Insulation Calculator
Calculate how much insulation you need, expressed as square footage of a given R-value batt or roll product.
ft
ft
Target R-Value
Area to Insulate
300 sq ft
You Need
330 sq ft
of R-19 insulation
How to Calculate How Much Insulation You Need
Enter the length and width of the area you're insulating — an attic floor, a wall cavity, or a crawlspace — and pick the R-value you're targeting for that space. The calculator multiplies length by width to get the area, then adds a waste allowance for trimming around joists, wiring, and obstructions to tell you how many square feet of that R-value batt or roll insulation to buy.
This calculator keeps things simple by working in square footage of a given R-value product rather than modeling specific materials or thicknesses — batt and roll insulation is sold by R-value and coverage area, so "you need X sq ft of R-19 insulation" is exactly what you'll be shopping for.
Example
A 20 ft × 15 ft attic floor has an area of 300 sq ft. With a 10% waste allowance for cutting around joists and fixtures, you'd need about 300 × 1.10 = 330 sq ft of R-19 insulation.
Common Use Cases
- Estimating attic, wall cavity, or crawlspace insulation before an install or top-up.
- Comparing material needs across different target R-values.
- Budgeting a home insulation upgrade project.
FAQs
What R-value should I target?
Recommended R-values vary by climate zone and the part of the home — attics in colder climates often target R-38 to R-60, while walls commonly use R-13 to R-21. Check your local building code or an energy guide for your specific climate zone and application.
Can I mix R-values or add a second layer on top of existing insulation?
Yes — R-values add together, so laying a new batt over existing attic insulation increases the combined R-value. Calculate the additional area you're covering and the R-value of the new layer separately from what's already there.
Why does this use square footage instead of volume or thickness?
Batt and roll insulation products are manufactured and sold at a fixed thickness for a given R-value, so the amount you need to buy is simply the area you're covering — thickness is already baked into the product you choose, not something you calculate separately.