Ice is never completely safe. This calculator gives general estimates from published guidelines; check local conditions and advice, test the ice as you go and wear a flotation suit or life jacket.
Why thickness matters
Floating ice behaves like a plate resting on water. A load bends it, and if the bending is too strong the ice cracks. Its strength rises with the square of its thickness: doubling the thickness makes the ice roughly four times stronger. This relationship, measured in many field tests, is known as Gold’s formula, after the Canadian engineer Lorne Gold, who summarized it in 1971.
The calculation
- Equivalent clear ice = clear ice + half of the white ice
- Allowable load: P = 3.5 × h², with P in kilograms and h in centimeters
The coefficient 3.5 is a conservative value for good clear ice; it already includes a safety margin compared with the loads that cause failure. White or snow ice, full of air bubbles, is about half as strong, so it counts for half.
Example
The ice has 4 inches of clear ice covered by 2 inches of white ice:
- Equivalent clear ice: 4 + 2 ÷ 2 = 5 inches (12.7 cm)
- Allowable load: 3.5 × 12.7² = 565 kg, about 1,245 lb
- Enough for a snowmobile by the usual guidelines, not for a car
- A 1.5 t car would need about 8.1 inches (20.7 cm) of clear ice
Usual guidelines for new clear ice
- Under 4 in: stay off
- 4 in: walking and ice fishing on foot
- 5 to 7 in: snowmobile or ATV
- 8 to 12 in: car or small pickup
- 12 to 15 in: medium truck
Safety rules
- Measure the ice every few yards with an auger, chisel or spud bar: thickness can change quickly.
- Avoid areas near inlets, outlets, docks, bridges, reeds and dark spots, and all river ice.
- Ice that has thawed and refrozen, has cracks or is covered in slush is weaker.
- Never go out alone. Carry ice picks, a rope and a phone in a waterproof bag, and tell someone where you are going.
- If you fall in, turn toward where you came from, kick your legs to get horizontal, pull yourself out with the ice picks and roll away from the hole.