These are all measured by the outer diameter of your mounting bar, or in simpler terms, the cross-section of the tube. We make the following sizes: 1.125", 1.5", 1.75", 2"
The easiest way to measure these is with a set of calipers.
Not everyone has a set of calipers so another great way to do it is take a piece of string and wrap it around your bar, marking where the string meets itself. After pulling this off the bar, you can measure the length of the string with a ruler and this will give you an approximate circumference which you can use to infer the diameter. Typically your string will stretch as you pull it tight so your measurements will be slightly longer, by rounding down you can find what you need. You can see in the photo that my string measured right at 5 inches, so by referencing the table below (and rounding down to account for string stretching) i can see i have 1.5" OD tube cross bars.
- Here's an easy guide for diameter to circumference (length of string):
1.125" = 3.53"
1.5" = 4.7"
1.75" = 5.5"
2" = 6.3"