NOBLING has great advice, invest in an inexpensive caliper. Absolutely necessary for good scratchbuilding. My personal preference, both professionally and in my hobby work, is a dial caliper.
It would help to know what sizes you need in 1:1, but when working in 1:25 scale, the following Evergreen round stock will get you close;
- #211 measures .040", equals 1" in scale
- #221 measures 3/64" (.047"), equals just shy of 1.25" in scale
- #222 measures 1/16" (.062"), equals 1.5" in scale
- #212 measures .080", equals 2" in scale
- #213 measures .100", equals 2.5" in scale
There are other sizes both smaller and larger available, but these sizes should get you pretty close for most chassis work. If you look at the numbers above, you may notice a very handy conversion for working in 1/25 scale, and in most cases 1/24 scale as well. In 1/25 scale, 1" = .040". That makes the math easy; .020" = 1/2 inch, .060 + 1.5 inches, .080 = 2 inches, etc. Also, as has been pointed out on the forum in the past, 1 mm + .0397" (roughly). .0397" is only .0003" away from .040, a negligible amount in our hobby, so millimeters, for our intents and purposes, equals 1 inch in 1/25 scale. Very handy.