Spray Foam Calculator
Calculate how many board feet of spray foam you need for a wall, ceiling or crawlspace, the R-value at that thickness, and how many 600 or 200 board foot kits to buy.
By CalcBuilt Editorial TeamUpdated September 9, 2026Formulas checked against manufacturer specs, see how we calculate.
Results
- Board feet to order
- 704 board feet
- 640 before 10% waste; 58.7 cu ft of foam
- R-value achieved
- R-13
- Closed-cell at R-6.5 per inch, 2 in thick. At this thickness closed-cell foam also acts as a vapor retarder
- 600 board foot kits
- 2 kits
- Or 4 kits of 200 board feet
- Area to spray
- 320 sq ft
- 40 ft x 8 ft minus openings
- Cured foam weight
- 106.7 lb
- Closed-cell cures at about 2 lb per cubic foot
Spray Foam formula
Area (sq ft) = Length (ft) x Height (ft) - Openings (sq ft)
Board feet = Area (sq ft) x Thickness (in) (1 board foot = 1 sq ft at 1 in thick)
Order quantity = Board feet x (1 + waste %); Kits = Order quantity / kit size (600 or 200)
R-value = Thickness (in) x R per inch (closed-cell 6.5, open-cell 3.7)Worked example
A 24 x 24 ft garage ceiling sprayed with open-cell foam 5.5 in thick to fill 2 x 6 rafters: 24 x 24 = 576 sq ft. 576 x 5.5 = 3,168 board feet. With 10% waste, 3,168 x 1.1 = 3,485 board feet, so 6 kits of 600 board feet or 18 kits of 200.
The R-value is 5.5 x 3.7 = R-20.35, about R-20. The cured foam is 3,168 / 12 = 264 cubic feet, which at 0.5 lb per cubic foot weighs 132 lb. A job this size is usually cheaper from a spray foam contractor, who prices by the board foot.
Quick reference
| Thickness | Board feet | With 10% waste | Closed-cell R-value | Open-cell R-value |
|---|---|---|---|---|
| 1 in | 100 | 110 | R-6.5 | R-3.7 |
| 1.5 in | 150 | 165 | R-9.75 | R-5.55 |
| 2 in | 200 | 220 | R-13 | R-7.4 |
| 3 in | 300 | 330 | R-19.5 | R-11.1 |
| 3.5 in | 350 | 385 | R-22.75 | R-12.95 |
| 5.5 in | 550 | 605 | R-35.75 | R-20.35 |
How to calculate spray foam
Spray foam is bought and priced by the board foot: one square foot at one inch thick. Measure the area you will spray, subtract doors and windows, and multiply by the thickness in inches. A 2 x 4 wall cavity is 3.5 in deep, a 2 x 6 cavity 5.5 in, and a 2 x 10 roof rafter 9.25 in, but closed-cell foam is rarely sprayed full depth because 2 to 3 in already gives R-13 to R-20 and an air seal.
Add at least 10 percent for waste. Kit yields assume tanks at 75 to 85 F, a substrate above 50 F and steady 1 in passes. A kit that sat in a 40 F garage sprays off-ratio, comes out dark and crumbly, and a 600 board foot kit yields 450 to 500. Warm the tanks indoors overnight and shake them before you start. The nozzle clogs about 30 seconds after you stop spraying, so swap it (kits ship with 8 to 10) rather than fight it. Once the hoses are charged the kit has to be used up within about 30 days.
Closed-cell vs open-cell
Closed-cell foam is dense, rigid and gives the most R-value per inch. It stops water vapor at 1.5 to 2 in, adds racking strength to a wall and is the choice for rim joists, crawlspace walls, basements and anywhere the cavity is shallow. Open-cell is soft, roughly half the R-value per inch and less per board foot, and it is used to fill full-depth wall and roof cavities where thickness is not the limit.
Both are combustible. IRC R316.4 requires a 15 minute thermal barrier, 1/2 in gypsum, between foam and living space. Attics and crawlspaces entered only for service need an ignition barrier instead (R316.5.3, R316.5.4): 1-1/2 in mineral wool, 1/4 in plywood, 3/8 in gypsum, or an intumescent coating tested with that foam. Spray in a full-face or supplied-air respirator with a suit and gloves; the isocyanate A side can sensitize your lungs for life after one bad exposure. Run exhaust fans and keep everyone out for 24 hours after a kit and 24 to 72 after a high-pressure rig, per the manufacturer's data sheet. Closed-cell goes on in 2 in lifts 10 to 15 minutes apart; a thicker pass overheats as it cures and can char or ignite. Above about 1 in over anything bigger than a rim joist, a rig contractor is typically cheaper per board foot and warrants the job. Kits typically run $1.25 to $1.50 per rated board foot before lost yield.
