What does Nominal measurement mean?
When you see the word 'Nominal' next to something regarding lumber, it means that the actual dimensions of the wood are smaller than what the sales name implies.
If you've recently purchased 4 in. x 4 in. wood or timber from a home supply store, you've most likely got a piece of wood that is actually 3.5 in. x 3.5 in. which is a nominal measurement.
The reasons for this are based on how wood has been shaped through the years. Before precise machinery, a 4 in. x 4 in. was 'green wood', or wood that was rough and unfinished. After drying and planing, the wood would be approximately 3.5 in. x 3.5 in. Although this can often cause consumers considerable frustration, these standards are a result of lumber industry agreements and create a reliable way to ensure the correct sizes of lumber are obtainable.
What does Actual (real) measurement mean?
As the name suggests, an actual measurement is the actual dimensions of the lumber. So, if you purchase a 4 in. x 4 in. (actual), you are getting a piece of lumber that is genuinely 4 in x 4 in.
Are my fence/deck posts Nominal or Actual?
The easiest way to be certain is to measure one!