How much spray foam do I need?
Spray foam is sold and estimated in board feet. One board foot is one square foot of surface covered to one inch thick (1 ft² × 1 in = 1/12 ft³), so a 2-inch coat over 100 ft² is 200 board feet. Enter each surface's area and either a foam thickness or a target R-value, and the calculator returns the board feet you need, how many open- or closed-cell DIY kits or pro sets that works out to, and an estimated cost — cavity or surface, metric or imperial, entirely in your browser.
Closed-cell foam runs about R-6.5 per inch and air-seals rim joists and below-grade or exterior walls; open-cell runs about R-3.6 per inch and is common on roof decks and interior partitions. The calculator keeps a separate board-foot pool per foam type — a closed-cell kit can't fill open-cell board feet — and divides each pool by a real-world yield efficiency (typically 65–80%) to get the number of kits to actually buy.
The board-foot formula
Every surface reduces to the same identity: board feet is the sprayed area in square feet multiplied by the coat thickness in inches. A whole number of board feet is exactly one square foot at one inch, so the unit is easy to picture and it is the number printed on every kit box worldwide.
Every surface reduces to the same identity:
The board-foot formula
board feet = area (ft²) × thickness (in) × (1 + waste)- Area comes from your dimensions or a direct entry; thickness is entered directly or derived from a target R-value as target R ÷ R-per-inch.
- Kits to buy = ceil(pool board feet ÷ (rated board feet × yield efficiency)); leftover board feet is what a whole number of kits leaves spare.
Open-cell vs closed-cell spray foam
The biggest difference in this calculator is R-per-inch. Closed-cell foam is denser and delivers roughly R-6.5 per inch, so it hits a target R in far less thickness — and far fewer board feet — than open-cell foam at about R-3.6 per inch. At the same R-19 target, closed-cell needs about 2.9 inches while open-cell needs about 5.3 inches, as the two bays below show.
Closed-cell also acts as an air and vapour retarder and adds rigidity, which is why it is the usual choice for rim joists, crawlspaces and exterior walls; open-cell is cheaper per board foot, more flexible, and common on roof decks and interior sound-damping walls. Because thicker open-cell means more board feet, the kit count and cost move with the foam type you pick per surface.
How many kits or pro sets to buy
A kit's rated board feet is a theoretical maximum. On a real job you lose foam to cold substrate, technique, over-spray and hose purging, so the yield efficiency you actually get is usually 65–80%. The calculator divides each foam-type pool by that efficiency before rounding up to whole kits: kits to buy = ceil(pool board feet ÷ (rated board feet × yield efficiency)).
Yield efficiency is a separate control from the waste percentage on each surface. Waste covers extra material bought for offcuts and complex geometry on the area side; yield efficiency covers the kit under-performing its label. For large jobs, switch the kit picker to a two-drum pro set instead of a DIY kit — the same math sizes how many sets you need.
Rim joists and quick air-sealing jobs
The default project is a small closed-cell rim-joist band — the most common first spray-foam job. Rim joists and roof decks are sprayed as an open slab against the substrate rather than inside a bounded stud cavity, so the calculator draws them in surface mode with a depth ruler and no over-fill warning. A single small DIY kit usually covers a whole rim-joist perimeter.
Closed-cell foam is limited to roughly two inches per pass; thicker single lifts trap curing heat and can scorch, so the tool flags an advisory when a closed-cell coat exceeds that per-pass limit. It never changes the quantity — deeper coats simply go on in multiple passes.
Using the spray foam calculator
Six steps take a project from an empty sheet to a shopping list of kits and a cost. Everything recalculates as you type and persists in your browser between visits.
Estimate spray foam in six steps
- Choose units and currencyPick imperial or metric and your currency in the header. Board feet stays the purchase unit in every locale, with a litres readout beside it.
- Add each surfaceAdd a row per wall, ceiling, floor, roof deck or rim joist, and enter its area directly or as dimensions.
- Pick the foam typeChoose closed-cell, open-cell or custom per surface. Each foam type gathers into its own board-foot pool and kit list.
- Set a thickness or target REnter the coat thickness directly, or switch to R-target and let the calculator derive the thickness from the foam's R-per-inch.
- Choose kits and yield efficiencySelect a DIY kit or pro set per foam type and set the real-world yield efficiency (75% by default) that sizes how many to buy.
- Read the resultsThe rail shows board feet by foam type, kits to buy with spare board feet, volume and an optional cost or installed estimate.
Worked examples
Three quick examples show the board-foot math, an R-target deriving its own thickness, and the kit count including yield efficiency.
