Unfortunately, most rain boots don't come in half sizes. Every manufacturer has its own sizing chart and that's where you should start. You might have to measure the length, width, and possibly circumference of your foot. If you fall between sizes, some manufacturers make recommendations whether to size up or down. You might also have to consult reviews to see if the boots run large, small, or true to size. In general, rain boots tend to run a little small, but there are exceptions.