Just building a mate’s Marin bike up – seatpost is 27.2mm so what would that make the clamp size??

Cheers.

Depends on whether it is old skool marin with Tange steel tubing or a modern alloy tubed marin, at a guess from what i know of marins i’d say you need a 31.8mm seatpost clamp (i’ve owned a few steel marins with 27.2mm seatposts)

Google the frame specs. Google will know.
I don’t think clamp size is directly relative to post size.

My cove HJ, and Dialled Love/ hate both have 27.2mm seat posts and use 30.0mm seat clamps

Yeah i guess it could be 29.8mm/30mm – google the frame as someone else somewhere in the world will have asked the same question at some point in the past.

The post size is meaningless when trying to work out the clamp size. For example both a Jones steel frame and a Jones titanium both take a 27.2 post, but the clamp on the steel is a 31.8mm and the titanium is a 35mm.

Just measure across the top of the seat tube to work out the clamp size. Have a look here.