# 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.

Calculate bags or rolls of insulation needed for any area and R-value — covers blown-in fiberglass, cellulose, batts, and spray foam.

**Interactive version:** https://onlinecalculator.me/construction/insulation/
**Category:** Construction

## 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 questions

### 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.

## Sources

- [R-value (insulation) — thermal resistance](https://en.wikipedia.org/wiki/R-value_(insulation)) — Wikipedia
- [Recommended Home Insulation R-Values](https://www.energystar.gov/saveathome/seal_insulate/identify-problems-you-want-fix/diy-checks-inspections/insulation-r-values) — ENERGY STAR (U.S. EPA / DOE)

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