How to work out spray foam
Measure the area you will spray, wall by wall or bay by bay. Then decide on the thickness. Foam is sold in board feet, and a board foot here means 1 square foot at 1 inch thick, so you multiply the two and stop there. You may see a sum that divides by 12, which is wrong for this unit.
A kit's name is its yield at 1 inch. DuPont's Froth-Pak kits are rated at 200 and 630 board feet, and it says these are theoretical, worked out in laboratory conditions without the loss of blowing agent or the variations in how people spray. It prints no loss figure. Our 5 percent allowance comes from one retailer's listing, read only as an excerpt, of 620 from a 650 kit.
The foam type sets the R-value. DOE's Building America guide gives R-3.6 per inch for open cell, and R-6 typical or R-6.1 aged for closed cell. It was last updated in 2009, and we could not open a more current DOE page. DuPont's own table shows 6.2 at 1 inch and 12.2 at 2 inches, aged 180 days, so R per inch falls slightly with thickness on that product. These figures do not cover cans of sealant, which are a different product.
A worked example
Here is 500 square feet of wall at 2 inches of closed cell, using 200 board foot kits. These lines come from the same formula the calculator above runs.
500 sq ft at 2 in, closed cell
- 500 × 2 = 1000 bd ft
- 1000 × 1.05 = 1050 bd ft with our allowance
- 1050 ÷ 200 = 5.25, so 6 kits
- 2 × 6.1 = R-12.2
With 630 board foot kits, the same job takes 2 kits. At 2 inches, a 200 kit covers 100 square feet and not 200. To compare foam with fiberglass or cellulose, try the insulation calculator or the blown-in calculator.
Spray foam by thickness
DOE's guide says closed cell installed to 5 inches in a 5-1/2 inch cavity would exceed R-30, which the 5 inch row shows. A 200 board foot kit's coverage is before any loss.
| Thickness | R, open cell | R, closed cell | Board feet per 100 sq ft | A 200 bd ft kit covers |
|---|---|---|---|---|
| 1 in | R-3.6 | R-6.1 | 100 | 200 sq ft |
| 2 in | R-7.2 | R-12.2 | 200 | 100 sq ft |
| 3 in | R-10.8 | R-18.3 | 300 | 67 sq ft |
| 3.5 in | R-12.6 | R-21.3 | 350 | 57 sq ft |
| 4 in | R-14.4 | R-24.4 | 400 | 50 sq ft |
| 5 in | R-18.0 | R-30.5 | 500 | 40 sq ft |
| 5.5 in | R-19.8 | R-33.5 | 550 | 36 sq ft |
Mistakes with spray foam kits
- Reading the kit's name as coverage. The yield is at 1 inch, so at 2 inches a 630 kit covers half the area.
- Assuming full yield. DuPont calls it theoretical. Real conditions come in below it.
- Ignoring the fire rating. DuPont's Class A rating covers up to 2 inches. Going thicker can change what your code asks for.
- Treating foams as interchangeable. DOE's guide warns that the choice has moisture consequences in the wall.
- Going by R-value alone. DOE's figures are aged, and products differ, so compare the aged numbers on the sheets you are buying from.
Questions
How many board feet of spray foam do I need?
Your area in square feet times the thickness in inches. A 500 square foot wall at 2 inches is 1,000 board feet. There is no division by 12, which is a common error, since a board foot here is 1 square foot at 1 inch thick.
Open or closed cell?
Closed cell gives more R per inch, about 6.1 aged against 3.6 for open cell, by DOE's guide. They also differ on moisture: DOE says open cell is air-impermeable but not vapor impermeable, and closed cell is a vapor retarder at typical thicknesses. Pick for the assembly.
How much spray foam do I need for 1,000 square feet?
2,000 board feet at 2 inches, which is 11 kits of 200 board feet or 4 kits of 630, with our 5 percent allowance. At 1 inch it is half that. The kit's rated yield assumes laboratory conditions, so real coverage comes in lower.
How much spray foam is in a can?
Read the yield on its label, because we do not cover cans here, as sealant is a different product from these kits. This page covers two-component kits rated in board feet, and the sum for a can would not be the same.
How much does spray foam insulation cost?
Multiply the kits this shows by your kit's price, since we hold no sourced figure for what kits cost. A hired crew charges in its own way, so ask for a quote per board foot. The insulation calculator compares other types.
This spray foam count is an estimate. A kit's yield is theoretical, worked out in laboratory conditions at 1 inch, so real coverage runs lower, and our waste figure is only an assumption. Read the label and data sheet for thickness limits and fire ratings, and follow its safety steps. Open and closed cell handle moisture differently, so check with your supplier.
Sources and review
- Which spray foam is right for you?, U.S. Department of Energy, 2009.
- Froth-Pak product data, DuPont, 2025.
Checked in November 2026. Spotted a wrong number? Report it and it goes in the corrections log.
How this is calculated
Every figure below is read from the code that produces the answer, so what is written here is what the calculator ran. The constants were last checked against their sources in November 2026.
The arithmetic
Board feet
Board feet = Area × Thickness in inches
- Thickness
- In inches, with no division by 12
Worked through: 500 × 2 = 1000 bd ft
R-value
R = Thickness × R per inch
- R per inch
- 3.6 for open cell and 6.1 for closed cell, aged
Worked through: 5 × 6.1 = 30.5
The constants, and where they come from
R_PER_INCH
Published figureAged R-value per inch, for open and closed cell foam
- open
- 3.6
- closed
- 6.1
Applies to: A DOE guide last updated in 2009. Products vary, so check your own sheet
Source: Which spray foam is right for you?, U.S. Department of Energy, published 2009-04, read 2026-11. This source is about 17 years old, and it is the one the figure rests on.
Also from: Froth-Pak product data, DuPont, published 2025, read 2026-11.
The guide says R-3.6 for open cell. For closed cell it says R-6 typical and R-6.1 aged. DuPont's kit tests at 6.2 at 1 inch and 12.2 at 2 inches, and an excerpt of another DOE guide says up to 6.5.
DEFAULT_WASTE_PERCENT
Modelled from published dataExtra added since real yield falls short of the rating
Applies to: Our own assumption, which you can change
Source: Froth-Pak product data, DuPont, published 2025, read 2026-11.
DuPont says its yield is theoretical, from perfect laboratory conditions, and prints no loss. A retailer's listing, read as an excerpt, shows 620 from a 650 kit, or about 5 percent.
What the labels mean
- Published figure:
- taken directly from the source named beside it.
- Modelled from published data:
- derived rather than measured, calibrated against the source named beside it.
If a figure here looks wrong, it may be. Tell us what you found and we will check it against the source and publish a correction if it needs one. How constants are chosen and how often they are reviewed is set out in our methodology.
How to use the spray foam calculator
- 1
Measure the area you are spraying
- 2
Decide how thick it goes
- 3
Choose open or closed cell
- 4
Read the yield from the kit
- 5
Check the data sheet for thickness limits before you spray
What this calculator assumes
Every estimate here rests on the assumptions below. Where your job differs from one of them, the figure will differ too.
- A board foot is 1 square foot, 1 inch thick
- R-values are aged figures from a DOE guide last updated in 2009
- A 5 percent waste allowance is our own assumption
- This page covers two-component kits, and not cans of sealant
Pro tips
- Check the sheet for thickness per pass and the fire rating
- Follow the maker's safety instructions, and ask your supplier about protective gear
- Keep the moisture behavior of the wall in mind before you choose