Pilot tools
Top of descent calculator
Enter your cruise altitude, the altitude you need to reach and your groundspeed. You get how many nautical miles out to start your descent and the vertical speed that holds a 3° path.
Feet, or a flight level like FL350
Where you need to be, such as 3,000 ft to start the approach
Your average groundspeed on the way down
3° is the standard path
Start descent
101nm
before the point where you need to be at 3,000 ft.
- Vertical speed
- 2,100fpm
- Time to descend
- ~15 min
- Altitude to lose
- 32,000ft
Show calculationsHide calculations
Flying with Stratos? Switch on Pause TOD and it pauses your sim at the top of descent for you.
For flight simulation only. Don't use it for real-world flight planning or navigation.
How it works
A 3° descent loses about 318 ft for every nautical mile flown. The calculator divides the altitude you need to lose by the tangent of your descent angle to get the distance, and multiplies your groundspeed in feet per minute by the same tangent to get the vertical speed.
distance (nm) = altitude to lose (ft) ÷ tan(angle) ÷ 6,076
vertical speed (fpm) = groundspeed (kt) × 101.3 × tan(angle)
In the cockpit, two rules of thumb get you close enough at 3°: 3 nm for every 1,000 ft to lose, and groundspeed × 5 for the vertical speed. Both come out a little under the exact figures. The 3-to-1 distance is a slightly steeper path, about 3.1°, so if you need to slow down on the way, start a few miles earlier. Our phases of flight guide covers where the descent fits in the rest of the flight.
The groundspeed already includes the wind, so the calculator doesn't ask for it. Use your average groundspeed for the descent rather than in cruise: an airliner slows as it comes down, often to 250 kt below 10,000 ft, so its average is usually well under its cruise groundspeed.
Questions
How do you calculate top of descent?
Take the altitude you need to lose in thousands of feet and multiply it by 3. Coming down from 35,000 ft to 3,000 ft is 32,000 ft, so start your descent about 96 nm before the point where you need to be at 3,000 ft. That is the 3-to-1 rule, and it gives a path of about 3°.
What descent rate should I fly?
For a 3° path, multiply your groundspeed by 5. At 400 kt that is 2,000 fpm; the exact figure is closer to 5.3 times groundspeed, or about 2,120 fpm. As you slow down on the way down, the rate comes down with it.
How does wind affect top of descent?
Wind doesn't move the start of a 3° path, but it changes the descent rate that holds it. A tailwind raises your groundspeed, so you need a higher rate; if the aircraft can't come down that fast at idle, start your descent a little earlier. A headwind does the opposite. Enter the average groundspeed you expect on the way down, wind included.
Why start your descent earlier than the calculator says?
The calculation assumes you come down at a steady speed. In practice you also have to slow down, often to 250 kt below 10,000 ft and again for the approach, and an airliner can struggle to descend and slow down at the same time. Many pilots add about 10 nm, or a mile for every 10 kt they need to lose.
Does the FMS work out top of descent for me?
Yes. In an airliner with a flight plan in the FMS, the top of descent appears on the navigation display and the FMS builds the path from the arrival's altitude and speed restrictions. This calculator is for checking the FMS, for aircraft without one, and for planning before you load the route.
Stratos by Skyvex Software
Free ACARS for every pilot.
On Windows, macOS and Linux.
Stratos reads Microsoft Flight Simulator and X-Plane directly. It counts down to your top of descent, can pause the sim when you reach it, and files the PIREP before you leave the cockpit.