Various brands of swimwear have different sizes depending on gender and age. 


The best way to know what size to order is by looking on the brands website size chart. Below are a few links to various brands size charts:


Funkita - https://www.funkita.com/content/12-size-guide-funkita


Funky Trunks - https://www.funkytrunks.com/content/8-size-guide


Amanzi - https://www.amanziswimwear.com/size-chart


Tyr - https://www.tyr.com/shop/sizing/


Speedo sizes can be all found here - https://www.speedo.com/uk/en/womens-size-guide.html