Calculate spray foam insulation board feet, R-value, kits, and estimated cost from area and thickness for open or closed cell foam, or the area a kit covers.
Spray Foam Insulation Formula
Spray foam is measured in board feet. One board foot is one square foot of surface covered at one inch thick. The core calculation multiplies the area you are covering by the thickness you plan to spray:
BF = A * T
Because trimming, overspray, and uneven passes waste some product, add an overage percentage to get the amount you should actually buy:
BF_total = BF * (1 + W / 100)
Variables:
- BF is the board feet of spray foam for the job
- A is the area you are covering, in square feet
- T is the installed thickness, in inches
- BF_total is the board feet to buy after allowing for waste
- W is the overage percentage for trim and waste, often 10 to 15
Thickness sets your R-value. Each inch of foam adds a fixed amount of thermal resistance, so the R-value achieved and the thickness needed for a target R-value come from the R-value per inch:
R = T * Rpi T = R_target / Rpi
You can also run the math backward. If you already own a kit or a set with a known board foot yield, divide that yield by the thickness to see how much surface it covers, and divide the total board feet by a kit size to see how many kits to buy:
A_cov = BF / T Kits = roundup(BF_total / K)
Variables:
- R is the R-value achieved by the finished layer
- Rpi is the R-value per inch, about 3.7 for open cell and 6.5 for closed cell
- R_target is the R-value you want to reach
- A_cov is the area a fixed amount of foam will cover, in square feet
- K is the board feet per kit
- Kits is the number of whole kits to buy, rounded up
The calculator runs these relationships in three modes. In the material mode you enter an area and either a thickness or a target R-value, and it returns board feet, R-value, an estimated cost range, and the number of kits. In the coverage mode you enter the board feet you already have and a thickness, and it returns the area that amount will cover. In the R-value mode you convert between thickness and R-value for either foam type.
Foam Type and Thickness Reference
Open cell and closed cell foam behave differently, so the same project needs different thicknesses and budgets depending on which you choose. This table compares the two.
| Property | Open cell | Closed cell |
|---|---|---|
| R-value per inch | 3.5 to 3.8 | 6.0 to 7.0 |
| Cost per board foot | $0.44 to $0.65 | $1.00 to $1.60 |
| Vapor barrier | No | Yes, near 2 in |
| Adds rigidity | No | Yes |
| Board feet per two-drum set | About 16,000 | About 4,000 |
| Best uses | Interior walls, vented attics, sound control | Crawl spaces, basements, exterior walls, roof decks |
This second table is the part most calculators leave out. It pairs a typical thickness for each application with the board feet and the approximate material cost for every 100 square feet of surface, so you can size a job at a glance before you open the calculator. Cost uses mid rates of $0.55 per board foot for open cell and $1.30 for closed cell.
| Application | Foam | Thickness | R-value | Board feet / 100 sq ft | Material / 100 sq ft |
|---|---|---|---|---|---|
| Roof deck (unvented) | Closed | 5 in | R-32.5 | 500 | $650 |
| Attic floor (vented) | Open | 10 in | R-37 | 1,000 | $550 |
| Exterior wall | Closed | 3 in | R-19.5 | 300 | $390 |
| Exterior wall | Open | 5 in | R-18.5 | 500 | $275 |
| Basement wall | Closed | 2.5 in | R-16.3 | 250 | $325 |
| Crawl space wall | Closed | 2 in | R-13 | 200 | $260 |
| Rim joist | Closed | 3 in | R-19.5 | 300 | $390 |
Treat these as starting points. Local building code, climate zone, and whether a wall is above or below grade all shift the required depth, and DIY kits deliver less than their rated yield, so buy a margin above the number you calculate.
Example Problems
Example 1: Foam and cost for an attic roof deck.
You are spraying an unvented attic roof deck that measures 1,000 square feet with open cell foam at 8 inches. Multiply area by thickness for board feet, then add a 10 percent overage:
BF = 1,000 * 8 = 8,000 board feet. With 10 percent overage, BF_total = 8,800 board feet. The R-value is R = 8 * 3.7 = R-29.6. At a mid rate of $0.55 per board foot the material runs about $4,840, and at 600 board feet per kit you need 15 kits.
Example 2: Foam for a basement wall.
You are spraying 500 square feet of basement wall with closed cell foam at 3 inches. BF = 500 * 3 = 1,500 board feet, or 1,650 board feet with a 10 percent overage. The R-value is R = 3 * 6.5 = R-19.5. At a mid rate of $1.30 per board foot the material runs about $2,145, and you need 3 kits of 600 board feet.
Frequently Asked Questions
How much spray foam do I need per square foot?
It depends on thickness, because foam is sold by the board foot rather than the square foot. One square foot sprayed one inch thick is one board foot, so a 100 square foot wall at 3 inches needs 300 board feet. Decide your thickness first from the R-value you want, then multiply by the area.
Is open cell or closed cell cheaper for the same R-value?
Open cell is usually cheaper for a given R-value even though it needs more inches. To reach R-30 you need about 8.1 inches of open cell or about 4.6 inches of closed cell. The open cell job uses more board feet but at a much lower rate per board foot, so it often lands lower in total material cost. Closed cell wins when you also need a vapor barrier, added rigidity, or you are short on cavity depth. The calculator shows both side by side for your exact numbers.
How many board feet are in a spray foam kit or set?
Small DIY kits are rated around 200, 300, and 600 board feet. A full professional set of two drums yields roughly 16,000 board feet of open cell or about 4,000 board feet of closed cell, since closed cell is far denser. Rated yields assume ideal temperature and technique, so plan for 10 to 20 percent less in real conditions.
