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.
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.
Last updated 2026-09-01. Formulas are planning methods — confirm with a supplier before you order.