Insulation quantity calculator

What an insulation quantity calculator does

An insulation quantity calculator works out how many packs of batts or rolls to buy for a given area. Enter the net area to insulate, the number of layers, the coverage printed on the pack and your waste allowance, and it returns the coverage needed, the packs to order, the coverage you actually buy and the equivalent length of roll at your batt width.

The arithmetic is short: required area = net area × layers × (1 + waste), and packs = required area ÷ pack coverage, rounded up. With the defaults, 1,000 ft² at one layer and 10% waste needs 1,100 ft², which is 14 packs of 80 ft², or 1,120 ft² bought.

The calculator counts material and, if you enter a target, how many layers of a product reach it. It does not choose an R-value for you, check the energy code or model heat loss through studs. Use it once you have chosen the product, with the pack coverage from the label.

How to use the insulation quantity calculator

1. Choose Imperial or Metric, then enter the Net coverage area: the wall, floor or ceiling area to insulate, less windows, doors and other openings. The floor and wall area calculator can work this out.
2. Enter the number of Layers. An attic insulated with one layer between the joists and a second layer across them counts as two.
3. Enter the Coverage per pack from the product label, in ft² (m² in metric).
4. Pick a Batt / roll preset: US 15 in or 23 in batts for 16 in and 24 in on-center framing, or AU/NZ 430 mm and 580 mm batts for 450 mm and 600 mm centres. Choose Custom to enter another Roll / batt width.
5. Set Framing to 'Framed at regular centres' and enter the Framing centres, Stud / joist width and Batt length to see the stud bay width, whether the batt fits it, and a batt count of one per bay length.
6. Set R-value target to 'Check layers against a target' and enter the Target R-value and Product R-value per layer to see how many layers reach it. Layer R-values add in series; use the R-value / U-value converter to switch between SI and US values.
7. Set a Waste allowance and an optional price per pack. Read the coverage including waste, packs to order, purchased coverage, equivalent roll length, batt count and layers needed. Download the PDF or CSV, or copy the result into an order.

Worked example

Imperial: 1,000 ft² with one layer, 10% waste and 80 ft² per pack requires 14 packs.

Metric: 100 m² with one layer, 10% waste and 8 m² per pack requires 14 packs.

Batt sizes for 16 in vs 24 in on-center framing

Batts are sized to friction-fit between framing members. Standard US batts are 15 in wide for framing at 16 in on center and 23 in wide for 24 in on center. The clear cavity between 1-1/2 in studs is 14-1/2 in at 16 in o.c. and 22-1/2 in at 24 in o.c., so each batt is about 1/2 in wider than the space it fills and holds itself in place.

Pre-cut wall batts are commonly 93 in long, to fit the 92-5/8 in precut studs of an 8 ft wall, and longer batts are sold for taller walls. For a quick count, a wall at 16 in o.c. has 7.5 bays per 10 ft of length and a wall at 24 in o.c. has 5. Openings, corners and extra studs change the real number, which is why ordering by area plus waste is the more reliable method.

Gross wall area, net area and pack coverage

Enter net area: gross wall area less windows and doors. That net area still includes studs, plates and headers, and batts only fill the cavities between them. At 16 in o.c. a 15 in batt covers 15/16 (93.75%) of the wall width; at 24 in o.c. a 23 in batt covers 23/24 (95.8%).

Check how the pack states its coverage. If the label gives the area of the batts themselves, ordering against net wall area builds in roughly 4% to 7% extra before waste, because the framing does not need insulation. That margin covers some of the cutting around outlets, blocking and short bays, so straightforward walls may not need a large waste allowance on top.

The calculator also reports an equivalent roll length: the required area divided by the batt width. It is useful for faced rolls and for checking against the length printed on a roll.

R-value per inch for common insulation types

R-value measures resistance to heat flow. A batt's rated R-value applies at its full thickness, and compressing it into a shallower cavity lowers the installed value. A common example is an R-19 batt, about 6-1/4 in thick, compressed into a 5-1/2 in 2x6 cavity, where it performs at about R-18.

The figures below are typical per-inch values in US units (h·ft²·°F/Btu). The label on the product governs. To convert between R and U, or between US and SI units, use the R-value / U-value converter.

Worked example: insulating the exterior walls of a house

A 36 × 28 ft single-story house has 8 ft walls framed at 16 in on center. The perimeter is 128 ft, so the gross wall area is 128 × 8 = 1,024 ft². Fifteen 3 × 5 ft windows (225 ft²) and three 20 ft² doors (60 ft²) leave a net area of 739 ft². Using 15 in batts from packs labelled 80 ft², with 10% waste:

• Coverage including waste: 739 × 1.1 = 812.9 ft².
• Packs to order: 812.9 ÷ 80 = 10.16, rounded up to 11 packs.
• Purchased coverage: 11 × 80 = 880 ft².
• Equivalent roll length: 812.9 ÷ 1.25 ft = 650.3 ft.
• Check: the batts only need to fill about 93.75% of the net wall, roughly 693 ft², so the order leaves a comfortable margin. With 5% waste the coverage is 775.95 ft², which is 10 packs.

Who it is for and when to use it

Method, formulas & assumptions

Required area = net area × layers × (1 + waste/100). Packs = ceil(required area ÷ pack coverage); roll length = required area ÷ batt width (preset or custom). Framed mode: bay = centres − stud width, batts = ceil(required area ÷ (centres × batt length)). Layers for a target = ceil(target R ÷ product R per layer).

Imperial inputs are converted with exact factors (1 ft = 0.3048 m, 1 lb = 0.45359237 kg, 1 US gal = 3.785411784 L) before the calculation runs in SI, then results are converted back.

Common mistakes to avoid

• Entering gross wall area without deducting windows and doors. In the worked example, openings are 285 ft², nearly 28% of the gross wall.
• Buying 23 in batts for 16 in framing, or 15 in batts for 24 in framing. Check the stud spacing before ordering.
• Compressing thick batts into shallow cavities. An R-19 batt squeezed into a 2x6 wall performs at about R-18.
• Forgetting the second layer. Attic top-ups and double layers need Layers set to 2.
• Treating the pack count as a thermal check. The calculator does not check R-values against your energy code, thermal bridging or air sealing.
• Mixing faced and unfaced products without checking the assembly. Faced batts include a vapor retarder; whether you need one, and on which side, depends on climate and the wall assembly.

Scope & limits

Supply net coverage and pack data; width presets are typical products. The R-value helper adds layer R-values in series only. This is not a thermal performance or code compliance check.

Frequently asked questions

Further reading

• Understanding R-value and U-factor
• Typical exterior wall assemblies
• IECC 2018 to 2021 changes
• All materials & quantities tools
• All free architecture & construction tools
• Harth resource library

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