I'm getting ready to fix up an old road bike, How would you measure the seatpost, inside diameter? or circumference?

Usually the existing seatpost is stamped with the size. It's usually something like 26.8 or 27.2 or something similar. It represents the diameter is metric measurement. If you can't find a stamped number, use a venier caliper to measure the O.D. of the seatpost, or the I.D. of the seatube.

L8R

just take a mesuring tape, and mesure the distence aroun the post.

Dude, he wanted diameter not circumference.

EevetS

You want diameter, to the nearest 0.1mm (those sizes are in mm). There are a few "standard" sizes - 27.2mm the most common on steel frames road bikes.

The best solution is to borrow or buy a calibrated caliper. A cheap plastic one with a tolerance of +/- .1mm is adequate for this task. You also may want to consult Sheldon Brown's website, where he has started to compile a list of seat post diameters by bicycle model and brand name.

Just divide by 3.14159265358979323846264338327950288419716939937510582097494459230781640628620899862803482534211706 79 ...

Sorry ... I just wanted to see if I still knew it ...

Any ideas on whether your likely to do any harm running a 30.8 post in a tube specced for 30.9 if the clamp holds the post firm?

Richard

If the bolt(locking clamp) doesn't bend, it would be fine. If it does(bend), it would take so much effort to tighten them and the thread will be ruined..

Don't mean to revive an old thread, but others might be coming across this thread by way of Google search (like me) and I wanted to add that basic high school mathematics will get you the answer quickly. The formula for the circumference of a circle is:

Circumference = π * diameter

Therefore, diameter = circumference / π

π = 3.1415

You can get the circumference by using a tape measure (or a ruler and piece of string) and wrapping it around the seatpost. If your seatpost circumference is 90mm,

diameter = 90mm / 3.1415
diameter = 28.6mm

This is another method of getting a seatpost diameter if you don't want to go out and buy more tools

Yet, even in the 21st century, there are still bike manufacturers that used oddly sized seat post. I just picked up a fat bike and the seat post is 30.4. Not unheard of, but choices are very limited.

Close but no cigar.

WRONG, WRONG, WRONG. You don't divide the inch measurement by Pi to get the metric measurement. PI is for finding the circumference, not diameter. To get mm from inches, divide the inch measurement by .03937. There are 25.4 mm's in an inch.
Jon

you misundertood Jon I think, the circumference is Pi * diameter, so he was saying how you could get the (metric) diameter from the (metric) circumference.

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...seatposts and the seat tubes they fit into vary considerably in terms of the accuracy of their sizing. As someone who has recently extracted a 27.2 post stuck in a frame that was obviously reamed for 27.0, and another that was pretty well jammed into a frame that was reamed for 27.2, but the post was just a little too large somewhere down deeper in the seat tube, thus whoever inserted it jammed it in there, I would advise continuing to measure with a proper post gauge (a stepped tube, in .2 mm increments) as a starting point, and then carefully using a seat post of the size indicated to see if you have a good fit.

I tried three different 27.2 posts in the second frame before I found one that was slightly undersized at about 27.13 that fit very nicely...good contact all around, but still sliding freely in the tube.

I realize that not everyone has a collection of posts with which to do this, but you really need to avoid the mindset that this is an exact science.