How thick does the ice need to be?  This depends on a lot of things including:

  • How much weight?
  • Is the load moving or stationary?
  • How spread out is the load?
  • How variable is the thickness?
  • Temperature?
  • How thawed?
  • What type of ice?

Most breakthrough accidents occur on ice that is thinner or weaker  than the surrounding ice.  If you check it in one place, it may not be the same somewhere else.  Having said that, the Minnesota DNR suggestions are fairly conservative for cold ice.  If the ice is a little thinner somewhere you did not measure, it still is likely to be strong enough. If the ice is thawed, broken (for example at a pressure ridge), or has other defects the following thickness recommendations do not apply.

Copyright 2012 Minnesota Department of Natural Resources. Reprinted with Permission

Skaters and sailors tend to spend more time on thinner ice than fishermen because the smoothest ice is often the newest ice.  They have a relatively high swim rate as a result, however, for the most part, they are better prepared and equipped to deal with getting wet than most ice users.  If you see skaters or sailors on new  ice, don't assume the ice is OK.  They may not have found any 'too thin'  ice yet or, if they have, they may know where it is and are avoiding it. The same thing can be said about using the presence of other vehicles on the ice as a basis for driving yours on. 

Since a high proportion of accidents occur on new ice, being prepared for getting wet and for rescuing yourself and others is especially important in the early part of the season. As the Minnesota ice accident statistics show December has a particularly high vehicle related mortality rate, making it an especially good time to leave your vehicle on shore.  January is the next worse month.  During and a few days after thaws is also a high risk time.  In the 2013 ice season many areas experienced several January and February thaws.  About half the deaths 53 lake ice deaths in the 2013 season in North America were attributable, at least in part, to warm weather.

For more detail on this subject see:

Body weigh breakthrough thickness

Bearing Strength of Ice