onlinecalculator.me

Free online insulation calculator

Insulation amount depends on your target R-value and each product's coverage rate. This calculator converts area and R-value into a bag or roll count for blown-in fiberglass.

About this calculator

How to use

  1. Enter the area to insulate in square feet.
  2. Choose the insulation type — blown-in fiberglass or cellulose, batts, or spray foam.
  3. Pick your target R-value from the dropdown.
  4. Set a waste factor (5% is typical for blown-in; batts rarely need more than 5%).
  5. Read bags or rolls needed, thickness required, and coverage per unit.

Formulas

Area with waste:

area needed = ceil(area × (1 + waste% / 100))

Blown-in fiberglass (R-2.2/in):

thickness   = R-value / 2.2  inches
coverage    = 1000 / 17 × (30 / R-value)  sq ft/bag
bags        = ceil(area needed / coverage)

Blown-in cellulose (R-3.5/in):

thickness   = R-value / 3.5  inches
coverage    = 25 × (30 / R-value)  sq ft/bag
bags        = ceil(area needed / coverage)

Fiberglass batts:

thickness: R-11/13/15 → 3.5 in, R-19 → 6.25 in, R-21 → 5.5 in,
           R-30 → 9.5 in, R-38+ → 12 in
coverage per roll ≈ 40 sq ft
rolls = ceil(area needed / 40)

Spray foam (open-cell, R-3.5/in):

thickness = R-value / 3.5  inches
coverage  = 200 / thickness  sq ft per kit (at 1-inch yield × thickness)
kits      = ceil(area needed / coverage)

Worked example

Attic 1,000 sq ft, blown-in fiberglass, R-30, 5% waste:

area needed   = ceil(1000 × 1.05)          = 1050 sq ft
thickness     = 30 / 2.2                   ≈ 13.6 in
coverage/bag  = (1000/17) × (30/30)        ≈ 58.8 sq ft
bags          = ceil(1050 / 58.8)          ≈ 18 bags

Notes

  • Blown-in insulation settles over time — installers typically overfill by 10–15% to reach the settled R-value target.
  • Always check your local building code and the DOE Climate Zone map for minimum R-value requirements.
  • Hire a certified contractor for spray foam — off-ratio mixing is a common DIY failure that leaves foam soft and non-insulating.
  • For walls, factor in studs. Studs conduct heat around batt insulation; continuous rigid foam on the exterior eliminates most of that thermal bridging.

Frequently asked

What R-value do I need?
The DOE recommends R-30 to R-60 for attics in most US climate zones, R-13 to R-15 for exterior walls, and R-19 to R-25 for floors over unheated spaces. Higher R-values save more energy but cost more upfront.
How many bags of blown-in fiberglass do I need for R-30?
A common estimate is 17 bags per 1,000 square feet at R-30. The calculator scales this linearly for other R-values and adds your waste factor.
What is the R-value per inch for common insulation types?
Blown-in fiberglass: about R-2.2 per inch. Blown-in cellulose: about R-3.5 per inch. Open-cell spray foam: about R-3.5 per inch. Closed-cell spray foam: about R-6 per inch. Fiberglass batts: R-3.1 to R-4.3 per inch depending on density.
Should I use blown-in or batts in my attic?
Blown-in insulation fills gaps and covers joists, providing higher effective R-values in most attics. Batts are faster to install in open walls during new construction but leave gaps if not cut carefully. Many pros recommend blown-in for finished attics.
What is the difference between open-cell and closed-cell spray foam?
Open-cell foam is softer, lighter, and cheaper — R-3.5/in. Closed-cell is rigid, acts as a vapor barrier, and reaches R-6/in. Closed-cell is the better choice for unvented roofs and below-grade applications.
How do I share my insulation estimate?
Click Share for the page link, or Share with my numbers to save your area, type, and R-value in the URL.