Free online insulation 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.