Typically speaking, flip flops and sandals don't come in half sizes. We typically recommend that if you are a half size, you choose the next size up for a more comfortable and roomy fit. You may size down if you prefer a fit that is more snug or are worried about excess room in the front or back.
Some brands my have an unfamiliar sizing scale. We have outlined them for you below
BIRKENSTOCK SIZING CHART:
WOMEN'S RAINBOW SIZING CHART:
MEN'S RAINBOW SIZING CHART: