Insulation Calculator
Estimate fiberglass batt count for walls, floors, or attics from net area and batt width. R-value is informational only.
Assumptions
- Standard 15 in or 23 in wide batts for stud spacing.
- Batt length assumed 48 in unless product differs.
- R-value does not change quantity — it reflects product choice.
- Default waste is 10% for cuts around obstructions.
- Does not include blown-in density calculations.
Coverage per batt = batt width × standard batt length (typically 48 in for walls). Batts needed = ceil(net area with waste ÷ coverage per batt).500 ft² wall area, 15 in batts
- Each batt covers 15 in × 48 in = 5 ft². Base batts = 500 ÷ 5 = 100. With 10% waste = 110 batts.
How to measure
Calculate net insulated area — wall area minus windows and doors, or attic floor footprint. Match batt width to stud or joist spacing (16 in OC → 15 in batts).
Common mistakes
- Using batts too wide for stud spacing (compression reduces R-value).
- Forgetting attic kneewalls and rim joists in the total area.
- Confusing R-value with square footage needed.
Frequently asked questions
15 in or 23 in batts?
15 in fits 16 in on-center framing. 23 in fits 24 in on-center (common in attics).
Blown-in vs batts?
This calculator is for batts. Blown insulation uses bag coverage by depth, not batt count.
Do I need a vapor barrier?
Depends on climate zone and assembly. Quantity here is insulation only.
Can I layer batts for higher R-value?
Only if the assembly is designed for it — do not compress batts to fit.