Seatpost size confusion.

I thought finding out seatpost size would be fairly simple - I did a lot of careful measuring, then decided to invest in some digital callipers, thinking that either they would confirm my thoughts, or at worst give me the definitive answer to the contrary. Alas.
Measurements range from 25.94 to 26.83 depending on the cross section I choose, whether I measure the very top or a bit further in, and whether I open the callipers so they're gently resting, or apply just a tiny bit of extra effort. I decided to measure the seat post I currently have (although I'm doing this as I think it may be a little small because it looks a little overtightened). Those measurements went from 25.93 to 26.81, but mostly settling closely around the 26.6 mark.
What the he** size seatpost should I go for?!

Thanks.

you've cleaned the inside of the seat tube I presume, if you are not exactly straight with the calipers it can be so easy to get differing readings.
I've used a 50p plastic caliper from tesco, actually takes quite accurate measurements compared to my digi ones..lol
Okay, so what is the manufacturer/tubing (if known)
It could be anything from 26.0 to 26.8 though 26.6 is far less common
26.2 is fairly common for old single butted 531, 26.8 for some older double butted 531 (I had a '71 Claud Butler that size) 26.4 is relatively common for continental frames.
Do you have any old posts you can try or would a LBS measure it for you for nowt if you smile nicely :o)

Digital callipers can usually be zeroed first by pressing the jaws together reasonably hard and then press the reset or zero button. Doing that first may help.

Otherwise make sure you are always using the bevelled part of the inside jaws (the small jaws on the back), not the flat part further into the jaws. Since you are measuring the inside of a round tube, using the flat part of the jaws will make the the tube seem smaller since the edges of the flat will be resting against the curvature of the tube. Hope that makes sense. And always apply a bit of pressure the make sure that the edge of the jaws are resting flat and parallel to the inside of the tube.

It's not uncommon for the measurements to vary a bit depending where you measure, because the tubing does deform due to over-tightening etc. So it might also help to measure at several points and then take an average. That said it shouldn't vary by 0.8mm.

Also if the frame has been refinished at some point and not had the seat tube reamed afterwards, you might end up with a thin, uneven layer of paint, often clear coat, near the lip of the seat tube. That will throw off the measurements. In that case it helps to have the tube reamed. But then that's going to cost you, and you need to know what the diameter was anyway.

Also all the things that Bump said.

Thanks for your help Mr Bump and Shanghaied.

Mr Bump, I gave the inside of the tube a wipe but didn't do a thorough clean.
I don't know the tubing, but I know it is a Woodrup from the early 90's. I've emailed the chap that made it, but seeing as I cant find the frame number it would be dependent on him being able to remember the finer details of one specific bike.

Shanghaied, I did zero them but they were only 15 quid from Maplin so maybe they're just not incredibly accurate! It has definitely been refinished at some point - I'll take a closer look when I get home and try to figure it out again.

Hopefully its creator will remember and that would make everything a lot easier!

Ah a woodrup, very very nice frames, lugs can be exquisite. Maurice was another of those old school artisans :o) Stephen his son took over in the 70s & I would think you'll be able to get a definitive answer from them for sure.
If I had 2k burning a hole in my pocket I'd get an 853 Maurice special..gorgeous :o)