How much insulation do I need?
Enter the size of the space, choose whether you're insulating the walls, the ceiling/attic, or both, and set a waste allowance — you'll get the area of insulation to buy and, if you add the coverage printed on the package, the number of rolls, batt packs or bags to order. Works in feet or metres, for fibreglass and mineral-wool batts and rolls or blown-in insulation.
Enter the coverage printed on your roll, batt pack or bag to get the number to order. For blown-in insulation that coverage depends on the R-value you fill to — read it off the bag's coverage chart.
Insulation to buy = area to insulate × (1 + waste), then ÷ the coverage per package, rounded up. Wall area = 2 × (length + width) × height; ceiling, attic or floor area = length × width. The waste figure covers offcuts and the trimming around joists, pipes and wiring. These are planning figures, not code values — check the recommended R-value for your area and climate with your local energy authority, and read the coverage off the package you actually buy.
Common questions
- How do I calculate how much insulation I need?
- Work out the area you are insulating, add a small waste allowance, then divide by the area one package covers. Walls are 2 × (length + width) × height; an attic, ceiling or floor is simply length × width. A 12 × 12 ft room with 8 ft walls has about 384 ft² of wall; with a 10% waste allowance that is roughly 422 ft² of insulation to buy. The calculator above does it in feet or metres and turns the area into packages once you add the coverage.
- How many bags or rolls of insulation do I need?
- Insulation is sold by the roll, batt pack or bag, each labelled with the area it covers, so divide the area you are buying (surface area plus waste) by that coverage and round up. For example, 422 ft² of wall insulation at an example 40 ft² per pack is 422 ÷ 40 = 10.6, which rounds up to 11 packs. Always read the coverage off the package you actually buy — for blown-in insulation it changes with the R-value (depth) you fill to.
- How much insulation do I need for a 1,000 sq ft attic?
- An attic floor is a flat area, so 1,000 ft² is your starting figure. Add about 10% for the bits trimmed around joists, hatches and vents and you are buying roughly 1,100 ft². At an example coverage of 40 ft² per bag that is about 28 bags — but blown-in coverage depends on the R-value you fill to, so check the coverage chart printed on the bag, which lists square feet per bag at each depth.
- How do I work out wall insulation versus attic insulation?
- They are different areas. For walls, use 2 × (length + width) × the wall height, because you are covering the vertical surfaces. For an attic, ceiling or floor, use length × width, because it is a flat plane. Tick both in the calculator and it adds the wall area and the ceiling/attic area together before applying the waste allowance and dividing by the coverage per package.
- How much extra insulation should I buy for waste?
- About 10% is a sensible allowance for most jobs — it covers the offcuts where batts are cut to length, the trimming around pipes, wiring and electrical boxes, and the odd squashed or torn piece. A simple open attic with few obstructions can get away with a little less; a wall full of windows, services and awkward cavities deserves a little more. Round the final figure up to whole packages, because you cannot buy part of one.
Want the full walk-through? Read the insulation guide →
Reference & education only. Not professional, engineering, or code-compliance advice. Estimates are based on published model codes; local amendments and your Authority Having Jurisdiction (AHJ) govern. Always verify against the current adopted code and a licensed professional before doing work.
Last reviewed 2026-06.