How is top-of-descent distance calculated?

The descent angle is converted to a gradient in feet per nautical mile using the tangent of the angle times 6076, the number of feet in a nautical mile. Dividing the altitude to lose by that gradient gives the distance before the target to begin descending.

How does this differ from the 3:1 rule?

The 3:1 rule is a fixed shortcut for a 3-degree path that loses 300 feet per mile. This tool accepts any angle, so you can plan steeper or shallower descents and see the exact gradient and vertical speed for that angle.

What vertical speed should I fly?

The tool shows the vertical speed in feet per minute for your groundspeed and angle, computed as groundspeed times 101.27 times the tangent of the angle. Holding that vertical speed keeps you on the planned path to the target.

Should I use true airspeed or groundspeed?

Use groundspeed, because the descent distance and time depend on how fast you cover the ground. A tailwind increases groundspeed, moving the top of descent farther out and raising the vertical speed needed for the same angle.

Why add margin to the calculated distance?

The calculation assumes a constant-speed descent at the chosen angle. If you must slow down or configure flaps and gear before the target, begin a few miles early so deceleration does not flatten your path and overshoot the altitude.

Top of Descent (TOD) Calculator

Knowing exactly where to start down lets you fly an efficient, idle descent that arrives level at a crossing restriction or approach altitude. This calculator solves for the top-of-descent point, vertical speed, and time for any descent angle you choose.

How it works

The descent angle sets a gradient, and the distance and vertical speed follow:

altitude to lose   = cruise altitude − target altitude
gradient (ft/nm)   = tan(angle) × 6076.12
TOD distance (nm)  = altitude to lose / gradient
vertical speed     = groundspeed × 101.27 × tan(angle)   ft/min
time to descend    = TOD distance / groundspeed × 60      minutes

A shallower angle pushes the top of descent farther out and lowers the vertical speed; a steeper angle does the opposite.

Example and notes

Descending from 33,000 ft to 4,000 ft at 3 degrees with a 420 knot groundspeed gives a gradient near 318 ft per nautical mile, a top of descent about 91 nautical miles before the target, and a vertical speed near 2,200 ft per minute. Start a little earlier if you must decelerate before the target, and remember to use groundspeed so a tailwind is accounted for in both distance and vertical speed.