Seatpost diameter on stuck seatpost

I need to buy a new seatclamp as the saddle is moving around. I can't buy a new seatpost as the old one is stuck.
So how do I measure the seatpost diameter without taking it out?
The seatpost diameter is smaller under the saddle where the clamp is so I wouldn't get the right measurement if I took the saddle of and measured the top of the tube.

It's an old Carlton with Reynolds 531 tubing.
According to Sheldon, it could be 27 if it's late 50's and 27.2 if its circa 1968.
I don't know when it was made.
Any ideas?

have you got a pic?

of the seatpost, or the bike?

If it's an old style seatpost with separate clamp then it's ,as far as i remember, one size fits all.

that is exactly what i was thinking :)

yeah, it is that kind.
Here's a pic of what it looked like before I did it up.
Actually, it still kinda looks like crap but now it has wheels etc

Hi Julio!

http://www.csgnetwork.com/picalc2.html

Wrap a tape measure round the seatpost. Then use this^ to calculate the diameter from the circumference.

it should be a standard size.

Hi!
Thanks, good idea!

I've been there pretty recently, trying to sort out band-on measurements for me new road bike...

thanks everyone.
Does anyone know where to get one?
I've googled but only seatpost clamps come up

any bike shop should have one, if not look for a abandand ??? bike and whip one off, usually see them on older bikes. good luck

hey julio, just tighten them bolts till you puke, that'd stop the spinning. i have the same on my polo bike and might have a spare clampy bit if you need a new one.

hi Matt
Yeah, I'll give it some welly, see if I can get it to stay put and then if not, maybe see if you can unearth the spare.
Cheers!

http://cgi.ebay.co.uk/Vintage-Racing-Bike-Parts-Saddle-Bracket_W0QQitemZ350062102700QQihZ022QQcategoryZ56197QQssPageNameZWDVWQQrdZ1QQcmdZViewItem

Should do the job