Pure saidWhat's your waist size? They do list a chart on their site comparing waist size with their sizing. Depends if you have a big ass/package too though, you may want to go one or two sizes up unless you like really really tight underwear.
You can like take your existing underwear brand and find the size chart for that and then compare it to the aussiebum chart to see where you fit. what pure said was true though because the size of your ass does matter but i havent tried on aussiebum so i dont know how they work.
Just to clarify, I usually get a Medium but they are pretty tight, so even though my waist is probably 30", if I wanted to have a slightly less restrictive fit I would probably get a large. But also it depends on what style and fabric the underwear or swimwear is made of - swimwear can be stretchier, though I have some board shorts that are not stretchy and even the Medium is a squeeze on my package and around my thighs.
Almost embarrassed to admit this, but the red trunks I have on in my pic are actually XL sized!