It seems like buying lumber should be one of the easier steps in building a project. But if you're not familiar with lumber dimensions, you'll soon discover that what you see is not exactly what you get. Most boards measure somewhat smaller than the dimensions posted on the shelf. To understand what's happening here, we first need to make an important distinction between lumber dimensions for softwood, and lumber dimensions for hardwood.
Softwood Lumber Dimensions
Most of the lumber you see at home improvement centers is made of softwood – like pine, spruce, and fir. Softwood is marked and sold in "nominal" dimensions – a somewhat misleading number that is larger than the actual size of the board. The nominal size describes the size of the board when it first arrived at the lumber mill. However, before being shipped out, both the width and the thickness is shaved off by as much as 3/4 of an inch. Here are some of the more common lumber dimensions for softwood:
Lumber Dimensions – 1x Nominal
These boards lose 1/4 inch in thickness and 1/2 inch in width before leaving the mill. That means a 1 x 4 board is actually 3/4 inches by 3 1/2 inches.
Lumber Dimensions – 2x Nominal
These boards lose 1/2 inch in thickness and 1/2 inch in width before leaving the mill. That means a 2 x 4 board is actually 1-1 / 2 inches by 3-1 / 2 inches. You'll find a couple of variations from this basic formula across different sizes of softwood, so be sure to check a lumber dimensions chart for the complete list of board sizes.
Buying Hardwood
Hardwood follows a completely different set of rules for determining the size of a board. Unlike pine and spruce, hardwood is often sold in random widths, thicknesses, and lengths. Why? Hardwood is much too valuable to chop up into uniform pieces. This creates too much waste. Instead, lumber mills use as much of the hardwood tree as possible, which produces boards in a variety of shapes and sizes. Since no two hardwood boards are exactly the same, prices are based on a unit of measurement called "board feet" – a more accurate way of estimating how much wood is in a particular board.
The basic formula for estimating board feet is to multiply Thickness x Width x Length – and then divide that number by 144 (which gives you the total board feet). Of course, buying rough hardwood lumber in this fashion leaves plenty of work for you in the shop – like ripping, crosscutting, and planning all the boards before you even start the project. Most woodworkers prefer this approach, though. It lets them decide exactly where on the board they want to cut the individual project pieces, paying special attention to the location of knots, interesting grain patterns, and colors.