How to Calculate Board Feet (And Why Your Lumber Bill Uses Them)

A board foot is a unit of volume: 144 cubic inches of wood. Picture a piece one inch thick, twelve inches wide, and twelve inches long. That is one board foot.

The formula is simple enough to do in your head:

Board feet = (thickness in inches × width in inches × length in feet) ÷ 12

Or, if you have all three dimensions in inches:

Board feet = (thickness × width × length in inches) ÷ 144

The arithmetic is not what confuses people. What confuses people is knowing which numbers belong in it.

Why volume instead of length

If you buy 2x4s, you buy sticks. They are all the same size, so counting pieces works fine.

Hardwood does not work that way. A hardwood log gets sawn into the widest, longest boards it will yield, which means a stack of walnut contains boards of varying widths and lengths. You cannot price that by the piece, because one board might contain three times the wood of another.

Board feet solve it by pricing the actual volume of wood. A supplier quoting walnut at $12 per board foot can price any random-width board in the pile with the same number.

Worked examples

A 2x4, 8 feet long:

2 × 4 × 8 ÷ 12 = 5.33 board feet

A 1x6, 10 feet long:

1 × 6 × 10 ÷ 12 = 5 board feet

A 2x10, 16 feet long:

2 × 10 × 16 ÷ 12 = 26.67 board feet

A 6x6 post, 8 feet long:

6 × 6 × 8 ÷ 12 = 24 board feet

A hardwood board, 1 inch thick, 7-1/2 inches wide, 9 feet long:

1 × 7.5 × 9 ÷ 12 = 5.63 board feet

Multiply by the price per board foot and you have your material cost for that piece.

The nominal versus actual problem

Here is where it gets genuinely confusing. A 2x4 does not measure 2 inches by 4 inches. It measures roughly 1-1/2 by 3-1/2 inches. The board was 2x4 when it was rough sawn, then it was dried and surfaced on four sides, and it lost material in the process.

So when you calculate board feet for a 2x4, which numbers do you use?

For softwood dimensional lumber, use nominal dimensions. The industry prices 2x4s as 2x4s. Using actual dimensions would give you a smaller, technically-accurate number that does not match how anyone sells the material, and your calculation would not reconcile with the invoice.

For rough hardwood, use actual dimensions, because rough lumber has not lost anything yet.

For surfaced hardwood, convention bills on the pre-surfacing thickness. A 4/4 board surfaced to 13/16 inch is still sold as 4/4. You are paying for the wood the mill started with, including what came off as shavings. This surprises people, but it is standard practice throughout the hardwood trade.

Quarter thickness: 4/4, 6/4, 8/4

Hardwood thickness is specified in quarters of an inch of rough thickness:

| Notation | Spoken | Rough thickness | Typical surfaced |

|---|---|---|---|

| 4/4 | four quarter | 1 in. | ~13/16 in. |

| 5/4 | five quarter | 1-1/4 in. | ~1-1/16 in. |

| 6/4 | six quarter | 1-1/2 in. | ~1-5/16 in. |

| 8/4 | eight quarter | 2 in. | ~1-3/4 in. |

| 10/4 | ten quarter | 2-1/2 in. | ~2-1/4 in. |

| 12/4 | twelve quarter | 3 in. | ~2-3/4 in. |

The practical consequence: if your finished project needs 3/4-inch material, order 4/4. If it needs a full 1-1/2 inches, order 8/4 and expect to lose thickness in milling. Ordering 6/4 and hoping is how projects end up short.

This is also why you should not order rough lumber at exactly your finished dimension. Rough boards are not flat. Jointing and planing them flat consumes thickness, and cupped or twisted stock consumes more.

Adding waste

Board foot math tells you how much wood is in a board. It does not tell you how much to buy.

Realistic waste allowances:

• Simple projects, straight cuts, standard widths: add 10%
• Furniture and casework with grain and color matching: add 20 to 30%
• Rough lumber you will surface yourself: add 30% or more
• Anything with figure or where appearance matters: add generously and buy it all at once

That last point matters more than people expect. Wood from different lots differs in color and grain, sometimes dramatically. Running short on a dining table and going back for two more board feet three weeks later frequently produces a visible mismatch. Buy the whole project at once from the same lot.

Hardwood grades affect your usable yield

Hardwood grading is based on how much clear, defect-free material a board yields. FAS is the top grade with the highest clear yield. Select and Common grades yield progressively less usable clear material per board foot.

This means the cheapest price per board foot is not always the cheapest project. A lower grade at a lower price can cost more in practice once you cut around knots and defects, especially on projects needing long clear pieces. For short parts, lower grades are often excellent value.

When lumber is not sold by board foot

Plenty of products use different units:

• Decking is commonly priced per lineal foot, since profiles are fixed
• Flooring is priced per square foot
• Trim and moulding is priced per lineal foot
• Plywood and sheet goods are priced per sheet
• Timbers may be quoted either way

The rule of thumb: fixed-profile products get lineal or square foot pricing, and random-width random-length lumber gets board foot pricing. If a quote is ambiguous, ask — confusing per-lineal-foot with per-board-foot pricing on decking produces a badly wrong budget.

Estimating a whole project

Work through it methodically:

1. List every part with finished dimensions

2. Add milling allowance to thickness and width, and a few inches to length for cutting

3. Convert each part to board feet

4. Total them

5. Add your waste percentage

6. Round up to the nearest whole board

One caution: suppliers sell boards, not board feet. You cannot buy 43.7 board feet of cherry — you buy whole boards that total at least that. On random-width stock you may end up with meaningfully more than your calculated number.

Just use a calculator

The math is easy but tedious across a long parts list, and arithmetic slips are how orders come in short. Our board foot calculator handles the conversions, and the lumber weight calculator is useful when you need to know whether your order fits in your vehicle.

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Pricing a lumber order? Send your parts list and species through our quote form — suppliers can quote from a board foot total, and it is the fastest way to compare real pricing.