Board Feet Formula: Calculating Lumber Volume from Thickness, Width, and Length
Board feet measure lumber volume, not just length or area. The formula requires three dimensions: thickness in inches, width in inches, and length in either inches or feet.
BF = (Thickness_in × Width_in × Length_in) / 144 BF = (Thickness_in × Width_in × Length_ft) / 12 For multiple pieces: multiply result by quantity
The divisor 144 comes from the definition: one board foot = 1" × 12" × 12" = 144 cubic inches. Any combination of dimensions that equals 144 cubic inches is one board foot.
Common examples:
1×6×8 ft: (1 × 6 × 8) / 12= 4.00 BF
2×4×8 ft nominal: (2 × 4 × 8) / 12= 5.33 BF
2×4×8 ft actual (1.5"×3.5"): (1.5 × 3.5 × 96) / 144= 3.50 BF
1×12×6 ft: (1 × 12 × 6) / 12= 6.00 BF
For roof framing and deck projects, calculate the actual surface area first before estimating board feet. The Roof Pitch Calculator gives you the true sloped surface area, which is larger than the plan-view footprint.
Board Feet to Square Feet: Converting Lumber Area and Volume
Board feet add a thickness dimension to square feet. The relationship is straightforward: one board foot of 1-inch material covers exactly one square foot of surface. Thinner material covers more area per board foot; thicker material covers less.
Square feet = Board feet / Thickness in inches Board feet = Square feet × Thickness in inches
Board Feet
Thickness
Square Feet Covered
100 BF
3/4" (flooring)
133.3 sq ft
100 BF
1" (decking)
100.0 sq ft
100 BF
2" (planks)
50.0 sq ft
200 sq ft needed
3/4"
150 BF required
500 sq ft needed
1"
500 BF required
This conversion is most useful for flooring, decking, and wall paneling, where you know the surface area but need to buy by the board foot. Find your room or project area first with the Square Footage Calculator, then multiply by thickness in inches to get board feet needed.
Calculating Board Feet in a Log: Doyle, Scribner, and International Rules
Log board feet are calculated differently from dimension lumber because logs are tapered, round, and lose volume to slabs and sawdust. Three scaling rules are in common use across North America, each with different accuracy and regional adoption.
Doyle Rule
BF = ((D - 4) / 4)² × L
D = small-end diameter inside bark (inches), L = log length (feet)
Simplest formula. Underestimates small logs (under 16"), overestimates large ones. Most common in eastern hardwood markets.
Uses log diameter and length to look up values in standardized tables
Standard in the western United States for softwoods. More accurate than Doyle for small logs. Results are in Scribner board feet (Scribner Decimal C rounds to nearest 10).
Example: 12" diameter, 16 ft log: approximately 80 BF (Scribner)
International 1/4-inch Rule
Complex formula with taper correction
Accounts for log taper, kerf width (1/4"), and saw efficiency
Most accurate of the three. Standard for USDA Forest Service timber sales and eastern hardwood exports. Produces consistently higher estimates than Doyle for the same log.
Example: 12" diameter, 16 ft log: approximately 95 BF (International)
When buying standing timber or logs, confirm which rule the seller is using before agreeing on a price. A 14-inch log graded by Doyle may yield 30% fewer board feet than the same log measured by the International rule.
Nominal vs Actual Lumber Dimensions and Their Effect on Board Foot Count
Lumber sold at retail is labeled by nominal size, but the actual piece is smaller. A 2×4 is cut to 2 inches by 4 inches in its rough state, then dried and planed down to 1.5 inches by 3.5 inches before sale. The nominal dimensions stick as the product name even though the wood no longer matches them.
Nominal Size
Actual Size
BF at 8 ft (nominal)
BF at 8 ft (actual)
1×4
3/4" × 3½"
2.67 BF
1.75 BF
1×6
3/4" × 5½"
4.00 BF
2.75 BF
2×4
1½" × 3½"
5.33 BF
3.50 BF
2×6
1½" × 5½"
8.00 BF
5.50 BF
2×8
1½" × 7¼"
10.67 BF
7.25 BF
Retail lumber yards price by nominal board feet. When a sign reads "$3.50 per board foot," the BF count is based on nominal dimensions. This is standard and expected. The gap between nominal and actual matters most for rough-sawn hardwood from sawmills, where thickness is often sold closer to the nominal size before any planing.
For framing projects where you need accurate lumber quantities, enter actual dimensions into the calculator above. For ordering and pricing, use nominal dimensions to match what you will see on the supplier's invoice. The Drywall Calculator handles sheet counts for the same wall framing projects where you need lumber quantities.
Common Board Feet Calculation Mistakes
Using nominal dimensions when actual dimensions are needed
A 2×4 is 1.5" × 3.5", not 2" × 4". Using nominal sizes overstates your board feet by about 34% for a 2×4. For framing quantity estimates, use actual dimensions. For ordering lumber at a yard, use nominal.
Forgetting to add a waste factor
Real projects generate waste from end cuts, defects, and mistakes. Add 10 to 15% for straightforward projects. For complex layouts with diagonal cuts or figure-matching, 20% is more realistic.
Confusing board feet with linear feet
Linear feet measures length only. A 2×4×8 is 8 linear feet but 5.33 board feet (nominal). Board feet account for thickness and width. When suppliers quote prices in linear feet, verify the cross-section to compare correctly.
Mixing inches and feet in the same calculation
The formula uses inches for thickness and width, and either inches or feet for length (with different divisors). This calculator uses inches for all three dimensions. Entering length in feet when the formula expects inches overstates BF by 12x.
Not confirming the pricing basis before ordering
Ask whether the price per board foot is based on nominal or actual dimensions. Most retail yards use nominal, but some specialty hardwood dealers price on surfaced (actual) dimensions. The difference on thick slabs can be 20 to 30%.
HR
Hassaan Rasheed
Developer and Researcher, CalculatorFlux
Researches and verifies the formulas, methodology, and source data behind each calculator on CalculatorFlux. All tools are built and checked against the cited references before publication.
American Lumber Standard Committee: Voluntary Product Standard PS 20-15
Frequently Asked Questions
A board foot (BF) is a unit of lumber volume equal to 1 inch thick by 12 inches wide by 12 inches long, or 144 cubic inches total. It is the standard unit for buying and selling rough lumber in North America. One board foot is not a fixed shape. A piece 2 inches thick, 6 inches wide, and 12 inches long also equals one board foot, because the volume is the same.
Lumber yards price on nominal dimensions. When comparing prices between suppliers, check whether the price per board foot uses nominal or surfaced (actual) dimensions. For thick hardwood slabs, the difference can reach 20 to 30% of the quoted price.