Measure the width of one of the boards that you plan to use for your fence. Most board fences consist of boards that are 4 or 6 inches wide.
Divide 12 (the inches in 1 foot) by the width of the board in inches. For example, if the board's width is 4 inches, divide 12 by 4 to get 3. In that case, you need three boards for each foot of fence.
Multiply the number of boards needed for each foot of fence by the total length of the fence. This information will help you determine the total number of boards that you need for the entire fence length. For instance, if the fence will be 100 feet long and you need three boards for each foot of fence, then multiply 100 times 3 to get 300 boards.
Multiply the number of boards needed for the project by the cost of one board if you want to estimate the cost of the fence's lumber. For example, if you need 300 boards and one board costs $1, then multiply 300 times $1 for a total of $300.