Difficult to measure even with expensive digital calipers ,only the top is the diameter you need ,the seattube opens out as you move down the tube then it narrow's again as you get towards the bottom:roll:
I think the makers should stamp the seatpost size on the bottom bracket,its the only measurement that's hard to work out.
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A good way would be to have access to a bunch of different sized posts,a shop wont do this for you as it will damage any new one they try
Ask them if you can check the bikes they're repairing, seatposts they use, to gauge your own as they are already damaged[scratched]
Took me months to find out my torus is exactly 30mm ,not the 29.4/something it measured at
