Fence post cap sizes can be confusing. So far this year approximately half of our customers have called asking "what's the difference between a nominal size and an actual or full size?"
In USA, a nominal size, such as a Nominal 4x4 is just a name. You may be surprised to learn that it is not actually a 4 inch square cap nor your fence post a full 4 inches square. Many lumber yards cut the posts to be a little under 4 inches, sometimes as small as 3.5 inches although we have found much variance across the country.
Nominal 4x4 - Actual Size 3.5 inches
Full 4x4 - Actual Size 4 inches
Nominal 6x6 - Actual size 5.5 inches
Why the difference?
Years ago, a 4x4 did indeed measure a full 4 inches. Over time, builders & engineers realized that they could get just as much strength from a slightly smaller piece of wood. We have found, however, that lumber yards may differ even within towns & cities, let alone state to state and some nominal 4x4 posts may be 3.75 inches.
In addition, untreated lumber such as cedar posts tend to be closer to the true stated size, such as a full 4 inches or 6 inches.
I recommend measuring your fence posts before you buy your caps. Take a random sample of your posts and measure with a ruler or tape that shows 1/8 inch. If your post measures 3.5 inches, you want a nominal 4x4 cap.
Also, think about selecting a cap up to 1/8 inch bigger than your post. This will give you a small clearance of 1/16" around each side to slide the cap on more easily.