Top of Descent Calculator

About this calculator

The top of descent (TOD) is the point in a flight where a pilot must begin descending to reach the target altitude smoothly by the time they arrive at their destination or a fix. It's commonly estimated with the 3:1 rule: distance in nautical miles = (altitude to lose in feet ÷ 1000) × 3 — so descending from 35,000 ft to sea level needs roughly 105 NM of descent distance.

Paired with a chosen groundspeed, the same numbers give the required rate of descent in feet per minute needed to fly that 3-degree-ish glide path. This rule-of-thumb math is standard practice for pilots doing quick mental flight planning and cross-checking what a flight management system (FMS) calculates automatically, especially in general aviation cockpits without an FMS.