Top of Descent Calculator
Fill in the values and the answer updates as you type.
How the Top of Descent Calculator works
Distance = altitude to lose ÷ (tan(angle) × 6,076 ft per NM). Required descent rate = groundspeed × 6,076 ÷ 60 × tan(angle), which is close to groundspeed × 5 ft/min for a 3° path. The 3:1 rule says to allow 3 NM for every 1,000 ft to lose. Add extra distance if you must slow down or expect winds to change on the way down. Example: with Current altitude 35000 ft, Target altitude 3000 ft, Groundspeed 450 kt, and Descent angle 3 °, the result is Distance to start descent (NM) = 100.5; Required descent rate (ft/min) = 2,388; Descent time (minutes) = 13.4.
Inputs and results
The Top of Descent Calculator takes 4 inputs and gives 4 results.
What you enter
- Current altitude (ft) — starts at 35000.
- Target altitude (ft) — starts at 3000.
- Groundspeed (kt) — starts at 450.
- Descent angle (°) — starts at 3.
What you get
- Distance to start descent (NM)
- Required descent rate (ft/min)
- Descent time (minutes)
- 3:1 rule of thumb distance (NM)
Example: Top of Descent Calculator with the starting values
With current altitude 35000 ft, target altitude 3000 ft, groundspeed 450 kt and descent angle 3 °, the calculator returns:
| Distance to start descent (NM) | 100.5 |
|---|---|
| Required descent rate (ft/min) | 2,388 |
| Descent time (minutes) | 13.4 |
| 3:1 rule of thumb distance (NM) | 96 |
Change any value above and the results update straight away.
Top of Descent Calculator FAQ
How do I use the Top of Descent Calculator?
Enter current altitude, target altitude, groundspeed and descent angle in the fields, then read distance to start descent (nm), required descent rate (ft/min), descent time (minutes) and 3:1 rule of thumb distance (nm) in the result panel. The answer updates as you type, and you can press Calculate or Enter to refresh it. Use “Copy link” to share your exact numbers with someone else.
What does the Top of Descent Calculator calculate?
Where to start descending for a chosen descent angle. It returns distance to start descent (nm), required descent rate (ft/min), descent time (minutes) and 3:1 rule of thumb distance (nm) from the 4 values you provide.
What values does the Top of Descent Calculator start with?
It opens with current altitude 35000 ft, target altitude 3000 ft, groundspeed 450 kt and descent angle 3 ° so you can see a working result straight away. Replace them with your own numbers, or press Reset to return to these starting values.
Is the Top of Descent Calculator free and private?
Yes. It is free, needs no account, and runs in your browser, so nothing you type is sent to or stored on our servers. See the Privacy Policy for details.
Can I rely on the Top of Descent Calculator for important decisions?
Treat the result as a well-rounded estimate. Real situations often have details a general calculator cannot know, so confirm important decisions with a qualified professional. See the Disclaimer for more.