How to calculate board feet

Imagine you want to build your dream tree house, one with a great view and a cozy environment to spend some quality time with your friends. To do so, you not only have to select a proper tree, but also proper materials and quantities.

Carpentry might be a thrilling and creative activity, but it also requires you to carefully plan, calculate, and get hold of the necessary materials in the correct amounts, especially if you want to stick to a budget.

In this field you will find several specialized units of measurement. A very important one —at least in Canada and the United States— is the board foot, which refers to a certain volume of lumber, or wood that has been processed and cut into boards. Knowing how to express the correct amount of lumber you need for your project is critical. Let’s learn more about this convenient measurement unit.

How to calculate board feet

1. Measure the thickness (t) of the wood board you need in inches (in).
2. Measure the width (w) of the wood board in inches (in).
3. Measure the length (l) of the board in feet (ft).
4. Use the following equation to calculate the equivalent board feet of lumber:

What is board foot?

Lumber refers to differently shaped pieces of wood which have been cut from a tree log and, in some cases, surfaced on one or more faces. These pieces are then sold as raw materials for a great variety of applications, like construction, furniture making, decoration, etc.

Logs are usually sawn into boards of specific dimensions. Those that comply with standardized sizes are therefore called dimensional lumber and are commonly found at most wood stores.

Wood boards are basically 3-dimensional rectangles, or rectangular cuboids, which have a specific width (w), thickness (t), and length (l). Although these labels are completely interchangeable, given they all represent a length, there is a widely accepted way of using them on a piece of lumber.

In the case of a long board, width and thickness often refer to the two dimensions of a cross-section of the lumber, whereas the length corresponds to the longer remaining dimension. In the case of rectangular boards, width and length refer to the sides of the lumber when viewed from the top, while the thickness refers to the dimension perpendicular to them. Look at the following image to clarify this:

The specific characteristics of a piece of lumber, whether mechanical —like the forces it can withstand, how hard its surface is, or how much it will bend before breaking— or aesthetic —like its tone or its grain—, depend on the specific hardwood used to obtain it. Given that many different types of wood are currently used for carpentry, suppliers could have a hard work selling lumber if all these previous variables had to be considered to quote a simple board.

This is precisely where the unit of board feet comes in handy. Since it represents a certain volume of wood, which already takes into account the lumber’s width, thickness and length, it reduces the number of variables to consider. Keep in mind, board feet refer to volume and not length, even if you are used to using feet in the context of measuring a certain distance.

The volume of a wood board is defined as: V = w × l × t.  One board foot of lumber is therefore defined as the volume of a wood board with a width of 1 foot, a length of 1 foot, and a thickness of 1 inch. The mix of different units (feet and inches) comes from the fact that, in boards as the one shown on the left in the previous image, both width and length are most likely measured in feet, while thickness —the smallest dimension— is measured in inches.

If the units are kept for each dimension of the lumber, any wood board’s board feet can be mathematically calculated as:

Where the brackets indicate the units in which each variable must be measured in order to comply with the definition of board feet. Given that 1 foot = 12 inches, equation 2 is most often written as:

Keep in mind that, in this equation, the width is measured in inches, not in feet. This is most likely to happen when dealing with boards such as the one shown on the right in the previous image.

Now, to calculate the amount of board feet you need for a project, first measure each board’s width, thickness and length using the appropriate units, and then use equations 2 or 3 to calculate BF. Keep in mind that, if you need several boards with the same dimensions, you just need to measure one of them and then multiply the resulting board feet by the number of boards you need.

Use this simple calculator to obtain the total board feet required for any of your projects, just by inputting the basic dimensions of each wood board and the total number of boards you require.

If you want to learn more details about hardwood, its standards and uses, visit the website of the National Hardwood Lumber Association

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