Rule of threes (survival)

In survival, the rule of threes involves the priorities in order to survive.^[1][2][3] The rule, depending on the place where one lives, may allow people to effectively prepare for emergencies^[4] and determine decision-making in case of injury or danger posed by the environment.^[1]

Training in use of a liferaft – the rule will apply when exposed at sea

Rule

Normally, the rule of threes contains the following:

• You can survive three minutes without breathable air (unconsciousness), or in icy water.
• You can survive three hours in a harsh environment (extreme heat or cold). You need shelter!
• You can survive three days without drinkable water.
• You can survive three weeks without food. But, many people have gone for over 40 days during fasting and have survived.

Each line assumes that the one(s) before it are met. For example, if you have a large quantity of food and water but are exposed to the environment, then the harsh conditions rule applies. The rule may sometimes be useful in determining the order of priority when in a life-threatening situation, and is a generalization (or rule of thumb), not scientifically accurate.^[5]

Additional generalizations may be presented with the rule, though they are not considered part of the "Rule of threes" and are also usually not scientifically accurate. For example, it might be said that it takes a three-second psychological reaction time to make a decision during an emergency,^[1] that one can survive 3 minutes in icy water,^[5] or that an individual can last three months without hope (or companionship).^[6][5][7]

Accuracy

Water

The amount of time a person can survive without a source of water (including food which contains water) depends on the individual person and the temperature. As temperature increases, so does water loss, decreasing the amount of time a person can survive without water. The longest anyone has ever survived without water was 18 days.^[8] The likely source of the "3 days" number is an experiment by two scientists in 1944, when they ate only dry food for a period of time; one ended the experiment at 3 days in, and the other at 4 days in. However since they stopped the experiment before being in any real danger, the actual survival time at room temperature likely exceeds 3–4 days.^[8] However, at temperatures greater than room temperature, dehydration can occur very quickly, and death can occur in a matter of hours rather than days. In these cases people typically die of heat stroke first, not terminal dehydration.^[8] One person was purported to survive seven days in the desert, six of these without water, without suffering heat stroke, as the temperature reached no higher than 103.2 °F (39.6 °C) during his ordeal.^[9] However, he had reached the third stage of dehydration, which is 80–90% fatal; this likely represents an upper limit of survival at high temperatures.^[9]