How it works
The calculator finds the area to spray, multiplies by the thickness to get board feet, and divides by the yield of one kit.
Area = Length × Height − Σ (Width × Height × Count of openings)
Board feet = Area (sq ft) × Thickness (in) = Volume (cu in) ÷ 144
Kits = ⌈ Board feet × (1 + Extra ÷ 100) ÷ Kit yield ⌉
R-value = Thickness (in) × R-value per inch from the data sheet
⌈ ⌉ means rounding up to the next whole kit.
Worked example in words
A 40 ft run of 8 ft wall is 320 sq ft. Two 3 × 4 ft windows take off 24 sq ft, leaving 296 sq ft. Sprayed 2 in thick, that is 296 × 2 = 592 board feet, or about 49.3 cu ft of foam. One DAP 600 kit has a theoretical yield of 600 board feet, so one kit covers it – but only just, with no margin for cool conditions or overspray. With DAP’s aged R-value of 6.4 per inch, the 2 in layer is R-12.8.
What a board foot is
Spray foam kits are sized in board feet: one board foot is a piece 12 in × 12 in × 1 in thick, or 144 cubic inches – the same unit used for lumber. A kit’s board feet divided by the thickness in inches gives the area it covers. DAP’s data sheet also states its kits in metric: 600 board feet is about 56.6 m² at 25 mm.
Theoretical yield and real output
Both kit makers listed here print a theoretical yield and say real output depends on conditions. DAP’s data sheet says theoretical yields are calculated in perfect laboratory conditions, without counting the loss of blowing agent or differences in application, and asks for chemicals and surfaces in a set temperature range. Use the extra allowance to plan for this; there is no published figure for how much to add.
Worked example
Inputs
- Measure by
- Wall length × height
- Wall length
- 40 ft
- Wall height
- 8 ft
- Doors & windows (not sprayed) 1
- Width 3 ft, Height 4 ft, Count 2
- Foam thickness
- 2 in
- Kit
- DAP Touch ’n Foam 600 Low GWP – 600 board feet
- Extra allowance
- 0 %
Result
- Area to spray
- 296 sq ft
- Foam volume
- 49.33 cu ft
- Board feet
- 592
- Kits to buy
- 1
- R-value of the layer (US)
- 12.8
- RSI of the layer (m²·K/W)
- 2.25
Common mistakes
- Treating the kit yield as a promise. Kits are labelled with a theoretical yield; cold chemicals, cold surfaces or spraying too thick at once lower what you actually get.
- Forgetting the thickness. A 600 board-foot kit covers 600 sq ft only at 1 in – at 2 in it covers 300 sq ft, at 3 in 200 sq ft.
- Mixing up board feet and square feet. Board feet are square feet times inches of thickness.
- Using a low-pressure kit’s R-value for a different foam. Open-cell and closed-cell foams, and different brands, have different R-values per inch – use your product’s data sheet.
- Leaving foam exposed where codes require a covering. Many codes require a thermal or ignition barrier over foam plastic insulation in living spaces, attics and crawl spaces.
Assumptions and limits
- In dimension mode the walls form one rectangle of total length × height; doors and windows are rectangles of width × height × count.
- The foam has one even thickness over the whole area. Studs and joists are not subtracted, which slightly overestimates foam between framing.
- Kits are counted from the maker’s theoretical board-foot yield plus your extra allowance, and rounded up to whole kits.
- R-values are the aged values per inch from the kit makers’ data sheets, multiplied by the thickness. RSI is the same value converted to metric units (1 US R = 0.1761 m²·K/W).
- This is a quantity estimate. It does not choose a foam type, a thickness or an R-value, and it does not cover safety equipment or ventilation.
Frequently asked questions
How do I calculate board feet of spray foam?
Multiply the area in square feet by the thickness in inches. One board foot is 12 × 12 × 1 in, so 296 sq ft of wall sprayed 2 in thick needs 592 board feet. In metric, cubic metres × 423.78 gives board feet.
How much 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 – in theory. DAP and Foam it Green both describe these yields as theoretical, measured in ideal conditions, so plan for less in a cold basement or garage.
What R-value does spray foam give per inch?
It depends on the product. DAP’s data sheet for its 200 and 600 Low GWP kits gives an aged R-value of 6.4 at 1 in and 12.8 at 2 in; Foam it Green’s gives 5.8 aged (6.5 initial) at 1 in. Check the data sheet of the kit you buy.
How thick should I spray the foam?
The R-value you need is set by your local energy code and depends on your climate zone and where the foam goes. Kit instructions also limit how thick each pass can be. This calculator works with whatever thickness you enter.
How many kits do I need for a rim joist?
Add up the area of the rim joist bays – length × joist depth – choose “known area” and enter the thickness. For example, 150 sq ft at 3 in is 450 board feet, so one 600 board-foot kit, even with 15 % extra.
Sources
- DAP Spray Foam Kits 200 & 600 Low GWP – Technical Data Sheet (1/9/2025) — DAP Products Inc.. Theoretical yield "Up to 200 board feet" / "Up to 600 board feet"; a board foot is "a 12” x 12” square at 1” thick"; ASTM C518 aged R-value 6.4 at 1 in, 12.8 at 2 in.
- Foam it Green Low GWP (HFO) Flame-Retardant Low-Pressure Spray Foam Systems – TDS — A&B Filling, Inc. (Foam it Green). Foam it 602 yield "50 f3 | 600 f2 @ 1” thick"; R-value at 1 in (ASTM C518) 6.5 initial, 5.8 aged; published yields are theoretical.
- Measuring Wood: Board Feet (FEM-042) — University of Wisconsin–Madison Division of Extension. One board foot = 144 cubic inches (12 × 12 × 1 in).
- NIST Handbook 44, Appendix C – General Tables of Units of Measurement — National Institute of Standards and Technology. Exact unit definitions: 1 in = 2.54 cm, 1 yd³ = 27 ft³, 1 US gal = 231 in³, 1 lb = 0.45359237 kg.
Method reviewed November 5, 2026