Ice Safety: How Thick Does Ice Need to Be?

No ice is ever 100% safe. As a minimum, the Minnesota Department of Natural Resources says to stay off new, clear ice that is less than 4 inches thick, and to double its guidelines on white or snow ice, which is only about half as strong. In New York City parks, the rule is simpler: don't walk on any frozen lake, pond or river at all.

Frozen water is one of winter's great pleasures, and one of its quietest dangers. This guide uses official guidance from a state where people walk, fish and drive on ice every winter, plus the rules for NYC parks.

No ice is ever 100% safe

The Minnesota DNR is blunt: there is no such thing as 100 percent safe ice (opens in a new tab). It adds that the DNR does not measure ice thickness on Minnesota lakes. Your safety is your responsibility (opens in a new tab).

Why you can't trust your eyes:

• You can't judge the strength of ice just by its appearance, age, thickness, temperature, or whether or not it's covered with snow. (opens in a new tab)
• Ice seldom freezes uniformly. It may be a foot thick in one location and only an inch or two just a few feet away. (opens in a new tab)
• New ice is usually stronger than old ice. (opens in a new tab) Four inches of new, clear ice may hold a person, while a foot or more of old, partly thawed ice may not.

Most deaths after a fall through ice are drownings, not hypothermia. The Minnesota DNR reports that of 269 ice-related deaths in the state since 1976, only 5 were a result of hypothermia (opens in a new tab). The cold water takes away your breath and strength fast. See cold water shock.

Thickness guidelines from an official source

The Minnesota DNR publishes these guidelines for new, clear ice only (opens in a new tab):

The DNR publishes thicker guidelines for heavier vehicles on its ice thickness page (opens in a new tab). Its numbers assume solid, clear ice and average equipment weight (opens in a new tab), and it warns that many factors other than thickness affect ice strength (opens in a new tab), including air temperature, wind, snow, streams, sun and shade.

Double

How to measure. The DNR suggests an ice chisel, an auger or a cordless drill with a long wood auger bit, then measuring the hole with a tape measure, hooking the bottom edge of the ice (opens in a new tab). It advises you to start by checking thickness near shore and to stop and check frequently as you go (opens in a new tab), and warns: don't judge ice thickness by how easily a chisel or drill breaks the surface (opens in a new tab).

Clear ice, white ice and slush

Not all ice is the same. From the Minnesota DNR:

• Clear ice is the strongest. It typically forms at the start of the ice season on top of calm water (opens in a new tab), and can look blue or black.
• White ice forms when snow melts on top of a sheet of ice and refreezes (opens in a new tab). It is only about half as strong as new, clear ice (opens in a new tab), so double the guidelines. When you measure, look at how much of the thickness is clear and how much is white.
• Honeycomb ice in spring is one of the most dangerous types of ice, associated with many fall-throughs during the late-ice season (opens in a new tab). The DNR says ice thickness guidelines do not apply to honeycomb ice. If ice is honeycombed, stay off (opens in a new tab).
• Snow on top slows the freezing process, and its extra weight reduces how much the ice can support (opens in a new tab).

Where ice is weakest

Even on a thick lake, some spots are dangerous. The Minnesota DNR lists:

• Moving water. Ice formed over flowing water and currents is often dangerous, especially near streams, bridges and culverts (opens in a new tab). Ice on the outside of river bends is usually weaker.
• Near shore, especially in spring. Ice near shore can be weaker than ice farther out (opens in a new tab), and in spring ice typically melts fastest near the shore (opens in a new tab).
• Around objects. Near logs, rocks, bridges and docks (opens in a new tab), which absorb heat from the sun, and in shallow water with heavy plant growth (opens in a new tab).
• Pressure ridges. Pressure ridges are always dangerous (opens in a new tab), and they can form in a few hours.
• Wildlife and fish. Schools of fish or flocks of waterfowl can adversely affect ice (opens in a new tab).

Stormwater and retention ponds are a special risk. A county health department in Indiana warns that their ice is often thin and unstable because of fluctuating water levels, varying depths and water flow (opens in a new tab). Teach children never to walk on these ponds. See quarries, ponds and reservoirs.

In New York City

NYC Parks is clear. Its winter safety rules:

• Do not attempt to walk on any lakes, ponds or rivers that appear frozen. (opens in a new tab)
• Parents and caregivers should make sure children are never unattended near ice. (opens in a new tab)
• If you hear cracking, lie down immediately to try to distribute your weight. (opens in a new tab)
• If you witness someone falling through ice, never attempt to make a rescue by yourself: call 911 (opens in a new tab), and give the exact location.

NYC Parks points out that ice skating rinks throughout the city offer a safe way to celebrate the cold weather (opens in a new tab).

Pets on ice: don't go after them

When a pet goes through the ice, the urge to go after it is strong. The Minnesota DNR says: do not risk your life to attempt to save a pet or other animal (opens in a new tab).

Keep dogs leashed near frozen water. If a pet goes through, call 911 and stay on shore. For people, see what to do if you fall through ice and reach or throw, don't go.

Check, ask and stay off when unsure

Before you step onto ice:

• Ask local experts, such as park staff or a bait shop, about current conditions. The DNR advises talking to local experts (opens in a new tab) about known ridges and holes.
• Measure the ice near shore, and keep measuring as you go.
• Never go alone, and tell someone where you'll be.
• Wear a foam life jacket or flotation suit, and carry ice picks and a whistle. The Minnesota DNR calls a foam life jacket or flotation suit maybe the No. 1 item that will keep you alive if you fall through the ice (opens in a new tab), and ice picks the second most important item (opens in a new tab). It says inflatable life jackets are not recommended for cold-water activities (opens in a new tab).
• Stay off if the ice is honeycombed, slushy, near moving water, or simply not thick enough.

What you can do today

• Find the official ice safety rules for the lakes and ponds near you. In NYC, the answer is: stay off.
• Teach children that frozen ponds and stormwater ponds are never for walking or playing.
• Keep dogs leashed near frozen water all winter.
• If you fish or skate on natural ice, get a foam life jacket, a pair of ice picks, a whistle, an ice chisel or auger, and a tape measure.
• Read the self-rescue steps in what to do if you fall through ice.