Top of Descent Calculator (Where to Start Descending, Rate of Descent)
How far from your destination to start descending, and the rate of descent you need, for a given altitude to lose, path angle and ground speed.
Results
The dashed line marks the value you entered.
What this tool does
A steady descent along a path of angle γ loses 6,076 × tan γ feet for every nautical mile, which for the usual 3° is close to 318 ft/NM. So 7,000 ft to lose needs about 22 NM, and the familiar rule of thumb of 3 NM per 1,000 ft gives about 21 NM, a touch short. The rate of descent follows your ground speed, which is why it must be raised in a tailwind and can be lowered in a headwind: at 120 kt a 3° path needs about 640 ft/min. Airspace and procedure constraints always come first.
Formula
distance = Δh ÷ (6076 × tan γ) · rate of descent = GS × 6076 ÷ 60 × tan γ · rule of thumb: 3 NM per 1000 ft
Variables
| Symbol | Meaning | Unit |
|---|---|---|
a1 | Altitude you are at | ft |
a2 | Altitude you want to reach | ft |
an | Descent path angle | ° |
gs | Ground speed | kt |
DT | Distance from the target at which to start the descent | NM |
VS | Rate of descent needed | ft/min |
TM | Time in the descent | min |
GR | Gradient of the path | ft/NM |
R3 | Distance by the '3 times thousands of feet' rule | NM |
Worked example
- Altitude you are at9500 ft
- Altitude you want to reach2500 ft
- Descent path angle3 °
- Ground speed120 kt
- Distance from the target at which to start the descent22.0 NM
- Rate of descent needed637 ft/min
- Time in the descent11.0 min
- Gradient of the path318 ft/NM
- Distance by the '3 times thousands of feet' rule21.0 NM
Limitations
- For work that must comply with a standard or be signed off, check the result against the applicable code and have it reviewed by a qualified engineer.
- The formula assumes ideal conditions: no friction losses, no air resistance and no efficiency losses unless you enter them.