Measure insulated surfaces
Calculate each wall, floor, or ceiling area receiving the same insulation and subtract openings where appropriate.
Keep assemblies with different R-values or framing spacing separate.
Match batt width to framing
Batt dimensions are designed around specific wood- or steel-framing layouts.
Keep the assumption visible so it can be replaced when project or product data is available.
Use package coverage
Divide net demand plus justified cutting allowance by the exact square feet covered per package.
Round up to whole packages only at the end.
Keep R-value separate from quantity
More packages do not compensate for choosing an insulation product that does not meet the required thermal specification.
Select the required assembly and product first, then estimate quantity.
Allow for real cuts
Odd cavities, blocking, penetrations, and small remnants can create unusable offcuts.
Use a project-specific allowance instead of a universal percentage.