2.7: Board Feet and Lumber Cost

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2.7 Board Feet and Lumber Cost

Hardwood lumber is commonly measured and priced by the board foot.

If you plan to work professionally with wood, board-foot calculation is a basic trade skill. It allows you to estimate material requirements, compare prices, prepare bills of materials, and determine project cost.

Learning Outcomes

After completing this section, you should be able to:

Define a board foot.
Calculate board footage for one or more pieces.
Convert feet to inches when necessary.
Use rough thickness when calculating hardwood volume.
Estimate lumber cost from a price per board foot.

What Is a Board Foot?

One board foot equals 144 cubic inches of wood.

The standard example is a board measuring:

1 in. × 12 in. × 12 in.

That equals:

144 cubic inches = 1 board foot

But a board does not need to have those exact dimensions.

These pieces also contain one board foot:

2 in. × 6 in. × 12 in.
1 in. × 6 in. × 24 in.
1 in. × 4 in. × 36 in.

The shape changes, but the total volume remains 144 cubic inches.

Board foot examples.png — Six boards of different dimensions, each containing one board foot of lumber, including 1 × 12 × 12 inches, 2 × 6 × 12 inches, 1 × 6 × 24 inches, 1 × 4 × 36 inches, 1 × 3 × 48 inches, and 2 × 3 × 24 inches.

Figure: Examples of One Board Foot

The Board-Foot Formula

When thickness, width, and length are all measured in inches:

Board Feet = Quantity × Thickness × Width × Length ÷ 144

Remember the standard lumber dimension order:

Thickness × Width × Length

Use the Rough Thickness

When calculating hardwood lumber, use the nominal rough thickness.

For example:

4/4 = 1 inch
5/4 = 1.25 inches
6/4 = 1.5 inches
8/4 = 2 inches

A surfaced 4/4 board may physically measure less than 1 inch thick, but board footage is normally calculated from its nominal rough thickness.

Example 1: One Board

Suppose you have one 4/4 walnut board that is:

1 inch nominal rough thickness;
6.5 inches wide; and
8 feet long.

First convert the length to inches:

8 ft × 12 = 96 in.

Then calculate:

1 × 6.5 × 96 ÷ 144 = 4.33 board feet

The board contains approximately:

4.33 bd. ft.

Example 2: Multiple Pieces

Suppose you need four pieces of 4/4 lumber, each:

1 inch nominal thickness;
6 inches wide; and
36 inches long.

Calculate:

4 × 1 × 6 × 36 ÷ 144 = 6 board feet

The calculated material requirement is:

6 board feet

This is before adding an allowance for defects, milling loss, grain selection, and other waste.

Calculating Cost

Once you know the board footage:

Lumber Cost = Board Feet × Price per Board Foot

For example, if the 4.33-board-foot piece from Example 1 were priced at a hypothetical $8.00 per board foot:

4.33 × $8.00 = $34.64

Estimated lumber cost:

$34.64

Actual purchasing methods may involve rounding or other supplier practices, so always understand how your lumber supplier calculates the sale.

Add for Waste and Selection

The exact board-foot volume of your finished parts is not necessarily the amount you should purchase.

Suppose your cut list contains 10 board feet of calculated material and you decide to allow 30% additional material.

10 × 1.30 = 13 board feet

You would estimate approximately:

13 board feet to purchase

The appropriate amount of extra material depends on the project, the lumber, and how selective you need to be about defects, grain, color, and figure.

Check Whether the Answer Makes Sense

Board-foot calculators and reference charts can save time, but you should still understand the formula.

A calculator can give you an answer.

Understanding the formula helps you recognize when that answer does not make sense.

For example, if a calculator tells you that a single 4/4 board measuring 6 inches wide and 8 feet long contains 40 board feet, you should immediately recognize that something is wrong.

Basic estimation is part of professional judgment.

Practice Your Board-Foot Skills

If you would like more practice, calculate the following:

One piece of 4/4 lumber measuring 8 in. wide × 72 in. long.
Two pieces of 5/4 lumber measuring 7 in. wide × 48 in. long.
Three pieces of 8/4 lumber measuring 5 in. wide × 36 in. long.
A 6.5-board-foot board priced at $9.50 per board foot.
A project requiring 12 board feet of calculated material with a 25% additional allowance.

Check Yourself

How many cubic inches are in one board foot?
What dimensions make up the traditional one-board-foot example?
Which thickness should you use for hardwood board-foot calculations?
What must you do if the lumber length is given in feet but the formula uses inches?
How do you calculate lumber cost?
Why should the amount you purchase usually be greater than the exact board footage of the project parts?

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