All our swimsuits are already designed specifically for small chests (those in the IBTC with 1-4 inch difference between your underbust and bust) so you do not need to size down just because you are concerned about the cup size. Please follow your underbust measurement for your size. The only time we would recommend sizing down is if you prefer a snug fit or if the shoulder straps are not adjustable and you have a short torso.
WHAT SIZE AM I?
We have a very useful size guide available for you, including a size generator. We also have our models' measurements (underbust, bust, waist, hips) displayed on each product page. However, if you have specific questions about your size please contact info@louswim.com.
Please note the FIO bikini top runs small so we recommend you view the information we have on the band size and maximum stretch to see what size will fit you best and whether you need to size up. For example if you are 71cm underbust usually you are a size XS but a size XS FIO top is 64cm band with 72cm at maximum stretch so we would recommend you size up to a size S