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How to calculate top of descent: the 3 to 1 rule
Plan your descent so you arrive at the right height and speed. The 3 to 1 rule gives a distance in seconds, and the planner does it for your route.
The rule
A standard three-degree descent covers about 3 nautical miles for every 1,000 feet you need to lose. Multiply the height to lose, in thousands of feet, by 3. From FL350 to a sea-level airport that is 35 x 3 = 105 nautical miles before the airport.
Adjust for slowing down
Add about 10 miles if you must slow from cruise speed to 250 knots below 10,000 feet, and more for headwinds or tailwinds. Subtract the airport elevation from your cruise altitude before you multiply.
Descent rate
To hold the three-degree path, a quick estimate is descent rate in feet per minute equal to ground speed times five. At 300 knots that is about 1,500 feet per minute.
Let the planner do it
The route planner shows the top of descent distance from your destination, the descent time, and a typical rate for the cruise level you choose. Import your SimBrief plan and it uses your own route.
Questions
What is TOD?
Top of descent is the point where you leave cruise altitude to start the descent.
Is the 3 to 1 rule exact?
No. It is a planning rule of thumb. Winds, speed changes and the arrival procedure all move the real point.
More tools and guides
- Takeoff speedsV1, VR, V2, trim, flex
- Landing speedsVref, Vapp, distance
- Runway visualizerWatch the roll
- 737 FMCRoute, legs, performance
- Route plannerDistance, TOD, fuel
- AirportsRunways for 6,000+ airports
For flight simulation use only. The results come from a simplified physics model, not from manufacturer data, and must never be used for real-world flight planning.