Aviation Calculators
Top of descent.
To find where to start descending, divide the altitude to lose by about 300 to get the distance in nautical miles for a 3 degree path. Enter your altitudes, ground speed and angle to get the distance, the descent rate and the time, exactly and by the rule of thumb. Runs in your browser; nothing is sent or stored.
Planning aid, not for flight operations
This tool is for study, trip planning and curiosity. It is not approved flight-planning or performance data. For an actual flight, use the aircraft's approved Pilot's Operating Handbook or Aircraft Flight Manual, current weather and charts, and your own judgement as pilot in command. Results can differ from your aircraft's real figures.
Inputs
Result
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Worked example
Cruising at 7,000 ft and wanting pattern altitude of 1,000 ft at a ground speed of 120 knots. The altitude to lose is 6,000 ft. The FAA rule of thumb divides by 300: 6,000 ÷ 300 = 20 NM, and the descent rate is ground speed × 5 = 600 ft per minute. The exact 3 degree path drops 318.4 ft per NM, so the distance is 18.8 NM, the rate is 637 ft per minute and the descent takes 9.4 minutes. Leave some extra room, as the FAA text advises.
How it works
A 3 degree path loses 6,076.1 ft × tan(3°) = 318.4 ft per nautical mile, the descent gradient the FAA handbook quotes for a typical arrival. So distance = altitude to lose ÷ feet per NM, and the rate of descent in feet per minute is ground speed × feet per NM ÷ 60, about 5.3 × ground speed for 3 degrees.
The FAA Instrument Procedures Handbook gives two rules of thumb, shown beside the exact figures: divide the altitude to lose by 300 to get the number of miles before your target point, and multiply ground speed by 5 for the 3 degree rate of descent (120 knots gives 600 fpm). It adds that you should leave some extra room so that you are at the target altitude before the point you need it.
Use ground speed, not airspeed: a headwind lowers your ground speed and so lowers the rate you need, while a tailwind raises it. The tool uses a constant ground speed for the whole descent.
Sources (each opened 2026-10-08): FAA, Instrument Procedures Handbook (FAA-H-8083-16B), Chapter 3: Arrivals.
FAQ
How do I calculate top of descent?
The FAA Instrument Procedures Handbook says to divide the altitude you need to lose by 300. Losing 6,000 ft therefore means starting 20 NM before the point where you want to be at the new altitude.
What descent rate gives a 3 degree path?
The same handbook says to multiply ground speed by 5. At 120 knots that is 600 feet per minute. The exact figure for 3 degrees is a little higher, about 5.3 times ground speed.
Why does the exact distance differ from the rule of thumb?
A 3 degree path drops 318 ft per nautical mile, not 300, so the exact distance is a bit shorter than dividing by 300. The rule of thumb is a little conservative, which also leaves some extra room.
Does wind change it?
Yes, through ground speed. Into a headwind your ground speed falls, so the descent rate for the same path is lower; with a tailwind it is higher. Enter the ground speed you expect.
Does it include slowing down?
No. It assumes a constant ground speed. Slowing during the descent changes the rate needed, so leave margin for it.
Can I rely on it for ATC crossing restrictions?
It is a learning and planning aid. Follow your clearance, the published procedure and your aircraft's approved data.