Deckorators post caps come in many sizes, but the most common sizes are 4" x 4" and 6" x 6", which are made to fit standard 4x4 or 6x6 posts. The inside dimensions of the standard 4" x 4" post caps measure 3-5⁄8”, and the inside dimensions of the standard 6" x 6" post caps measure 5-5⁄8”.
2-1⁄2” x 2-1⁄2” — inside dimensions: 2-1⁄2” x 2-1⁄2” (ALX railing)
4" x 4" — inside dimensions: 3-5⁄8” x 3-5⁄8” (made to fit a standard 4x4 post)
6" x 6" — inside dimensions: 5-5⁄8” x 5-5⁄8” (made to fit a standard 6x6 post)