acalculator

How much spray foam do I need?

Type the area to cover, how thick to spray, and the foam type. The spray foam calculator shows the board feet to buy, how many kits that is, and the R-value you get.

Your numbers

Units
Foam type
Board feet to buy
1,100

Buy 1,100 bd ft of spray foam (kits: 2) for about R-12.6.

Board feet sprayed
1,000
Kits
2
R-value
12.6
Volume (ft³)
91.7
Volume (m³)
2.6

Board feet to buy: 1,100. Buy 1,100 bd ft of spray foam (kits: 2) for about R-12.6.

How to calculate

Computes the board feet of spray foam insulation for an area and thickness, with extra for waste, how many kits to buy, and the R-value of open-cell or closed-cell foam.

Example with the default inputs (Area to cover 500 ft², Thickness 2 in, Foam type Closed-cell, Extra for waste 10%, Kit yield (board feet) 600): Buy 1,100 bd ft of spray foam (kits: 2) for about R-12.6.

Method: Board feet = area (ft²) × thickness (in); to buy = board feet × (1 + extra ÷ 100); kits = to buy ÷ kit yield, rounded up; R-value = R per inch × thickness (in).

  • A board foot of foam is 1 ft² sprayed 1 in thick (144 in³).
  • Closed-cell foam is R-6.3 per inch and open-cell R-3.7 per inch, from DOE’s Building America figures (3 in closed-cell is R-19, 7.5 in open-cell R-28); your product’s label may differ.
  • The extra for waste covers overspray and uneven thickness; 10% by default.

Machine-readable copies: Markdown, JSON.

Worked examples

Each example is checked against the calculator on every build.

  1. Area to cover 500 ft², Thickness 2 in, Foam type Closed-cell, Extra for waste 10%, Kit yield (board feet) 600 gives Board feet to buy 1,100, Board feet sprayed 1,000, Kits 2, R-value 12.6, Volume (ft³) 91.666667, Volume (m³) 2.595711.Source: US DOE Building America Solution Center, Unvented Conditioned Attic with Spray Foam Insulation Below Roof Deck: 3 in of closed-cell spray foam is R-19 and 7.5 in of open-cell spray foam is R-28, https://basc.pnnl.gov/resource-guides/unvented-conditioned-attic-spray-foam-insulation-below-roof-deck (retrieved 2026-10-18); NIST SP 811 (2008), Appendix B: 1 ft = 12 in, 1 in = 2.54 cm exactly
  2. Area to cover 1,200 ft², Thickness 5.5 in, Foam type Open-cell, Extra for waste 0%, Kit yield (board feet) 600 gives Board feet to buy 6,600, Kits 11, R-value 20.35.Source: US DOE Building America Solution Center, Unvented Conditioned Attic with Spray Foam Insulation Below Roof Deck: 3 in of closed-cell spray foam is R-19 and 7.5 in of open-cell spray foam is R-28, https://basc.pnnl.gov/resource-guides/unvented-conditioned-attic-spray-foam-insulation-below-roof-deck (retrieved 2026-10-18)
  3. Area to cover 215.3 ft², Thickness 1.969 in, Foam type Other, R-value per inch 7, Extra for waste 0%, Kit yield (board feet) 200 gives Board feet to buy 423.776001, Kits 3, R-value 13.779528, Volume (m³) 1.Source: NIST SP 811 (2008), Appendix B: 1 ft = 12 in, 1 in = 2.54 cm exactly

How it works

  • Board feet sprayed = area in ft² × thickness in inches. A board foot is 1 ft² × 1 in = 144 in³.
  • Board feet to buy = board feet sprayed × (1 + extra ÷ 100).
  • Kits = board feet to buy ÷ kit yield, rounded up to a whole kit.
  • R-value = R per inch × thickness in inches. Closed-cell foam is R-6.3 per inch and open-cell R-3.7 per inch. DOE's Building America Solution Center gives whole-layer values, not values per inch: 3 in of closed-cell foam is R-19, and 7.5 in of open-cell foam is R-28. The page divides each by its thickness, 19 ÷ 3 = 6.33 and 28 ÷ 7.5 = 3.73, and rounds to 1 decimal. Choose "Other" to type your product's value.
  • Volume in ft³ = board feet to buy ÷ 12; in m³ = ft³ × 0.028316846592.

An area in m² is divided by 0.09290304 to get ft², and a thickness in cm or mm is divided by 2.54 cm to get inches (exact NIST factors).

Exact arithmetic. Each value is read as the exact decimal you typed, in its unit, so every step is exact; kits are rounded up from the exact value.

Rounding. Board feet show as whole numbers, the R-value and ft³ with at most 1 decimal, m³ with at most 2, each rounded half up, trailing zeros left off.

Assumptions

  • The foam goes on at an even thickness over the whole area.
  • The extra for waste is 10% by default, from 0% to 100%.
  • The area is more than 0 and at most 1,000,000 m²; the thickness more than 0 and at most 1 m; the kit yield 1 to 1,000,000 board feet; a typed R per inch more than 0 and at most 20.

Worked examples by hand

500 ft² at 2 in of closed-cell foam, 10% extra, 600 board-foot kits. 500 × 2 = 1,000 board feet sprayed; × 1.1 = 1,100 to buy. 1,100 ÷ 600 = 1.83, so 2 kits. R-value 6.3 × 2 = R-12.6. Volume 1,100 ÷ 12 = 91.7 ft³ = 2.6 m³.

A 1,200 ft² roof deck with 5.5 in of open-cell foam, no extra. 1,200 × 5.5 = 6,600 board feet ÷ 600 = 11 kits. R-value 3.7 × 5.5 = 20.35, shown as R-20.4.

20 m² at 5 cm of a foam rated R-7 per inch, no extra, 200 board-foot kits. 20 ÷ 0.09290304 = 215.278 ft²; 5 ÷ 2.54 = 1.9685 in; 215.278 × 1.9685 = 423.78 board feet ÷ 200 = 2.12, so 3 kits. R-value 7 × 1.9685 = R-13.8. Volume 20 × 0.05 = 1 m³.

Other questions people ask

How do I calculate board feet of spray foam?

Multiply the area in square feet by the thickness in inches. A board foot is 1 ft² sprayed 1 in thick, so 500 ft² at 2 in is 500 × 2 = 1,000 board feet. Add about 10% for overspray: 1,100 board feet.

How many spray foam kits do I need?

Divide the board feet by the kit’s yield and round up. Kits are sold by the board feet they cover, often 200 or 600. 1,100 board feet ÷ 600 = 1.83, so buy 2 kits of 600.

What R-value does spray foam give?

Multiply the R-value per inch by the thickness. DOE’s Building America Solution Center shows 3 in of closed-cell foam as R-19 and 7.5 in of open-cell foam as R-28, about R-6.3 and R-3.7 per inch, so 2 in of closed-cell is about R-12.6. Use your product’s label value if you have it.

Should I use open-cell or closed-cell foam?

Closed-cell foam has about 1.7 times the R-value per inch and, at normal thickness, slows water vapor, but it costs more per board foot. Open-cell foam is lighter and cheaper, stops air, and lets water vapor through. Check what your building code and climate need.

How thick should spray foam be?

It depends on the R-value your building code asks for. For R-13, closed-cell foam needs about 13 ÷ 6.3 = 2.1 in, and open-cell about 13 ÷ 3.7 = 3.5 in.

How do I convert board feet to cubic feet?

Divide by 12. A board foot is 144 cubic inches, and a cubic foot is 1,728 cubic inches, so 1,200 board feet is 100 ft³ (2.83 m³).