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Top of Descent Calculator
Find where to begin your descent. Enter the cruise and target altitudes, the ground speed and either the descent rate or the path angle, and get the distance from the fix to start down, the vertical speed for a 3° path, the time it takes and a profile with checkpoints.
Last updated:
Altitudes
ft
ft
e.g. pattern altitude or a crossing restriction
kt
nm
e.g. 5 nm before the airport to enter the pattern; 0 for at the fix
Descent planned by
ft/min
°
3° is the standard ILS-like path
nm
Tells you how far until you should start down
Enter your values to see the result.
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How to use the Top of Descent Calculator
- Enter the cruise and target altitudes in feet.
- Enter the ground speed you expect in the descent.
- Choose descent rate or angle and enter the value (for example 500 ft/min or 3°).
- Add a level-off distance before the fix and your current distance to the fix if you want.
- Read the top of descent distance, then use the checkpoints to monitor the descent.
Top of descent formulas
- Altitude to lose: Δh = cruise altitude − target altitude.
- By descent rate: time (min) = Δh ÷ fpm, distance (nm) = GS × time ÷ 60.
- By angle: distance (nm) = Δh ÷ (6,076 × tan θ), required fpm = GS × 101.27 × tan θ (101.27 = 6,076 ft ÷ 60 min).
- Top of descent from the fix = descent distance + any distance you want to be level before the fix.
Worked example: cruise 9,500 ft, pattern altitude 2,500 ft, 140 kt ground speed, 500 ft/min. Δh = 7,000 ft, time = 14 min, distance = 140 × 14 ÷ 60 = 32.7 nm. To be at pattern altitude 5 nm out, start down 37.7 nm from the airport. That descent is a shallow 2.0° path.
Rules of thumb
| Path angle | Feet per nm | nm per 1,000 ft | fpm at 120 kt | fpm at 250 kt |
|---|---|---|---|---|
| 2.0° | 212 | 4.7 | 424 | 884 |
| 2.5° | 265 | 3.8 | 531 | 1,105 |
| 3.0° | 318 | 3.1 | 637 | 1,327 |
| 3.5° | 372 | 2.7 | 743 | 1,548 |
| 4.0° | 425 | 2.4 | 850 | 1,770 |
Descent planning tips
- Plan for slowing down. Faster aircraft cannot descend and slow down at the same time easily; add a few miles for each speed reduction.
- Check the winds. Use the ground speed you expect in the descent; the E6B flight computer gives it from TAS and winds.
- Monitor with checkpoints. Compare your altitude with the checkpoint table; if you are high, increase the rate, and if low, level off briefly.
- Keep passengers comfortable. About 500 ft/min is a common target in unpressurized aircraft.
Frequently asked questions
How do you calculate top of descent?
Divide the altitude to lose by the descent rate to get the time, then multiply the time by the ground speed to get the distance. For example, losing 7,000 ft at 500 ft/min takes 14 minutes; at 140 knots that is 140 × 14 / 60 = 32.7 nm. Add any distance you want to be level before the fix.
What is the 3-to-1 rule?
Plan 3 nautical miles of descent for every 1,000 feet of altitude to lose. It approximates a 3° descent path. Losing 10,000 ft needs about 30 nm. Many pilots add a few miles for slowing down.
What descent rate gives a 3-degree path?
Multiply the ground speed in knots by about 5 (more precisely 5.3). At 120 knots a 3° path needs about 640 ft/min, and at 90 knots about 480 ft/min. The exact formula is ft/min = GS × 101.27 × tan(angle).
What is a comfortable descent rate for passengers?
In unpressurized light aircraft many pilots use about 500 ft/min to keep ears comfortable. Faster aircraft and pressurized cabins commonly descend at 1,000–3,000 ft/min because the cabin altitude changes more slowly than the aircraft altitude.
Why use ground speed instead of airspeed?
Distance over the ground depends on ground speed. A tailwind in the descent increases ground speed, so you cover more distance per minute and must start down earlier or descend faster.
How do I convert a descent angle to feet per nautical mile?
Feet per nm = 6,076 × tan(angle). A 3° path is about 318 ft per nm, 2.5° is about 265 ft per nm and 4° about 425 ft per nm.
Are all the tools on FreeToolCabin tested for quality and accuracy?
Yes! Every tool on our site undergoes automated Q/A testing in real browser environments to ensure high precision, correct output, and zero errors. If you happen to spot a bug or have a suggestion, please contact us at [email protected] and our team will review it right away.
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