Work out how much spray foam insulation a job needs. Enter the area and thickness, choose open-cell or closed-cell foam, and get the board feet, how many kits or drum sets to buy, the R-value you will reach and an estimated cost.
Last updated:
Units
Area and depth
sq ft
Wall, roof deck or rim joist area (length × height)
m²
Wall, roof deck or rim joist area
in
mm
Foam type
Leave blank for 3.7 (open) or 6.5 (closed); use the value on your product's label
Framing members take up part of the area
%
10–20% is typical; real yield is below the rated board feet
Shows the thickness needed, e.g. 13, 20 or 38
Kits and cost
bd ft
Cost estimate
$
Editable; installed open-cell often runs about $0.40–$0.70 (2025–2026)
$
Editable; installed closed-cell often runs about $1.00–$1.75 (2025–2026)
$
Enter the price of the kit you picked
Enter your values to see the result.
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How to use the Spray Foam Calculator
Enter the area to insulate and the foam thickness you plan to spray.
Pick open-cell or closed-cell foam and whether you are filling stud bays or an open surface.
Choose the kit size and a loss allowance for overspray and trimming.
Add a price per board foot or per kit for a cost estimate.
Read the board feet, kits, R-value and the thickness table.
Spray foam formulas
Board feet = foam area (sq ft) × thickness (in)
Foam area = wall or roof area × (1 − framing fraction), about 9.4% for studs 16 in o.c. and 6.3% for 24 in o.c.
Board feet to buy = board feet × (1 + overspray and yield loss)
Kits = board feet to buy ÷ rated kit yield, rounded up
R-value = thickness (in) × R per inch; RSI = R × 0.1761
Example: 400 sq ft of 2×4 wall, 16 in on center, with 2 in of closed-cell foam. Foam area = 400 × 0.906 = 362.5 sq ft. Board feet = 362.5 × 2 = 725, or 834 with 15% loss. That is two 600 board-foot kits, and the wall reaches about R-13 from the foam.
R-value by thickness
Thickness
Open-cell (R-3.7/in)
Closed-cell (R-6.5/in)
Board feet per 100 sq ft
1 in
R-3.7
R-6.5
100
2 in
R-7.4
R-13
200
3 in
R-11.1
R-19.5
300
3.5 in (2×4 cavity)
R-13
R-22.8
350
5.5 in (2×6 cavity)
R-20.4
R-35.8
550
7.25 in (2×8 rafter)
R-26.8
R-47.1
725
9.25 in (2×10 rafter)
R-34.2
R-60.1
925
Tips before you spray
Check the required R-values for your climate zone and the part of the house you are insulating; local energy codes set the minimums.
Kit chemicals spray best in a narrow temperature range. Cold kits and cold surfaces cut yield sharply, so read the kit's instructions.
Spray foam must usually be covered by an approved thermal barrier, such as 1/2 in drywall, in living spaces. Wear the protective gear the maker specifies and ventilate well.
Open-cell foam is normally overfilled and trimmed flush with the studs, so allow extra for the trimmed waste.
Frequently asked questions
What is a board foot of spray foam?
A board foot is one square foot of foam one inch thick (144 cubic inches). Multiply the square footage by the thickness in inches: 400 sq ft at 2 in is 800 board feet. Kits and drum sets are rated in board feet.
How many board feet does a spray foam kit cover?
DIY kits are sold by their rated yield, commonly 200 or 600 board feet, which is 200 or 600 sq ft at 1 in thick. At 2 in, a 600 board-foot kit covers about 300 sq ft. Ratings are theoretical, so real yield is usually 10–30% lower because of temperature, overspray and technique.
What R-value does spray foam have per inch?
Open-cell foam is about R-3.5 to R-3.8 per inch and closed-cell foam about R-6 to R-7 per inch. The calculator uses R-3.7 and R-6.5 by default; enter the value on your product's label for an exact figure.
How thick should closed-cell spray foam be?
It depends on the R-value you need. 2 in of closed-cell gives about R-13, 3 in about R-20 and 3.5 in (a full 2×4 cavity) about R-23. Closed-cell foam is normally sprayed in passes of about 2 in or less so it can cure properly.
Open-cell or closed-cell: which needs fewer board feet?
Closed-cell reaches the same R-value with roughly 40% fewer board feet, but each board foot costs more. Open-cell is cheaper per board foot and is often used to fill deep roof rafters, while closed-cell adds air and vapor control in thin layers and on rim joists.
Why subtract framing from the wall area?
Spray foam goes between the studs or joists, so the wood itself is not foamed. With 1.5 in wide studs 16 in apart the framing covers about 9.4% of the wall face; at 24 in spacing it is about 6.3%. Headers and plates add a little more.
How is the spray foam cost estimated?
Either board feet × price per board foot (typical for contractor quotes) or kits × price per kit (for DIY). Prices vary by region and year, so the defaults are editable starting points, not quotes.
Are all the tools on FreeToolCabin tested for quality and accuracy?
Yes! Every tool on our site undergoes automated Q/A testing in real browser environments to ensure high precision, correct output, and zero errors. If you happen to spot a bug or have a suggestion, please contact us at [email protected] and our team will review it right away.