How To Measure For Slip-Over Post Caps
Always pre-measure your post and order a post cap that matches your post's dimensions.
For example: A 4" x 4" post is primarily considered "finished lumber" so the dimensions are actually 3.5" x 3.5" and will require 3.5" x 3.5" post caps. The proper size cap will easily slip-over your post. There will be a 1/16" gap between the post and the cap to allow for expansion due to temperature and humidity changes within the outdoor environment.
Please watch this short video for a detailed illustration on proper measurement techniques.