How do I measure my mattress for the right pocket depth?

Strip the bed and use a tape measure from the very top of the mattress straight down to the bottom seam, including any pillow-top or built-in topper but excluding the box spring or platform. Add 1–2 inches if you stack a separate mattress topper. Buy a pocket depth at least equal to that total; reviewers across r/Bedding repeatedly report that buying 'right at' the measurement leads to corners popping after a few washes as elastic relaxes.