How Much Insulation Do I Need?
Insulation is sold by the bag or roll, but you measure the job by area - the square foot or square metre. Here is how to turn an area - a wall, an attic, or a plain square-footage figure - into the batts or bags to buy, at the R-value you need.
By the Calculate My Reno Team / Published
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Open the Insulation Calculator
Insulation is priced by the bag or roll but the job is measured by area - in square feet or square metres - so the work is turning your area into a count of bags, at the right R-value, with a little extra for waste.
The formula
area = sum of (length × width) for each wall / ceiling
net area = area - openings (windows, doors)
bags = ceil( net area × wastage ÷ coverage per bag )
Add up the area you are insulating, take out the openings, add a wastage allowance, then divide by the area one bag or roll covers. The insulation calculator does all of it - including the individual batt count for your stud spacing - but the formula is what makes the number make sense. It works in either unit, so a coverage in m² and an area in m² give the same answer as the imperial figures below.
Coverage per bag is the number that matters
Unlike drywall, where every 4 × 8 sheet covers 32 sq ft (3 m²), insulation coverage changes with the product and the R-value:
| Type | Typical coverage per bag | Notes |
|---|---|---|
| R-13 batts | ~106 sq ft (9.8 m²) | 2×4 wall |
| R-15 batts | ~88 sq ft (8.2 m²) | high-density 2×4 |
| R-19 batts | ~75 sq ft (7.0 m²) | 2×6 wall |
| R-30 blown-in | ~60 sq ft (5.6 m²) | attic, settled |
| R-49 blown-in | ~37 sq ft (3.4 m²) | attic, deeper fill |
Batt bags carry a roughly fixed face area; blown-in has a fixed yield, so the deeper you fill (higher R-value) the less area a bag covers. Always read the coverage off your bag - it is printed on the label, in sq ft or m² - and enter it in the calculator.
Starting from a square-footage figure
Already know your area? Skip the room math:
bags = ceil( area × wastage ÷ coverage per bag )
For 1,000 sq ft (93 m²) of R-30 blown-in at ~60 sq ft (5.6 m²) per bag with 5% wastage: 1,000 × 1.05 ÷ 60 = 17.5, so 18 bags.
Worked example
A 4 ft × 12 ft (1.2 × 3.7 m) wall section, R-13 batts, 16 in OC (400 mm) studs, 10% wastage:
- Area: 4 × 12 = 48 sq ft (4.5 m²) (no openings).
- With wastage: 48 × 1.10 = 52.8 sq ft (4.9 m²) to cover.
- Bags: 52.8 ÷ 106 = 0.5, rounded up to 1 bag.
- As individual 15 × 92 in (380 × 2,340 mm) batts, 9.6 sq ft (0.9 m²) each: 52.8 ÷ 9.6 = 5.5 → 6 batts.
Walls, attic, and the rest
The single biggest swing in the count is where the insulation goes, because each area takes a different R-value:
- Attic / ceiling - the highest values (R-30 to R-60) and usually the best payback. See how much attic insulation do I need.
- Exterior walls - R-13 to R-21 depending on the stud depth.
- Floors and crawlspaces - R-19 to R-30 between the joists.
Pick the right target for each and the insulation R-value chart lists the depth each one needs. In Australia, New Zealand and the UK these targets are labelled in metric R-values (R1.5-R6.0) rather than the US scale - the chart lists the equivalents, and the calculator shows the metric scale on those regional pages.
What will it cost?
The bag count is also the basis for a budget. Drop your numbers into the insulation calculator and add a price per bag for a material total, plus a cost per square foot (or metre) you can hold against a quote - or switch to an installed rate to price a fitter. Insulation pricing swings too much by product, R-value and region to publish a useful average, so the calculator works from the prices you enter rather than a baked-in figure. The estimate covers the insulation material only; vapour barrier, fixings and labour are extra.
Tips
- Buy a spare bag - running one batt short stops the job, and the offcuts rarely add up to a full bay.
- Match the batt width to your framing: 15 in for 16 in OC and 23 in for 24 in OC, or 430 mm and 580 mm batts for 450 and 600 mm centres in metric.
- Choosing a product? See blown-in vs batt insulation before you buy.
Frequently asked questions
How much insulation do I need for 1,200 sq ft?
For 1,200 sq ft (about 111 m²) of attic at R-30 blown-in, with a bag covering about 60 sq ft (5.6 m²) and 5% wastage: 1,200 × 1.05 ÷ 60 = about 21 bags. For the same area in R-13 wall batts at ~106 sq ft (9.8 m²) per bag with 10% wastage that is about 13 bags. The coverage per bag is the figure that changes the count, so read it off your product or use the calculator.
How much insulation do I need for a 12x24 shed?
A 12 × 24 ft (3.7 × 7.3 m) shed with 8 ft (2.4 m) walls has about 576 sq ft of wall (perimeter 72 ft × 8 ft) plus a 288 sq ft ceiling - roughly 864 sq ft (about 80 m²) before you deduct the door and windows. At R-13 batts covering ~106 sq ft (9.8 m²) per bag with 10% wastage that is about 9 bags for the lot. Insulate only the surfaces you want to keep conditioned.
How much insulation do I need for a 24x24 garage?
A 24 × 24 ft (7.3 × 7.3 m) garage with 9 ft (2.7 m) walls has about 864 sq ft of wall plus a 576 sq ft ceiling - about 1,440 sq ft (134 m²), less the garage and entry doors. At R-13 wall batts and R-30 ceiling, expect roughly 8 wall bags and 10 ceiling bags depending on coverage. Treat the walls and ceiling as separate areas at their own R-values.
How much insulation do I need in my walls?
Measure each exterior wall as length × height, add them up, and take out the windows and doors. A standard 2×4 wall takes R-13 to R-15 and a 2×6 wall R-19 to R-21. Divide the net area by the coverage of one bag and add about 10% wastage. The insulation calculator counts both the bags and the individual batts for your stud spacing.