You can get that same vernier caliper from Harbor Freight for around $15 when its on sale. Its branded Cen-Tech instead, but same manufacturer.

Anyone who can't figure out how to accurately use vernier calipers and instead needs a "sizing tool" shouldn't be allowed to exercise their franchise in a democratic society, IMO.

For accurate sizing, you really want to test the size by clamping down on a post or a sizing rod, noting how much clamping movement is needed to secure the post.
The caliper won't measure easily when the seat tube opening or seat post (think steel post or one that's been well-used/abused) isn't quite round. You'll have to get the guage tips aligned with the tube to get the biggest reading for seat tubes, and likely have to take an average of several readings, so far from the best way.
Those Chinese steel dial calipers seem to be much more than accurate enough if used for things that such a tool is designed for.

Digital Caliper-


Dial Caliper-


Vernier Caliper -


Any of the three of these should be more than accurate enough in my experience. If you know how to read the vernier you probably have the skill needed. As far as actual post diameter being under by .1mm or so, it has to be (that's only .004"). A "size to size" fit would surely result in a stuck post.

That would have gotten you a severe reprimand in any of the shops I've been associated with. With very few exceptions, gauges are not intended to used as tools.

"If you don't quit using that screw-driver as a pry-bar, it'll never work as a chisel when you need it!"

Try Bike Tools Etc.

+1000.

Seat posts are typically undersized a bit (plus there is roundness and cylindricity to consider). Cheap posts are usually undersized more than high-end ones (for obvious reasons). I have found Campagnolo posts to be the most true-to-size and consistent of all brands of vintage posts. A 27.2 Campagnolo post will typically be about 27.18 or so (when new, and used/worn, sanded-down, and beat up doesn't count, sorry), for example.

I've twice had Campag SR posts that were problematic.
One I had to switch to another brand (Avocet) to keep from slipping.
The other I went to the next larger size, sanded it lightly and honed out the seat tube to make it reasonably easy to install.

I just measured 5 new NR posts, in the 26.8 and 27.2mm sizes. The undersizing was as follows:

.05 .06 .07 .08 .12

The last dimension was for a new NR post that measures 27.08mm, or .12mm undersize.

I also measured a new-looking Chorus titanium post, it measured a very consistent 27.12mm, very round and straight just like the alloy ones.

I've measured many posts over the years, and the .08mm undersize seems to me to be about the average of all of them.

Very interesting. I wonder if Campy took into account Reynolds and Columbus +/- tolerances, various lug dimension discrepancies, and/or frame builder idiosyncrasies ...or otherwise did this on purpose.

If think I had five NOS Nuovo Record seat posts, I'm not sure I'd care. But just the kind of very interesting minutiae that I enjoy.

It's actually quite easy to measure inside and outside tube diameters w/ a vernier calipers (they have seperate jaws for ID and OD measurements as shown in the pics. If you need to measure deeper into a tube, there are spring loaded inside gauges that you insert into the tube, allow to expand, then lock and remove, and you measure these with your calipers or micrometer. Unlike the $70 tool, this technique will allow accurate measurements of intermediate diameters rather than just a few steps, and you can use the tools for all other measurements. But if you want another single-use bike tool, go for it.

When I bought the peanut butter wrench to remove my Campy cranks, it took me about 5 minutes to open my wallet and pull out the $30. But at least it says Campy on it and can do multiple tasks. The thought of paying $70 for a gauge that only measures seat tubes is beyond my comprehension. If I had that kind of cash to waste, I would have already clicked BIN on the 7-11 Eddy Merckx.