Originally Posted by
FBinNY
I think there was a typo here, the 1-1/4 size is usually rounded to 32mm or 31.8. If you have a 31.6mm seatpost you don't need to measure. The seat tube diameter has to be larger, so it's surely 1-3/8" (34.9 or 35mm). Note metric sizes vary because bicycle tube ODs are in imperial sizes, and the metric variation is the result of rounding differently.
I also thought it might be a typo as 31.6 mm seatposts have become a new standard these days. However, seatposts being what they are, almost any size is possible.
I was sure it wasn't the size seatpost clamp diameter that the OP needed and 34.9/35 mm is almost certainly the right one.