How to measure your rims? (CB350)

Hi All,
so I have a 71 CB350 and I wanted to lace a new rim for it. I'm pretty sure these originally had a 19" rim on them but I measured the diameter of the rim that came off the bike and it is 20' across. What gives? How are these rims measured? The tire that was on it says 19"
Thanks !
Patrick-

It should have 18" front and rear (CL had a 19" front). What size are your tires?

The diameter is the Id of the tire bead. The sidewalls of the rim are about 1/2 inch high. So 19" with an additional sidewall height is 20".

As previous poster said, CB350 came with 18 inch rim, CL came with 19....