Seat clamp sizing question

I've used the fucking search and used my fucking brain but I seek wisdom and experience from others.

I picked up a new frame last night and it came without a seat post clamp so I'm looking to buy one. Would I be right in thinking that when manufacturers talk about size that this refers to the external diameter of the seat post. My seat tube is approx 32 mm in diameter so I think I should be looking for a 31.8 mm clamp. Does that make sense?

Fookin noobs

I know - thick as mince.

I think there is hardly a relation between clamp diameter and seat post diameter.
Seat post diam. is about the same as the internal diameter of the seat tube.
Clamp diam. is about the same as the external diameter of the seat tube.
It all depends on the material of the frame (wall thickness).
I'm still not talking about use of shims between seat post and seat tube...

Take a pair of vernier calipers and mesure the outer diameter of the seat tube (top end). This is the clamp diameter you need.
If your seat post is 32mm (31.8mm?), I guess the clamp is around 34.9mm or even bigger.

Thanks. My description wasn't clear as I muddled seat tube and seat post - I've fixed that now.

The internal diameter of the tube is 26.4, measured with vernier calipers. The external diameter is 32 mm, again measured with vernier calipers.

By my reckoning, I need a 26.4 seat post and and 31.8 mm seat clamp. Does that sound right?

Yes.

I have a black 31.8mm seat clamp here doing nothing.

If you want it it's yours. Could even stick it in an envelope and post if you like.

FYI - 31.8mm seat clamps are usually paired with 27.2mm seat posts but you may have thicker tubing or something.

i have a spare 31.8 clamp if you wish, serotta one

BUMP

I have an 853 Raleigh frame. Wall is NOT butted at the end. I guess my seatpost is normal size (Dura-Ace NJS stamped thing).

I have no callipers.

I need help