If there is any round shape bangle that fits you properly, just take that bangle, Then use a scale or a ruler and measure the diameter of the bangle in inches. The standard bangle size is 2.25 inches.
Bangle sizes are usually in the range of 2.0 inches - 3.0 inches. Please do NOT use a measuring tape as they are not accurate. Before you take the measurement using a scale/ruler, please take a moment to look at the scale/ruler and see how many divisions are there on it. You can do this by counting the number of lines or divisions between any one inch. Some scales/rulers will have 8 or 10 or 16 lines (divisions/markings). If the scales/rulers has 8 or 10 lines (divisions/markings) between any one inch, it is NOT very accurate and it is not recommended to use this. Try to find a scale/ruler that has 16 lines (divisions/markings) between any one inch, this is the most accurate one.
When you take the measurement, say for example, it comes to 2 inches and 4 lines or divisions, then the bangle size would be 2-4/10 inches (if the ruler has 10 lines or divisions between any one inch) which will make the bangle size 2.40 inches. (DO NOT confuse this with INDIA bangle size of 2-4 or 2-4 anna, that is a completely different size. Please read the question below for additional information on 2-4 anna). Basically, if the scale/ruler has 16 lines or divisions, and when you measure it comes to 2 inches and 4 lines, then the bangle size will be 2-4/16 or 2.25 inches - this is the same as India bangle size 2-4 or 2-4 anna.
Lastly, if you have any confusion in figuring out the bangle size- please take a bangle that fits you properly; keep a scale/ruler and call us. We can be over the phone with you to instruct and help you figure out the correct and exact bangle size.