Buying Lumber and Board Feet
Separate sales size from measured size and calculate board footage using an explicit dimension convention.
Nominal size is a name; actual size is what fits; Practice
Nominal size is a name; actual size is what fits
A nominal lumber size identifies a product category. Dry, surfaced construction lumber sold as a 2 × 4 commonly measures 1-1/2 × 3-1/2 in in cross-section. Rough, green, specialty, and reclaimed stock can differ. Measure the piece when fit matters.
Drying and surfacing help explain the difference, but nominal names are not a universal subtraction rule. Do not assume that removing the same amount from every nominal size will give its actual dimensions.
Use Measurement Language for units and Dimensional Analysis for conversions. Keep length units explicit: an 8 ft board is 96 in long, not 8 in.
Quarter thickness and surfacing describe different things
Quarter thickness and surfacing describe different things
Hardwood thickness is often named in quarter-inch increments before final surfacing: 4/4 means nominal one-inch stock and 8/4 means nominal two-inch stock. Neither guarantees that much finished thickness. Drying, flattening, and planing reduce the usable material.
- Rough
- Sawn surfaces remain; allow material for flattening and sizing.
- S2S
- Surfaced on two sides, normally the two broad faces. Do not assume the edges are straight or ready for a joint.
- S4S
- Surfaced on all four long sides: both faces and both edges. Ends are not included in the four.
Ask for the supplied thickness, width, length, moisture condition, grade, and surfacing. A thickness label alone is not a complete purchase specification.
A board foot is volume, not length
A board foot is volume, not length
One board foot (BF) is the volume of a piece 1 in × 12 in × 12 in: 144 cubic inches. With thickness and width in inches and length in feet:
BF = thickness (in) × width (in) × length (ft) / 12.
If all three dimensions are in inches, divide by 144 instead. Multiply by the number of identical boards only after finding the volume of one.
Why the mixed-unit formula divides by 12
Convert the length in feet to inches by multiplying by 12. Dividing the resulting cubic inches by 144 gives 12/144 = 1/12. The formula counts 144-cubic-inch units; it is not a linear-foot conversion.
State the measuring convention. A physical-volume calculation uses actual dimensions. A sales tally may use nominal thickness or other trade rules and may round individual board measurements. Ask the seller; do not replace a quoted tally basis with finished dimensions without saying so. The examples here explicitly specify their basis and use unrounded arithmetic until the final answer.
Keep purchase tally and finished volume separate; Interactive; Practice
Keep purchase tally and finished volume separate
Suppose a seller specifies a tally using nominal 1 in thickness, measured 7 in width, and 9 ft length. The tally is 1 × 7 × 9 / 12 = 5.25 BF.
If that board finishes at 3/4 in thick with the same width and length, its physical volume is 0.75 × 7 × 9 / 12 = 3.9375 BF, or 3.94 BF to two decimals. These answers describe different quantities. The finished volume does not change the stated sales agreement.
A board tally is not a finished-parts yield. Allow for saw cuts, defects, flattening, trimming, and the layout of the parts. Appearance grades and structural grades answer different questions; an attractive board is not automatically suitable for a load-bearing member.
Build a list of boards below to compare the sales tally with actual volume, calculate cost, and estimate a material allowance.
Say which allowance you mean. This calculator treats the allowance as the share of the material you buy that will not end up in parts: BF to buy = needed BF ÷ (1 − allowance), so a 20% allowance means buying 1.25 times what you need. Many shop guides instead say “add 20%” to what you need (× 1.20), which is a different definition and buys less. Both are used; check which one a source or an instructor means before you compare numbers.
Practice: sales tally Practice: physical volume Practice: several boards Practice: all-inch dimensions Practice: material allowance