How this is calculated
Roll count is net area plus waste, divided by the coverage printed on the packaging. Standard US batts often cover about 40 ft² per roll; UK rolls vary more — change the assumption to match your product.
This does not pick an R-value for you. Buy the thickness that meets your code or target; the calculator only handles how many rolls that thickness requires.
Measure each wall or ceiling plane separately, then add the areas. Door and window openings are often left in the gross area for batts because you still cut and fit around the opening — only subtract large openings if your installer works that way.
Attic and floor jobs may need two layers or a different product (blown cellulose, rigid board). If the label coverage assumes a single batt pass, do not force this roll math onto a blown-in quote.
Formula & methodology
Rolls = ceil((Area × (1 + waste %)) ÷ coverage per roll).
Worked example
A 14 ft × 12 ft wall, 10% waste, 40 ft² rolls
- 1.Area = 168 ft²
- 2.With 10% waste = 184.8 ft²
- 3.184.8 ÷ 40 = 4.62 → 5 rolls
Frequently asked questions
What coverage per roll should I use?
Read the label on the pack you are buying. A common US R-13 batt roll covers about 40 ft²; UK packs differ by brand and thickness.
Walls vs attic?
Same math — measure the flat area you are filling. Attic jobs often use multiple layers or blown insulation, which this tool does not model.
How much waste?
10% handles routine cuts around studs. Irregular framing or lots of outlets may need 12–15%.
Vapour barrier?
Not included. Follow local code on whether you need a separate vapour retarder and on which side of the batt it goes.
Does R-value affect roll count?
Thicker batts cover less area per roll for the same width. Always use the coverage on the exact product SKU you plan to buy.
Metric rooms?
Switch to metres. Enter roll coverage in m² from the EU/UK label.
Should I buy all rolls from one lot?
Where possible, yes — especially if you are matching faced batts. Running out mid-wall and switching SKUs is how gaps and thickness mismatches appear.
Does this include labour or PPE?
No. It is a materials count only. Fibreglass work needs gloves, mask, and long sleeves regardless of how tidy the roll math looks.
Last updated 2026-09-01. Formulas are planning methods — confirm with a supplier before you order.