The Basic Formula
Bags = (Square footage ÷ Coverage per bag) × 1.10
The 1.10 factor adds 10% for cuts, waste, and the occasional damaged bag. Most contractors order exactly this number — enough to cover the job without excess.
Step 1: Measure Your Square Footage
For Walls
Measure the total linear footage of exterior walls, multiply by ceiling height, then subtract openings (windows and doors):
(Linear feet × Height) - (Window area + Door area) = Net wall sq ft
Example: 150 LF of exterior wall × 9' ceilings = 1,350 sq ft, minus 200 sq ft of windows/doors = 1,150 sq ft net.
For Attics
Measure the floor area of the attic (same as the conditioned floor area below). For a 2,000 sq ft house, the attic floor is approximately 2,000 sq ft.
For Floors Over Unconditioned Spaces
Measure the floor area directly above the unconditioned crawl space or garage.
Step 2: Find Coverage Per Bag
Coverage per bag varies by R-value and batt width. Use this reference table:
| Product | Width | Coverage/Bag | Framing OC |
|---|---|---|---|
| R-11 | 15" | ~48 sq ft | 16" |
| R-13 | 15" | ~40 sq ft | 16" |
| R-13 | 23" | ~60 sq ft | 24" |
| R-15 HD | 15" | ~36 sq ft | 16" |
| R-19 | 15" | ~32 sq ft | 16" |
| R-21 HD | 15" | ~28 sq ft | 16" |
| R-30 | 16" OC | ~25 sq ft | 16" |
| R-30 | 24" OC | ~38 sq ft | 24" |
| R-38 | 16" OC | ~19 sq ft | 16" |
| R-38 | 24" OC | ~29 sq ft | 24" |
| R-49 | 16" OC | ~14 sq ft | 16" |
| R-49 | 24" OC | ~21 sq ft | 24" |
Step 3: Run the Calculation
Example: 2,400 sq ft house, zone 5, 16" OC framing
- Exterior walls (R-21): 1,200 net sq ft ÷ 28 sq ft/bag = 42.8 → 43 bags × 1.10 = 48 bags
- Attic (R-49): 2,400 sq ft ÷ 14 sq ft/bag = 171.4 → 172 bags × 1.10 = 190 bags
Or skip the math entirely — use the free calculator at prestigeinsulationco.com/calculator.html.
Common Mistakes to Avoid
- Ordering by gross wall area without subtracting windows and doors — you'll end up with 10–15% too much on a typical residential project
- Using the wrong batt width — 16" OC framing uses 15" batts, 24" OC uses 23" batts. Ordering the wrong width means wasted product and re-cutting every batt
- Forgetting the waste factor — 10% is standard. On a 200-bag order that's 20 bags — enough to matter if the job runs short
- Not accounting for multiple R-values — most projects use different products in different areas. Calculate each assembly separately
Ready to Place Your Order?
Call or text (929) 466-1426 with your bag count and R-value. We quote same-day and deliver next business day on stocked products.