All figures are imperial and rounded for readability.
Closed-cell wall by thickness
thickness mode- Given
- Area: 240 ft² (20 ft × 12 ft wall)
- Foam: closed-cell, 2 in thick
- Waste: 10%
- Formula
bf = area × thickness × (1 + waste)- Steps
- 240 ft² × 2 in = 480 board feet
- 480 × 1.10 (waste) = 528 board feet
- Achieved R = 2 in × 6.5 = R-13
- Result
- ≈ 528 board feet of closed-cell foam
Roof deck to a target R-value
R-target mode- Given
- Area: 400 ft²
- Foam: open-cell (R-3.6 per inch)
- Target: R-21, waste 10%
- Formula
thickness = target R ÷ R-per-inch, then bf = area × thickness × (1 + waste)- Steps
- 21 ÷ 3.6 = 5.83 in of foam
- 400 ft² × 5.83 in = 2,333 board feet
- 2,333 × 1.10 = 2,566 board feet
- Result
- ≈ 5.8 in and ≈ 2,566 board feet of open-cell foam
How many closed-cell kits
kit count- Given
- Closed-cell pool: 528 board feet
- Kit: Froth-Pak 620 (rated 620 bf)
- Yield efficiency: 75%
- Formula
kits = ceil(pool bf ÷ (rated bf × yield))- Steps
- Effective yield = 620 × 0.75 = 465 board feet per kit
- 528 ÷ 465 = 1.14 → round up to 2 kits
- Spare = 2 × 465 − 528 = 402 board feet
- Result
- Buy 2 kits, ≈ 402 board feet spare
Spray foam calculator FAQ
How much spray foam do I need?
Multiply each surface's area in square feet by the coat thickness in inches to get board feet, then add a waste allowance. Add up the board feet by foam type and divide by a kit's real-world yield to get the number of kits. The calculator does all of this per surface and totals it for you.
What is a board foot of spray foam?
A board foot is one square foot of surface covered to one inch thick — 1 ft² × 1 in, or 1/12 of a cubic foot (about 2.36 litres). Kits and pro sets are all rated in board feet, so it is the number you match against the box.
Should I use open-cell or closed-cell foam?
Closed-cell gives about R-6.5 per inch, air-seals, resists moisture and adds rigidity — good for rim joists, crawlspaces and exterior walls. Open-cell gives about R-3.6 per inch, is cheaper per board foot and more flexible — common on roof decks and interior walls. Closed-cell reaches a target R in roughly half the thickness.
How many spray foam kits do I need?
Divide the board feet for that foam type by the kit's effective yield (rated board feet × yield efficiency), then round up. For example 528 board feet ÷ (620 × 0.75 = 465) is 1.14 kits, so you buy 2 and have about 402 board feet spare.
What R-value per inch does spray foam have?
Open-cell is about R-3.5 to R-3.7 per inch; closed-cell is about R-6.0 to R-7.0 per inch fresh and settles toward R-6.0 to R-6.5 as it ages. The calculator defaults to R-3.6 and R-6.5 and lets you override both when your product data sheet differs.
How thick can spray foam be applied in one pass?
Closed-cell foam is limited to roughly 2 inches per pass — thicker single lifts trap curing heat and can scorch, so deeper coats go on in multiple passes with a short wait between them. Open-cell has no comparable residential limit. The calculator shows an advisory but never changes the quantity.
Is a DIY kit or hiring a pro cheaper?
Small jobs like a rim joist are usually cheapest with a DIY kit. Whole-house jobs are often better as a pro two-drum set or an installed quote. Enter a price per board foot or per kit for the DIY total, and an optional installed price per area for a rough hire-it-out comparison — clearly labelled an estimate, not a quote.
Are the kit yields and prices accurate?
They are secondary-sourced defaults that vary by brand, formulation, temperature and technique — and marketing names sometimes overstate the manufacturer's own rated board feet. Every yield, R-value and price is overridable, and you should confirm against the manufacturer and a local quote before buying.
Does the calculator work in metric?
Yes. Enter areas and thicknesses in metric or imperial and the geometry converts automatically. Board feet stays the purchase unit because that is how kits are labelled everywhere, with a litres and cubic-metre readout beside it for intuition.
Is the spray foam calculator private?
Yes. Every calculation runs in your browser and nothing you enter leaves your device. Your project is saved to local storage on your own machine so it is still there when you come back.
All kit yields, R-values and prices are secondary-sourced defaults that vary by brand, temperature and technique. Confirm against manufacturer data and local quotes before buying.