A board foot of spray foam is a volume measurement: one square foot of foam, one inch thick (1 ft x 1 ft x 1 in). To find board feet, multiply the square footage of the area by the number of inches of foam. So 1,000 square feet of wall sprayed 3 inches thick is 3,000 board feet. Spray foam contractors use board feet because foam is sold, sprayed, and tracked by volume. Once you understand the unit, a spray foam quote becomes much easier to read and compare. Why Spray Foam Use