Top of Descent Calculator

A stabilized, constant-angle descent is easier to fly and far less workload-intensive than diving late and chasing the glidepath, and this calculator finds exactly where to start one. It first computes the altitude you need to lose — cruise altitude minus destination field elevation minus your target pattern or approach altitude above the ground — then converts that vertical distance into a horizontal one using basic trigonometry: distance equals altitude divided by the tangent of your descent angle, with the result converted from feet to nautical miles by dividing by 6,076.12 ft per NM. That distance is Top of Descent — how far out from the destination you should begin coming down to arrive at your target altitude on a straight, constant-angle path.

From the same geometry, the calculator derives the descent rate in feet per minute your ground speed requires to hold that same angle, and the total time the descent will take. Alongside the precise trig-based numbers, it also reports the classic pilot rules of thumb — altitude to lose in thousands of feet times three for nautical miles, and ground speed times five for feet per minute — both of which are quick mental-math approximations built specifically around a standard 3° glidepath and drift from the exact geometric answer at steeper or shallower angles. Ground speed, not airspeed, drives both the distance and rate calculations, so failing to account for a headwind or tailwind component will throw off the actual descent point and required rate even though the geometry itself is exact.