About this tool
The Top of Descent (TOD) Calculator tells you how far before your target you should begin coming down, so you arrive at the right altitude without rushing the descent. Enter your cruise altitude, the altitude you want to level at, your ground speed and a descent ratio, and it works out the distance, the required descent rate, the time and the gradient. Everything runs locally in your browser.
It uses the classic 3:1 rule of thumb. The altitude to lose is cruise − target; the distance to start down is (altitude to lose ÷ 1000) × ratio, where a ratio of 3 means 3 nautical miles for every 1000 ft. The descent time is distance ÷ ground speed × 60 minutes, and the required rate of descent is altitude to lose ÷ time in minutes, which is equivalent to altitude × ground speed ÷ (60 × distance).
Raise the ratio for a shallower, more comfortable profile or lower it for a steeper one. The gradient output (feet per nautical mile) lets you cross-check against any charted descent gradient or a required crossing restriction.
Frequently asked questions
What is the 3:1 descent rule?
It plans 3 nautical miles of track for every 1000 ft of altitude to lose. So from 30,000 ft down to sea level you would start descending about 90 nm out. Change the ratio field to make the profile shallower (higher) or steeper (lower).
How is the required rate of descent found?
From the geometry: descent time in minutes is distance ÷ ground speed × 60, and the rate is the altitude to lose divided by that time, which equals altitude × ground speed ÷ (60 × distance) in feet per minute.
Does ground speed change the top-of-descent distance?
No. With the 3:1 rule the distance depends only on altitude and the chosen ratio. Ground speed sets how fast you cover that distance, so it changes the required rate of descent and the time, not where you start down.
Is this accurate enough for real flights?
It is a planning estimate. Real descents are affected by wind, speed changes, ATC restrictions and idle-thrust performance, so treat the numbers as a starting point and verify against your aircraft's official performance data and the FMS.
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