Top of descent calculator
Where to start down, how fast to come down, and what it costs in distance.
How it works
A 3° descent is very close to 318 ft per nautical mile, which is why the 300 ft/NM approximation is so durable. Distance = altitude to lose ÷ gradient. Rate = groundspeed × gradient ÷ 60.
The rules of thumb
Distance: altitude to lose in thousands × 3. Losing 8,000 ft needs about 24 NM.
Rate: groundspeed × 5 gives the feet per minute for 3°. At 120 kt that is 600 fpm.
Both are shown above so you can check them against the exact answer.
What to add
These give a continuous descent to the fix at the current groundspeed. Add distance for slowing down, for a level segment, and for any crossing restriction on the way — and remember groundspeed usually falls during a descent into a headwind, which stretches the distance you actually need.
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For planning and study, not for operational use. Aircraft performance comes from your POH, not from a generic formula.