Start with Water Temperature: Use the chart as a baseline.
| Water Temperature | Recommended Thickness | Common Wetsuit Type |
| 75°F+ (24°C+) | 0.5 mm - 2 mm | Shorty or Top |
| 65°F - 75°F (18°C - 24°C) | 2 mm - 3/2 mm | Springsuit or Fullsuit |
| 58°F - 68°F (14°C - 20°C) | 3/2 mm | Fullsuit |
| 52°F - 58°F (11°C - 14°C) | 4/3 mm | Fullsuit |
| 45°F - 52°F (7°C - 11°C) | 5/4 mm or 5/3 mm | Fullsuit |
| Below 45°F (7°C) | 6/5 mm+ | Fullsuit (often with integrated hood, boots, gloves) |
Define the User & Activity: Adjust the baseline thickness up (for less activity / cold-natured users) or down (for high activity / warm-natured users).
A. High-Activity (Swimming, Triathlon): Prioritize flexibility. A triathlete in 65°F (18°C) water might choose a sleeveless 2mm or a very flexible 3/2 mm suit to allow for a freestyle stroke, whereas a surfer might prefer a 4/3 mm for the extra warmth during periods of inactivity.
B. Activity (Scuba Diving, Snorkeling): Prioritize warmth. A scuba diver, who moves less and cannot easily surface, will choose a thicker suit, perhaps a 5 mm or even 7 mm for colder water.
C. Cold-Natured Individuals: If you know you get cold easily, consider a suit that is one step thicker than the temperature chart suggests.
Design the Zoning Map: Decide on the differential thickness pattern (e.g., 4/3 mm, 5/4/3 mm) to maximize core warmth while preserving mobility in key areas.
Select Neoprene and Construction Quality: Choose high-stretch neoprene and sealed seams for colder water or higher performance needs. This can sometimes allow you to use a slightly thinner suit without sacrificing warmth.