How thick a wetsuit do I need?
This is a common question. There are no easy answers as there are a great many variables that influence the answer to this question and why it can vary from one person to another. Let's look at some of these variables and how they can affect your decision.
Some people stay warmer than others at the same temperature. Just as we have different talents in life, there are physiological differences among individuals that enable some to be comfortable with a swimsuit while others are wearing wet suits. The same is true of those wearing thin shorty wet suits while their buddies are clad from head to toe in thicker suits. Mother Nature made us with different thermostats. Generally, women will chill faster than men, contrary to what some might assume. It's true that women have an extra layer of subcutaneous fat, put there by Mother Nature for embryo protection purposes, hence the assumption they should be warmer. However, they also have a higher surface area to mass ratio, or more simply put, a more efficient cooling system. You may notice this principle in effect in office buildings, the women are cold at temperatures that the men find barely comfortable (even if the men are not wearing suits or coats).
Heat loss areas
The three major heat loss areas are the inner thighs (near the femoral artery), under the arms (near brachial artery) and the head and neck. The latter two together account for less than 40% of heat loss as the head is proportionately higher because of the myriad of blood vessels it contains to keep the brain cool.
If you dive in cold water without a hood, it is roughly analogous to having the heat on in the cold of winter and leaving the front door and windows wide open.
Suits that fit poorly under the arms and around the thighs may allow too much water to collect there, robbing you of your body heat. These are areas where plenty of insulation is called for in cold water and where proper fit is essential.
Obviously, the colder the water, the greater the rate of heat loss. Wet suits work by the bubbles in the neoprene insulating you, to varying degrees, against the colder water outside the suit, by trapping a thin layer of water in the suit, which is warmed by your body, and by reducing the flow of water around your body. That flow, along with heat lost through the neoprene, dissipates your body's heat into the body of water you are diving in. Since your body cannot heat the surrounding water, eventually you will chill. The colder it is, the faster you will chill. Some waters, such as the North Atlantic, are so cold that an unprotected person, such as a shipwreck victim or downed pilot, would die in a matter of a few minutes from extreme hypothermia.
Thickness and fit of suit
The colder the water, thicker the suit needs to be to keep you comfortable. Increasing the thickness of your suit, either with a single suit or layers of neoprene, reduces your body's rate of heat loss. If your individual metabolism is such that you chill faster than others similarly geared, consider a thicker suit and the additional thermal protection it offers.
If your suit fits well, it will keep the water flow in and out of your suit to a minimum, thus reducing the rate of heat loss. If it doesn't fit properly, there's not much value in wearing it. The suit can't do its job.
Please note the following regarding thickness. The
neoprene sheets used in making the suits we sell comes in fraction
of an inch measurements. While we use the metric equivalent, it is
an approximation and not exactly equal. That is why sometimes you
see 1/4 inch suits described as either 6.5 or 7 mm, depending on
whether the measurement approximation is rounded downward or upward.
For your convenience, here is a chart of approximate equivalency.
Length and number of dives
It stands to reason, the longer you stay in the water and the more dives you make on each diving outing, the more heat loss you will have. Consider this in the equation. If you will make mostly shallow dives with a lot of bottom time, your heat loss will be greater than that of a deeper dive with its accordant shorter no-decompression time. If you make two or more dives in a day, the heat loss problem is exacerbated.
Depth of dives
The deeper you go, the more wet suits compress because of ambient pressure. The more they compress, the less thermal protection they offer. If most of your diving will be at greater depths, or if you wish to protect yourself against the occasional deeper dive, you will want a thicker suit than would be needed for shallower depths.
The weather in your usual diving locale plays a part in this. If you dive in an area with a wam climate and lots of sunshine, you have a chance to warm up between dives. If you dive in a cold, windy, overcast area, your heat loss will diminish although you may not recover enough heat to make your next dive comfortable.
The more active you are when diving, the more energy and heat will be generated by your body, and the more it will offset your heat loss. This probably shouldn't influence your choice of suit as a sport diver as it can be extremely variable from one dive to another.
Style of suit
The style of suit you buy has a definite bearing on your warmth and the thickness needed. Styles of wet suits include a one piece jumpsuit, two piece farmer john (farmer jane for women), long sleeve shorty, and short sleeve shorty. Some divers wear a 3 or 5mm short sleeve shorty over a 2 or 3mm one piece jumpsuit, giving them the greatest protection in the torso area, while affording a great deal of flexibility in the arms and legs. You will probably want to wear a hood as 60 percent of your heat loss in water is from your head and neck. You can choose from a standard hood, a cold water hood ( a hood with an extended bib), and a warm water hood, which cover the head but not the neck. These are sometimes referred to as Snoopy hoods, after the canine character from Charles Schultz's Peanuts comic strip (when Snoopy is the World War I ace on his Sopwith Camel).
You can even get some suits, particularly jumpsuits, with the hood attached for minimal water flow in the region of your neck. Other options include vests, and hooded vests, which if thin enough, may be worn underneath a wetsuit jacket. Consider a spine pad to fill in that space along your spine to prevent cold water from flowing down it.
There are other factors to be sure. However, these used in conjunction with your knowledge of self and the following water temperature/suit thickness guide should enable you to pick the right suit for yourself.