Some of your rims might have the ISO/ETRTO size printed or stamped into them. Usually, as they do on tires they put it in the tiniest print size possible. It will be written as nnn x nn or sometimes nn x nnn. Don't know for certain, but when they write it backwards I wonder if that is a sign that they don't comply fully with ISO standards. Though ISO is a voluntary standard so they certainly don't have to abide by ISO standards.
When you find the ISO size of your rim, then the three digit number will be the BSD or bead seat diameter. Knowing that, you'll know what size tire you can put on it since the ISO size will almost always be found somewhere on your tires. It too usually in the tiniest possible print embossed or stamped on the sidewall.
The two digit number in the ISO size of the rim is your internal rim width from the bead seat on one side to the other. On the tire it is the nominal width of the tire. Both in millimeters.
If any of your rims have a tire on them, then just look at the tire. Find the ISO size of the tire and you know the BSD of both.
Last edited by Iride01; 05-18-22 at 08:08 AM.