Top of Descent Calculator
Calculate when to start your descent, required descent rate, and time to descend for a smooth, constant-angle approach.
Inputs
ft MSL
ft MSL
ft AGL
kt
°
Standard: 3°, CDFA: 3°, Visual steep: 4–5°
Results
Start Descent At
20.4 NM from destination
Required Descent Rate
637 fpm
Descent Time10.2 min
Altitude to Lose6,500 ft
Rule of Thumb (×3)20 NM
Rule of Thumb (GS×5)600 fpm
How to Use This Calculator
- Enter your cruise altitude and destination field elevation.
- Set target altitude (pattern altitude or approach altitude AGL).
- Enter your descent ground speed.
- Use 3° for standard or adjust for steep approaches.
- Begin descent at the indicated distance from the destination.
How the result changes with Cruise Altitude
| Cruise Altitude | Start Descent At | Required Descent Rate |
|---|---|---|
| 6,500 | 15.7 NM from destination | 584 fpm |
| 10,000 | 24.5 NM from destination | 743 fpm |
| 35,000 | 98.9 NM from destination | 1,592 fpm |
What each input means
- Cruise Altitude
- Your current cruise altitude above mean sea level.
- Destination Elevation
- Destination airport field elevation.
- Pattern / Target Altitude
- Traffic pattern altitude AGL (typically 1,000 ft) or approach altitude.
- Ground Speed
- Current or expected descent ground speed.
- Descent Angle
- Standard glidepath is 3°. Steeper angles for obstacle clearance or noise abatement.
What each result means
- Rule of Thumb (×3)
- Quick estimate: altitude to lose in thousands × 3.
- Rule of Thumb (GS×5)
- Quick estimate for 3° path: ground speed × 5.
How this is calculated
Worked example, using the default values
- Altitude to LoseΔAlt = Cruise Altitude − Field Elevation − Pattern Altitude8,000 − 500 − 1,000 = 6,500 ft
- Top of Descent DistanceDistance = Altitude ÷ tan(Descent Angle) ÷ 60766,500 ÷ tan(3°) ÷ 6076 = 20.4 NM from destination
- Required Descent RateFPM = Ground Speed × ft-per-NM ÷ 60120 × 318 ÷ 60 = 637 fpm
- Descent TimeTime = Distance ÷ Ground Speed × 6020.4 ÷ 120 × 60 = 10.2 minutes
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