, and check your local energy code for the enforceable minimum.
Batts are pre-cut sections sold in packs, sized to fit standard 16 in or 24 in stud spacing. Rolls are continuous lengths you cut on site. Batts are faster for regular wall and floor bays; rolls work better for large open attic floors where you can unroll and cut freely with less waste. Coverage per pack or roll is printed on the label — use that number in the calculator.
For straightforward wall or ceiling jobs with consistent framing, 5–10% waste is typical. For rooms with many windows, doors, pipes, or irregular shapes, use 10–15%. Attic floors with few obstructions can often get away with 5%. It is always better to have one extra pack than to run short mid-job, particularly if the product is special-order or stock is limited.
Measure total wall area (length × height for each wall section), then subtract the area of windows and doors. For a room with 40 linear feet of exterior wall at 9 ft height, that's 360 ft² gross. Subtract two windows at 15 ft² each and one door at 20 ft², leaving 310 ft² of insulated area. Add your waste factor and divide by pack coverage to get your pack count.
Yes. For attic floor insulation, measure the floor area of the space you want to cover. For sloped rafter insulation, use the rafter length (not the floor span) multiplied by the rafter bay width and number of bays. If you're adding insulation in multiple layers to reach a higher R-value, run the calculation once per layer. Enter the total area, add waste, and enter the coverage per roll from the product label.

Insulation Installation Checklist

Reviewed by Sonia Cho (building performance consultant). Covers air sealing before insulation, vapour barrier requirements by climate, stud bay preparation, attic ventilation clearance, and common installation mistakes that reduce effective R-value.

Download Checklist (PDF)

Planning reference only. See Methodology and Data Sources. View all project checklists →

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Accuracy & Review

Reviewed by: Sonia Cho

Sonia is a building performance consultant with experience in residential energy auditing, insulation specification, and air sealing. She reviewed the R-value reference data, waste allowance guidance, and coverage calculation methodology used in this calculator.

Last updated:

See: Methodology · Data Sources · Review Board

Disclaimer: These estimates are for general planning purposes only and should not be used as a substitute for product specification, energy-efficiency design, or code compliance review. Actual quantities vary with framing layout, product thickness, installation method, and site conditions. R-value targets shown are based on US DOE recommendations — always check the enforceable minimums in your local energy code.

See Methodology and Data Sources for calculation assumptions.