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Why do 4 x 4 posts actually measure 3 ½” x 3 ½”?

Posted by Laura Kiralla on

This isn't 4" x 4"!

So your contractor told you he used 4 x 4 posts for your fence, but you’ve whipped out your tape measure and noticed that they actually measure 3 ½” x 3 ½”. You may be wondering if this is some sort of cost saving gimmick. Should you be concerned? Is your fence less structurally sound?

No need to worry! That’s just the way lumber is measured. It seems strange, but the reason for this discrepancy is simple, nominal 4 x 4 lumber is actually cut to those measurements from the green, unfinished wood. If interested, you can even find dimensional wood that is unfinished and these measurements will be accurate. However, dried and planed wood is reduced by a standard half inch leaving you with an actual measurement of 3 ½” x 3 ½”. (For wood sizes less than 2”, the lumber is only reduced by ¼”.)

(Image courtesy of www.buffalo-lumber.com)

So how do you know if you are looking at a nominal or actual lumber size? Actual sizes should include an " (inch) symbol in the measurements, whereas nominal sizes will omit the " symbol. But around construction sites you will almost always hear nominal sizes because it’s a lot easier to say "1 x 4" than "¾” x 3 ½”!"

If you’re unsure whether your posts are actual or nominal 4 x 4’s, we recommend measuring before placing an order for our post caps. It’s unlikely you have actual 4” x 4”s as most lumber yards do not stock this size. Most the time, they are custom cut by carpenters. But we think it’s best to be on the safe side to avoid reordering.

If you have any further questions we can help you with, we are always happy to assist! Feel free to contact us at (775) 298-5060 or online


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