5mm neoprene.

Perfect for integrating with dry suits, this is a top-quality hood for serious divers.

 
Features include:

• Made from tough high quality 5.0mm "CR" neoprene. NOTE: be wary of buying hoods made by "SVR" (polyester) neoprene which is cheaper but is not as durable, or as warm as "CR".  "SVR" is also highly flammable!
• Smooth skin face and neck seals to reduce water entry.

To find your size, measure around the widest point of head.