What is the application? Find out what size the 1:1 tube size is, and divide that by 12.
For instance, if the actual roll cage tube is 2 inches; 2 divided by 12 = .166 or 166 thou. The nearest common rod sizes are 3/16" (.187") for a bit fatter tube, and 5/32 (.156") or 4 mm (.157"), for a bit thinner tube.
For 2.5 inch tube; 2.5 divided by 12 = .208". The nearest common sizes are 7/32" (.218") for a fatter tube, and 5 mm (.197) for a thinner tube.
First though, you need to find the actual size of the tube you wish to replicate.