Top of Descent (TOD) Calculator
Calculate the ideal Top of Descent point, required descent rate, and time to descend for a given altitude drop and groundspeed.
Results
What is it?
The Top of Descent (TOD) point is where you begin your descent to arrive at a lower altitude at the correct distance from your destination. In commercial aviation, a 3 degree descent path is standard and aligns with most ILS glideslopes. Starting descent too late results in a steep uncomfortable descent; too early wastes fuel.
How to use
1. Enter your current cruise altitude in feet. 2. Enter your target altitude in feet. 3. Enter your current groundspeed in knots. 4. Select the desired descent angle (3 degrees is standard for most approaches). 5. The calculator shows how far from your destination to begin descending and the required vertical speed.
Example scenario
Cruising at FL150 (15,000 ft), targeting 3,000 ft, groundspeed 180 kts at 3 degrees. Altitude to drop = 12,000 ft. TOD distance = 12,000 / (tan(3) x 6,076) = 37.8 NM. Required descent rate = 950 fpm. Begin descending roughly 38 NM before your destination.
Pro tip
The classic 3-degree rule of thumb: multiply the altitude to lose (in thousands of feet) by 3 to get the NM distance to start descent. Losing 12,000 ft means start 36 NM out. For vertical speed: multiply groundspeed by 5 to get approximate fpm at 3 degrees. At 180 kts: 180 x 5 = 900 fpm.