Top of Descent Calculator
3:1 rule + 10% buffer
Rule of thumb: multiply altitude to lose (in thousands) by 3 to get distance in NM. Example: FL350 to 3,000 ft = 32,000 ft to lose = 96 NM.
About Top of Descent
Top of descent (TOD) is the point along your route where you begin descending from cruise altitude. Descending too early wastes fuel and speed; starting too late means you arrive above your target altitude and have to request an expedited descent from ATC. The standard 3:1 rule gives a quick estimate for a 3-degree descent profile.
Formula: TOD distance (NM) = altitude to lose (ft) / 1,000 x 3. Descending from FL350 to 3,000 ft means 32,000 ft to lose, giving a TOD of 96 NM from the target. Required descent rate (fpm) = groundspeed (kts) x 5 for a 3-degree gradient.
What is a standard descent gradient?
The 3:1 rule equates to a 3-degree descent angle and roughly 300 ft/min per 100 kts of groundspeed. Airlines typically plan 1,000-1,500 fpm for en-route descent, steepening to 700-800 fpm on the approach.
How early should I start descending?
Multiply the altitude to lose (in thousands of feet) by 3 to get the TOD distance in NM, then add 10% as a buffer for ATC routing or crossing restrictions. This calculator does that automatically in Normal mode.