Top of Descent Calculator

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.