What is a board foot of lumber?

A volume unit: a piece 1 inch thick, 12 inches wide and 1 foot long — 144 cubic inches of wood. Board feet = thickness (in) × width (in) × length (ft) ÷ 12. A 2-inch-thick, 6-inch-wide, 10-foot board is 2 × 6 × 10 ÷ 12 = 10 board feet.

What does 4/4, 6/4 and 8/4 lumber mean?

Hardwood thickness is quoted in quarters of an inch: 4/4 is 1 inch nominal, 5/4 is 1.25, 6/4 is 1.5, 8/4 is 2. These are rough-sawn dimensions — a 4/4 board surfaces down to about 3/4 inch, but hardwood is bought and priced by the nominal size.

Why does the calculator round my thickness up?

Because the hardwood trade does: thickness is measured rough-sawn and rounded up to the next quarter inch, and anything thinner than 4/4 is still counted and priced as 4/4. A 1.1-inch board is sold as 5/4. The checkbox lets you turn the rule off for exact arithmetic on surfaced stock.

How do I price lumber by the board foot?

Multiply total board feet by the price per board foot — the calculator does it when you fill the price field. Ten board feet at $12.50/BF is $125. Comparing suppliers, compare $/BF for the same nominal thickness; a 6/4 board carries 50% more wood per square foot of surface than 4/4 and should not match its price.

Board Foot Calculator (Hardwood Lumber)

Enter thickness, width, length and how many boards; get board feet per board, total board feet, and cost at your price per board foot.

Formula and example

Formula. Board feet = thickness (in) × width (in) × length (ft) ÷ 12, times the number of boards; one board foot is 144 cubic inches of wood.

Example. Three 4/4 (1 in) boards, 8 in wide and 8 ft long: each is 1 × 8 × 8 ÷ 12 = 5.33 board feet, so the lot is 16 board feet.

Thickness (in)

Nominal: 4/4 = 1, 5/4 = 1.25, 6/4 = 1.5, 8/4 = 2.

Width (in)

Rough-sawn or dressed — measure what you are paying for.

Length (ft)

Number of boards

Price per board foot ($, optional)

Apply the trade rule: thickness rounds up to the next ¼ in, minimum 4/4 (how hardwood is measured and sold).

The rule mirrors how hardwood suppliers tally: a 1.1 in board is 5/4; surfaced 3/4 in stock still counts as 4/4. Uncheck it for plain arithmetic on finished dimensions.

How to calculate board feet

A board foot is a volume — 144 cubic inches, pictured as a board one inch thick and a foot square. The formula divides by 12 because the length is in feet while thickness and width are in inches: thickness × width gives square inches of face, times length in feet equals twelfths of cubic inches, so ÷ 12 lands on cubic-feet-twelfths — which is exactly what a board foot is. The identical math with everything in inches divides by 144 instead. A 2 × 6 in board 10 ft long: 2 × 6 × 10 ÷ 12 = 10 board feet; double the thickness and you double the tally.

The trade conventions matter as much as the arithmetic. Hardwood thickness is quoted in quarters — 4/4, 5/4, 6/4, 8/4 — and those are nominal, rough-sawn sizes: a 4/4 board dresses out near 3/4 in after surfacing, but it is measured, tallied and priced as 1 inch. Suppliers also round thickness up to the next quarter inch (1.1 in sells as 5/4) and treat anything under 4/4 as 4/4, because the mill cut it that way. That is why the checkbox is on by default: with it, a surfaced 3/4 in board still tallies at 4/4, matching the invoice you will actually receive.

Where board feet trip people up is mixing measurement systems. Construction softwoods (the 2×6 wall stud world) are sold by the piece; hardwood is sold by the board foot; projects mix both freely. The other trap is width: hardwood is often random-width, so a real tally runs board by board — enter each width as its own row (or run the calculator per width group and add). For comparison shopping, everything reduces to $ per board foot at the same nominal thickness.

Sources & review date

Board-foot definition (144 cu in), quarter-size designations, nominal/roughsawn measurement and the round-up-to-next-quarter practice verified against Hardwood Industries' board footage page — a working hardwood supplier's own tally rules — fetched and checked October 2026.
Quarter notation cross-checked against lumber listings in daily trade use; 4/4 surfacing to ~3/4 in is the standard dressed-size relationship.

Last reviewed: October 2026. Spotted a stale number? info@orbitalfour.com. Dimensions, quantity and price are your inputs; the only rule on the page is the trade rounding, and it has an off switch.

Board foot FAQs

How many board feet is a 2x4 eight feet long?

Treating the nominal full 2 × 4 in: 2 × 4 × 8 ÷ 12 = 5.33 board feet. Framing studs are priced by the piece, not the board foot, but the volume conversion is useful when comparing lumber costs across stores.

Why is my invoice more board feet than I measured?

Almost always the nominal rule: the supplier tallies rough-sawn thickness (4/4 = 1 in) and rounds up to the next quarter, while you measured the surfaced board (3/4 in). Ten boards that tally 1 BF each bill as 10 BF, yet measure only 7.5 BF surfaced. The tally rule is on this page precisely so the invoice is no surprise.

Does length round up too?

Often to the next foot with rough lumber — a 10 ft 2 in board is tallied as 11 ft by many suppliers. That practice varies by yard, so it is left out of the arithmetic here; enter the length you are being billed for.

Board feet versus square feet?

Square feet measure surface (width × length); board feet measure volume and add thickness. A 100 sq ft of 4/4 lumber is 100 board feet; the same coverage in 8/4 is 200. Coverage questions (flooring, tabletops) start from square feet; buying questions start from board feet.