What this calculator leaves out
The result is foam only. Not included: the thermal or ignition barrier, which is drywall or an intumescent coating at typically $0.50 to $1 per sq ft applied. Nor extra nozzles, a spare 200 board foot kit for the yield shortfall, or plastic and tape to mask windows, floors and anything within 20 ft of overspray. Open-cell in zones 5 and colder also needs a vapor retarder. Also soffit baffles or vent chutes if the roof stays vented, and removal of old batts, since foam does not bond through them. Most jurisdictions require a permit and an inspection before the foam is covered.
Mistakes that cost money
- Tanks under 75 F spray off-ratio: crumbly or gummy foam and 20 to 30% less yield; warm the kit indoors overnight, substrate above 50 F.
- Closed-cell over 2 in in one pass overheats as it cures, chars and can ignite; lift 2 in, wait 10 to 15 minutes, spray again.
- Foam left exposed in a basement or garage fails IRC R316.4; it needs 1/2 in drywall or an approved intumescent coating before occupancy.
- Re-entering the same day: allow 24 hours after a DIY kit and 24 to 72 after a high-pressure rig, exhaust fans running the whole time.
- Over 1 in on anything bigger than a rim joist, hire a rig; kits typically run $1.25 to $1.50 per rated board foot.
- Open-cell alone on a roof deck in zone 5 and colder wets the sheathing; R806.5 wants R-20 of closed-cell under it, or all closed-cell.
Key facts
- A board foot of spray foam is one square foot sprayed one inch thick, so 12 board feet make a cubic foot.
- Closed-cell foam cures at about 2 lb per cubic foot and delivers R-6 to R-7 per inch; open-cell cures at about 0.5 lb per cubic foot and delivers R-3.5 to R-3.8 per inch.
- Two-component DIY kits are sold in 200 and 600 board foot sizes; the rated yield assumes a 1 in pass on a warm, clean surface.
- Closed-cell foam becomes a Class II vapor retarder at about 1.5 to 2 in, which is why it is used on crawlspace walls and rim joists without a separate vapor barrier.
- IRC R316.4 requires a 15 minute thermal barrier, normally 1/2 in gypsum board, between spray foam and living space; attics and crawlspaces entered only for service need an ignition barrier (R316.5.3, R316.5.4).
Frequently asked questions
How many square feet does a 600 board foot kit cover?
600 sq ft at 1 in thick, 300 sq ft at 2 in or 200 sq ft at 3 in. Real yield is usually 10 to 20% lower than the rating, so add waste.
How thick should closed-cell spray foam be?
2 in gives R-13 and is the common minimum in a 2 x 4 wall; 3 in gives R-19.5. In an unvented roof, IRC Table R806.5 sets the minimum air-impermeable layer against the deck: R-5 in zones 1 to 3, R-15 in 4A and 4B (R-10 in 4C), R-20 in zone 5, R-25 in 6, R-30 in 7 and R-35 in 8. That is about 1 to 5.5 in of closed-cell; the rest of the required R can be batts or open-cell under it.
Do I have to cover spray foam with drywall?
Inside living space, yes: IRC R316.4 requires a 15 minute thermal barrier, which is 1/2 in gypsum board, over any foam. An attic or crawlspace you only enter for service gets by with an ignition barrier (R316.5.3 and R316.5.4): 1-1/2 in mineral wool, 1/4 in plywood, 3/8 in gypsum, or an intumescent coating tested with that foam, typically $0.50 to $1 per sq ft sprayed. Rim joists and sill plates are the exception: R316.5.11 allows foam up to 3-1/4 in bare if it is rated flame spread 25 or under, which closed-cell kits are.
Do I need a vapor barrier with spray foam?
Not with closed-cell at 1.5 in or more; it is a vapor retarder on its own. Open-cell foam lets vapor through, so in cold climates it needs a separate vapor retarder or a coat of vapor retarder paint on the drywall.
Sources and references
- Froth-Pak Insulation Spray Foam product information sheet (R-value, density and board-foot kit yields) (DuPont)
- Which Spray Foam Is Right For You? Open-cell and closed-cell SPF properties, Building America (U.S. Department of Energy)
- Chapter 3 Building Planning, Section R316 Foam Plastic (thermal and ignition barriers), 2021 IRC (ICC)
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Results are estimates based on standard formulas and typical product specifications. Confirm quantities with your supplier and local code before ordering.