5/3 fullsuit wetsuits are designed specifically for cold water activities and can be used in some pretty darn cold water. There are two main types which are 5/3 and 5/4/3. The numbers represent the thickness of the neoprene used in different parts of the wetsuit. In the 5/4/3 model you have some areas which are 5 millimeters thick (usually legs & body), some that are 4 millimeters thick, and some that are 3mm. In 5/3 you only have 5mm and 3mm thickness. Different thicknesses are there for increased mobility. Thinner areas give you more free movement so you will find them in shoulders and arms.
Should you get a 5/3 or 5/4/3 wetsuit?
Actually there is not too much difference between the 5/3 and the 5/4/3 so the best thing to do is to focus on fit of the wetsuit, it’s more important.
How cold can it get for 5/3?
Combining a full 5/3 wetsuit with thick wetsuit boots and gloves and a neoprene hood can really help keep you warm even in waters as cold as 45 degrees. Of course that is not a set number because there are several other factors which go into identifying the safe temperature. Wind, air temperature and other things can make you lose your heat much faster than just cold water alone.
If you want to learn more about which wetsuit goes with which water temperature and what are the factors that influence the feeling of cold check this article.