Hanover, New Hampshire

For many of us in northern climates, working or playing on the frozen surface of a river or lake is part of winter. Knowing how to do so safely can be a matter of life or death. This handout presents general, common-sense precautionary measures that should be followed when you plan to be on a floating freshwater ice cover. Since it cannot cover every ice condition you may encounter, your judgement is critical. Remember: Only you are responsible for your own safety!


There are four things to focus on when planning an outing on the ice: your physical condition, your clothing, your equipment, and your procedures.

Physical condition

Anyone who goes out on the ice should be in reasonably good condition and be able to sustain periods of intense exertion if an emergency arises-either falling through the ice themselves or rescuing someone who does. Being able to swim, or at least being comfortable staying afloat, is important in an emergency and can reduce the chances for panic.


Naturally you should choose clothing that provides protection from low air temperatures, wind, and precipitation while at the same time allowing you mobility. But in addition, when you select clothing, keep in mind the possibility of falling through the ice. Clothing that would severely restrict your ability to swim or to stay afloat is not a good choice. Hip boots or waders should never be worn, as they can fill with water and restrict movement while adding weight. A personal flotation device (PFD) should be worn. This can be a vest or jacket, either inflatable or aturally buoyant.


Include items for testing and measuring the ice thickness, as well as items for rescue or self-rescue. In the first category are a heavy ice chisel, an ice drill or auger (manual or powered), a measuring tape or stick that can be hooked under the bottom edge of the ice in an auger hole, and possibly a perforated ladle for cleaning ice out of the auger holes. In addition to the PFD, bring a rope or rescue throw bag containing a rope that floats. Ice rescue picks sold for ice fishermen are an excellent idea. They thread through your jacket sleeves like children's mittens and are immediately available in an emergency for pulling yourself out of the water onto the ice.



Once on the ice it is time to begin more systematic observations of the ice sheet you want to use to support a load. There may be many variations in the structure, thickness, temperature, and strength of a floating freshwater ice sheet.

How thick is the ice?

This is determined by drilling holes with the drill or ice auger. The technique is to drill a hole and check the ice thickness every 150 feet or so along the intended path. This should be done more frequently if the ice thickness is quite variable. Note whether the ice in each hole is clear (sometimes called black ice) or white (due to air bubbles-sometimes called snow ice). Measure the thickness of both kinds.On rivers the ice thickness and quality can change a lot in a short distance; be particularly alert to variations in ice thickness due to bends, riffles or shallows, junctions with tributaries, etc. For both rivers and lakes, warm inflows from springs can create areas of thinner ice. The ice near shores can either be thinner (due to warm groundwater inflow or the insulating effect of drifted snow) or thicker (due to the candle-dipping effect of variable water levels).

Measure the snow cover thickness on the ice cover; significant variations in thickness may mean highly variable ice thicknesses.

How thick does it need to be?

A simple formula to estimate the minimum ice thickness required to support a load is:

formula graphic

where h is the ice thickness in inches and P is the load, or gross weight, in tons. You can also use the graph or table to determine the minimum thickness.

Remember that the load is the total load in tons (not a vehicle's load capacity).

loads on ice

Minimum ice thickness required to support a load

 Ice Load 
 Required ice thickness 
 Distance between loads 
0.1 2 17
1 4 34
2 6 48
3 7 58
4 8 67
5 9 75
10 13 106
20 18 149
30 22 183
40 26 211

The equation, graph, and table are valid when the load (such as a person on foot, or a wheeled or tracked vehicle) is distributed over a reasonable area of a continuous ice sheet. The larger the load, the greater the area it should cover for the calculation to remain valid. Neither large loads that are concentrated in relatively smaller areas, nor loads that are at or near the edge of a large opening in the ice, are safely described by the equation, graph, or table.In such cases, seek more advice.

The equation, graph and table assume clear, sound ice. If white, bubble-filled ice makes up part or all of the ice thickness, count it as only half as much clear ice.

Any recent large snowstorm creates a new load on the ice. If the new snow is heavy enough, the ice sheet will sag and its top surface will be submerged below the water level. Then water will flood the top of the ice sheet through cracks, saturating the lower layers of the snow. Until this slush is completely frozen, stay offthe ice sheet. When the saturated snow becomes frozen, it is an added thickness of white ice.

Contrary to what you would expect, a rapid, large air temperature drop makes an ice sheet brittle, and the ice may not be safe to use for 24 hours or more.

If the air temperature has been abovefreezing for at least 6 of the past 24 hours, multiply the load by 1.3 before you use the equation (or use the lower dashed line on the graph), obtaining a larger minimum ice thickness to account for any possible weakening. If the air temperature stays above freezing for 24 hours or more, the ice starts losing strength, and the equation, figure, and table no longer represent safe conditions. Stay off the ice!

You are likely to encounter cracks in the ice. Cracks are either wet or dry. If they are dry, they do not penetrate the ice sheet and are not a concern. If they are wet, multiply the load by 2, as shown on the graph, before you use the equation to obtain the required minimum ice thickness.If you plan to leave a load on the ice for extended periods, usually more than two hours, multiply the load by 2 (as shown by the upper dashed line in the graph) before you use the equation to find the required minimum ice thickness.



Be sure you understand this information. Don't hesitate to seek the advice of others whose experience you trust. Be safe out on the ice!

US Army Corps of Engineers
Cold Regions Research and Engineering Laboratory
72 Lyme Road, Hanover, New Hampshire 03755, USA
CRREL Public Affairs Office, 603-646-4386