In aviation, the rule of three or 3:1 rule of descent is a rule of thumb that 3 nautical miles (5.6 km) of travel should be allowed for every 1,000 feet (300 m) of descent. For example, a descent from flight level 350 would require approximately 35x3=105 nautical miles.

The "Rule of 3" is a classic aviation mental math formula used by pilots to determine the Top of Descent (TOD)—the point where they must leave cruise altitude to arrive at a target altitude at a specific location. The rule states that for every 1,000 feet of altitude you need to lose, you should allow 3 nautical miles (NM) of travel distance. For example, if a pilot is cruising at 30,000 feet and needs to descend to 10,000 feet (a loss of 20,000 feet), they would multiply 20 by 3 to get 60 NM. This calculation typically results in a standard 3-degree glide path, which is comfortable for passengers and efficient for the aircraft. Pilots also use a secondary part of the rule to determine the required Rate of Descent (ROD): multiplying their ground speed by 5 (or halving the ground speed and adding a zero). If flying at 300 knots, the pilot would aim for a 1,500 feet-per-minute descent. While modern flight computers handle this automatically in 2026, the Rule of 3 remains a vital backup skill for pilots to maintain situational awareness and ensure they don't "run out of sky" before reaching the airport.