Calculate top of descent with the 3-to-1 rule, including target altitude, worked examples, wind, deceleration and vertical-speed checks.
To calculate top of descent with the 3-to-1 rule, subtract the target altitude from your present altitude, divide the height to lose by 1,000, then multiply by 3. The result is the approximate number of track nautical miles before the target fix at which to begin a steady descent.
In our Aviation & Real-World Flying guidance, we treat this as a quick planning estimate rather than an exact idle-descent prediction. Wind, speed changes, restrictions and aircraft energy management can move the practical descent point.
What is the top-of-descent formula?
The 3-to-1 formula assigns three nautical miles of distance for every 1,000 feet of altitude to lose.
TOD distance (NM) = (present altitude − target altitude) ÷ 1,000 × 3
- Choose the target fix and altitude. Use a published restriction, assigned level-off altitude or sensible approach-gate altitude.
- Calculate the height to lose. Subtract the target altitude from your present altitude.
- Convert it to thousands of feet. Divide that figure by 1,000.
- Multiply by three. Measure the resulting distance backwards along the planned route from the target fix.
- Add an energy allowance. Start earlier if the aircraft must slow substantially, faces a tailwind or cannot maintain the required path near idle thrust.
Worked top-of-descent example
From FL350 to a crossing altitude of 3,000 feet, the aircraft must lose 32,000 feet.
((35,000 − 3,000) ÷ 1,000) × 3 = 96 NM
The basic top of descent is therefore about 96 track nautical miles before the crossing fix. A transport jet may need additional distance for slowing and meeting terminal restrictions. Our practical Boeing 737 descent-planning walkthrough shows how that allowance fits into an approach.
Which target altitude should I use?
Use the altitude you must reach at a specific fix, not automatically the runway elevation.
- On a STAR or instrument approach, calculate to the relevant altitude constraint.
- Under ATC, use the assigned level-off altitude and the point by which you expect to reach it.
- For a visual arrival, use the circuit altitude or another sensible approach-gate altitude.
- If calculating towards an airport, account for airport elevation; do not assume every runway is at sea level.
Distance must be measured along the route. Straight-line distance to the airport will give a late descent when the arrival contains turns or vectors.
How do wind and deceleration change top of descent?
The geometric 3-to-1 distance does not change with wind, but the practical point for an idle or low-thrust descent often does.
- Tailwind: the aircraft covers more ground while descending, so begin earlier.
- Headwind: it covers less ground, so the practical descent point may be closer.
- Large speed reduction: add distance rather than expecting the aircraft to descend and decelerate sharply at the same time.
- Restrictions or level segments: include extra distance because a continuous 3-to-1 descent is no longer possible.
For rough simulator planning, 10–20 NM can be a useful initial deceleration allowance for a transport jet approaching a terminal area, but it is not a universal figure. Aircraft type, weight, speed, wind and drag configuration decide what is actually required. In an Airbus, compare the mental calculation with the managed profile described in our A320 top-of-descent and flight-management guidance.
What vertical speed maintains a 3-to-1 descent?
Multiply groundspeed by about five for a three-degree descent, or by 5.5 for the slightly steeper, exact 3-to-1 gradient.
At 420 knots groundspeed, that means roughly 2,100 feet per minute for three degrees or about 2,300 feet per minute for an exact 333-feet-per-mile profile. Use groundspeed, not indicated airspeed, and adjust the vertical speed as groundspeed changes. Our guide to reading groundspeed and the vertical-speed indicator explains the cockpit indications involved.
Why did the 3-to-1 rule leave me high or low?
A 3-to-1 calculation usually fails because the wrong altitude, distance or speed assumption was used.
- Using altitude above sea level without subtracting the target altitude.
- Measuring directly to the airport instead of along the programmed route.
- Forgetting deceleration, level-offs or altitude restrictions.
- Holding one vertical speed while groundspeed changes significantly.
- Following an inaccurate simulator flight-plan distance or receiving late vectors.
Recalculate during the descent: multiply the remaining thousands of feet by three and compare that result with the track miles remaining. If the required distance is greater than the available distance, the aircraft is high; if it is smaller, the aircraft is low. Correct early and within aircraft limits rather than forcing an unstable approach. The final approach path is a separate phase, covered in our three-degree landing and approach guidance.
In real-world flying, the rule never overrides ATC clearance, terrain clearance, published restrictions or the aircraft operator’s procedures. Use it as a mental cross-check against approved charts, avionics and performance data.