The actual size of 2×4 lumber is 1 ½ x 3 ½ (38 x 76.2 mm), smaller than what is being advertised in hardware stores. The reason for this is that hardware stores sell lumber that has been planned. And the measurement of a 2×4 board is taken when it was freshly cut.
nominal size refers to the dimensions of the lumber before it’s smoothed for all sides. The actual size of a board is determined after it has been measured after planning.
Since 2×4 lumber at hardware stores is usually a little smaller than advertised, measure it carefully to ensure that you get the right 2×4 lumber for your project.
2×4 lumber Length
There is no difference in the length of a 2×4 board whether it is nominal or actual.
A 2×4 will have an actual length of 6 feet if its nominal length is 6 feet. The length of a nominally lengthened board of 12 feet will actually be 12 feet.
2×4 lumber dimensions
| 2×4 lumber | Nominal size in inches | Actual size in inches |
| 2 | 1 ½ | |
| 4 | 3 ½ |
| 2×4 Lumber | Nominal size in mm | Actual size in mm |
| 50.8 | 38 | |
| 304.8 | 76.2 mm |
Why is the actual size of a 2×4 lumber smaller than its nominal size?
Plans are generally measured when they are rough-sawn in their original shape. Later, the hardware stores buy those lumbers and treat them to make them smooth from all sides.
The planning process creates a uniform thickness by smoothing the surface of the wood. As thin layers of timber are sliced off by planning through each side of the timber, its thickness shrinks. Consequently, timbers with four-sided smooth surfaces lose thickness and width, reducing their actual dimensions.
In this way, a lumber’s dimension is reduced. In spite of this, they refer to the lumber’s measurements as their original dimensions, despite the fact that the new dimensions are less than the actual dimensions.
