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Batts are 1160 mm long and fit between the framing, so batts = area ÷ (1.16 × frame spacing). A 12 × 9 m ceiling at 450 mm centres is 108 ÷ 0.522 = 207 batts; divide by the batts per pack on the label.

Your job

Where

Area 1

Batts

Framing centres

R-value

The R-value your home needs depends on its climate zone: check the energy rules for your area.

Pack sizes vary by brand and R-value. Makers don't publish a wastage figure, so add your own if you're cutting around a lot.

Optional

Ask your insulation supplier. The greyed number is an example only and is never used in your result.

Joists at 450 mm: 430 × 1,160 mm batts, one per spaceR4.0 batt, about 195 mm thick105 mm above a 90 mm joist

One batt fills each space between the framing. Below, the batt's thickness against a 90 mm joist or stud.

Batts

207batts

Add batts per pack to get packs

Area to insulate

108.0m²

1 area

Batt size

430mm wide

× 1160 mm, for 450 mm framing

Thickness

195mm

Typical R4.0; check roof clearance

The working, with your numbers

  1. 1. Area12.00×9.00= 108.00 m²
  2. 2. One frame space1.16 m × 0.45 m= 0.522 m²
  3. 3. Batts108.00 ÷ 0.522, rounded up= 207
Typed those measurements in by hand?MetreMate measures them straight off your PDF plans. Free to start.

The formula, in plain words

  1. 1Area = length × width of each ceiling (or length × height of each wall), less windows, doors and spaces left uninsulated.
  2. 2Each batt fills one frame space: 1.16 m long by the frame spacing (0.45 or 0.6 m). Batts = area ÷ (1.16 × spacing), rounded up.
  3. 3Packs = batts ÷ batts per pack from the label, rounded up. Pack sizes vary by brand and R-value.
  4. 4Thickness grows with R-value: a typical R4.0 ceiling batt is about 195 mm, so it stands well above a 90 mm joist.

Worked example

A 12 × 9 m ceiling with joists at 450 mm, R4.0 batts.

  1. 1.Area: 12 × 9 = 108 m².
  2. 2.One frame space: 1.16 × 0.45 = 0.522 m².
  3. 3.Batts: 108 ÷ 0.522 = 206.9, so 207 batts, about 195 mm thick.

Answer: 207 batts (430 × 1160 mm); packs depend on the pack size on the label.

How to measure it

  • Measure the ceiling's length and width from inside the roof space, or the room below.
  • For walls, measure length and height and add up the doors and windows to take off.
  • Check the joist or stud spacing: 450 or 600 mm centres.
  • Leave out manholes and the clearance zones round downlights and flues.

Where the numbers come from

Every default in the calculator is a number a supplier or manufacturer publishes. We list the source next to it so you can check.

WhatValueSource
Timber-frame batts are 1160 mm long: 430 mm wide for 450 mm centres, 580 mm wide for 600 mm centres (R7.0 ceiling batts 460 and 620 mm)1,160 mm longKnauf Earthwool, Ceiling Batt data sheet (2024) (checked 2026-10-01)
Ceiling batt thickness: R3.5 175 mm, R4.0 195 mm, R5.0 210 mm, R6.0 275 mm, R7.0 330 mm195 mm at R4.0Knauf Earthwool, Ceiling Batt data sheet (2024) (checked 2026-10-01)
Wall batts for 90 mm studs: R2.0 at 90 mm90 mm at R2.0Fletcher Insulation, Pink Batts trade brochure (2026) (checked 2026-10-01)
Pack calculator results don't include wastage: discuss with your installer0 %CSR Bradford, pack calculator (checked 2026-10-01)

Manufacturers named here are the source of the number, not a recommendation. Confirm quantities with your supplier.

Frequently asked questions

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