What your result means
The big number is the total board feet of foam to buy, including your waste allowance. One board foot is one square foot of foam one inch thick, which is how spray foam is sold. The kit line shows the fewest 600 and 200 board-foot DIY kits that cover it. The pro set figure tells you how much of a contractor’s 55-gallon set your job would use, which is useful when you compare quotes.
If the job comes to more than about 1,500–2,000 board feet, DIY kits usually stop being the cheaper option. At that size a contractor with a proportioning rig is often cheaper per board foot. They also get better yield and a more even foam.
How to use this calculator
- Pick the foam. Closed-cell is denser, adds more R-value per inch, and blocks moisture. Open-cell is cheaper per board foot and is often used in rooflines and interior walls.
- Enter the area. Type the total square feet, or switch to length × height for a single wall or roof plane. Add separate surfaces together and enter the total.
- Say whether you’re spraying between framing. Studs and joists take up part of the surface, so you need less foam than the full area suggests.
- Choose the depth. Enter a thickness in inches, or enter the R-value you need (for example, from your local energy code) and the calculator works out the thickness.
- Set waste. Keep 10% for most jobs. Raise it for cold weather, small or awkward cavities, or if it’s your first time spraying.
How spray foam is calculated
The calculator uses the same three steps an estimator would:
Spray area = Area × (1 − framing share)
Board feet = Spray area × Thickness (in) × (1 + Waste %)
R-value = Thickness (in) × R per inch- R per inch: 3.7 for open-cell and 6.5 for closed-cell. These are midpoints of the typical published ranges (about R-3.5–3.8 and R-6–7 per inch). Your product’s data sheet has the exact figure.
- Framing share: a 1½" stud every 16" covers about 9.4% of a wall, and every 24" about 6.3%. Plates, headers and blocking take up more, so treat the framing deduction as the most you can save.
- DIY kits: two-component kits are sold by nominal board feet, most often 200 and 600. Nominal yield assumes ideal temperature and technique, so real yield is often lower. That’s what the waste allowance covers.
- Pro sets: one 55-gallon set (an A drum and a B drum) yields roughly 14,000–16,000 board feet of open-cell or 4,000–5,000 of closed-cell. We use 15,000 and 4,500.
Full details and sources are on the methodology page. To turn board feet into dollars, use the spray foam cost calculator.
Worked example: basement walls
Say you’re foaming the concrete walls of a basement. The perimeter is 140 feet and the walls are 8 feet tall. You want 2 inches of closed-cell foam straight onto the concrete, so there’s no framing to deduct.
- Area: 140 × 8 = 1,120 sq ft
- Board feet before waste: 1,120 × 2 = 2,240
- With 10% waste: 2,240 × 1.10 = 2,464 board feet
- R-value: 2 × 6.5 = R-13
- DIY kits: 4 × 600 + 1 × 200 (2,600 nominal board feet)
- Pro: about 0.55 of a closed-cell set
Five kits is a big DIY job. That’s exactly the size where it pays to get a contractor quote before you buy. Ask what they charge per board foot at 2", and compare it with your kit cost divided by 2,464.
Board feet and R-value per 100 sq ft
Board feet scale linearly with area, so this table gives you a quick check. Multiply by your area in hundreds of square feet. Figures are before waste and framing.
| Thickness | Board feet / 100 sq ft | Open-cell R-value | Closed-cell R-value |
|---|---|---|---|
| 1" | 100 | R-3.7 | R-6.5 |
| 2" | 200 | R-7.4 | R-13.0 |
| 3" | 300 | R-11.1 | R-19.5 |
| 3.5" (full 2×4 cavity) | 350 | R-13.0 | R-22.8 |
| 5.5" (full 2×6 cavity) | 550 | R-20.4 | R-35.8 |
Open-cell vs closed-cell: which numbers to use
The two foams need very different amounts for the same job. To hit R-13, closed-cell needs about 2.0" of foam. Open-cell needs about 3.5", which is nearly twice the board feet. Open-cell is cheaper per board foot, though, so the total cost is often closer than the quantities suggest.
- Closed-cell is usually the pick for basement and crawl space walls, rim joists, metal buildings and anywhere moisture is a concern. At around 1½–2" it also works as a vapor retarder.
- Open-cell is common for unvented attic rooflines and interior walls where sound control matters. It stays vapor-open, so check what your climate and code need before using it on a roof deck.
Spray thickness also has limits. Closed-cell is usually applied in lifts of about 2" or less, with cooling time between passes. Thicker passes can overheat, shrink, or pull away from the framing. The calculator flags this when your thickness needs more than one pass.
Tips that change how much you buy
- Warm the kit and the surface. DIY kits are rated at specific chemical and surface temperatures, usually printed on the box. Spraying cold can cut yield a lot.
- Don’t overfill open cavities. Foam that rises past the stud face gets trimmed off and thrown away. That’s board feet you paid for.
- Measure the real surface, not the floor plan. Pitched rooflines are bigger than the attic floor. Multiply the floor area by the roof-pitch factor (about 1.12 for a 6/12 roof).
- Plan for a thermal barrier. In living spaces, building codes generally require foam to be covered by a thermal barrier such as ½" drywall. Budget for it alongside the foam.
- Protect yourself. Wear a properly fitted respirator, eye protection and skin cover, and ventilate while spraying and curing. The EPA’s spray foam guidance explains why.
Frequently asked questions
How many board feet are in a 600 spray foam kit?
A “600” kit is rated for a nominal 600 board feet, which is 600 sq ft at 1" thick or 300 sq ft at 2". Real yield depends on temperature and technique, and is often lower. That’s why the calculator adds a waste allowance before matching kits.
How much area does a 200 board-foot kit cover?
Divide 200 by your thickness in inches: 200 sq ft at 1", 100 sq ft at 2", or about 57 sq ft at 3½". Small kits are best for rim joists, gaps and patch work.
How thick should spray foam be?
It depends on the R-value your code and climate need. A common target in walls is R-13 to R-21. With closed-cell that’s about 2–3", and with open-cell it’s a full 3½" or 5½" cavity. Use the target R-value mode above to convert.
How do I calculate spray foam for a pole barn or metal building?
Add up the wall and roof areas (subtract large doors), choose closed-cell, and enter the total with framing set to “No”, because foam is usually sprayed over the steel panels. Many metal buildings use 1–2" of closed-cell to control condensation.
Is it cheaper to DIY or hire a contractor?
DIY kits usually win on small jobs: rim joists, a single wall, patching. On large jobs, contractors buy foam in bulk sets and often charge less per board foot than kit prices, with better yield. Get a quote whenever your total passes about 1,500–2,000 board feet.
Does spray foam replace a vapor barrier?
Closed-cell foam at about 1½–2" generally works as a vapor retarder, while open-cell does not. What your wall or roof needs depends on your climate zone and local code, so check before you skip a separate barrier.
Sources
- U.S. Department of Energy, Guide to Home Insulation and Building America Which Spray Foam Is Right for You?
- ENERGY STAR, Recommended Home Insulation R-Values
- U.S. EPA, Spray Polyurethane Foam (SPF) Insulation and How to Use It More Safely
- Manufacturer technical data sheets for open- and closed-cell SPF products (R-value per inch, maximum pass thickness, set yield)