Top of Descent Calculator
Find how far out to start down, how long the descent takes and the vertical speed to fly.
Where you want to level off, for example the approach or pattern altitude.
1,000 ft for a light-aircraft circuit, 1,500 ft for many jets, or the approach height.
Use the true airspeed and the wind component along your track. A headwind steepens the path over the ground, so you can start later.
3° is the normal path for airliners and instrument approaches.
For example 250 kt below 10,000 ft where that speed limit applies. Leave blank to keep one speed all the way down.
Level flight to lose speed. A rough guide is 1 nm for every 10 kt.
From your position to the airport or fix, to see when to start.
How your top of descent is worked out
Find the height to lose, pick a slope, then see how far and how fast.
- 1
Height to lose
35,000 − 3,000
= 32,000 ft - 2
Pick the slope
3.0° path
= 318 ft per nm - 3
Distance to descend
32,000 ÷ 318
no extras
→ 100.5 nm - 4
Time and rate
Takes 13 min 24 s
32,000 ÷ 13.4 min
= 2,388 fpm
Read more: the formula, the 3:1 rule and a worked example
The top of descent (T/D or TOD) is the point where you leave the cruise and start down, so that you arrive at your target altitude at the right place without diving or having to level off early. Start too late and you need a steep, fast descent. Start too early and you spend longer low down, where an engine burns more fuel.
The formula
height to lose = cruise altitude − target altitude
slope (ft per nm) = tan(descent angle) × 6,076 (3° ≈ 318 ft per nm)
descent distance (nm) = height to lose ÷ slope
top of descent = descent distance + slow-down distance + distance before the airport
time (minutes) = descent distance ÷ ground speed × 60
vertical speed (ft/min) = slope × ground speed ÷ 60
If you fly a set rate of descent instead, the time is the height to lose ÷ the rate, and the distance is the ground speed × that time ÷ 60. A gradient in percent is the height lost ÷ the distance flown × 100: 3° is about 5.2%.
Worked example
Cruising at 35,000 ft with a target of 3,000 ft, the height to lose is 35,000 − 3,000 = 32,000 ft. On a 3° path the slope is tan 3° × 6,076 = 0.05241 × 6,076 = 318.4 ft per nm. The descent takes 32,000 ÷ 318.4 = 100.5 nm, so start down about 100 nm before the point where you want to be at 3,000 ft.
At a ground speed of 450 kt that is 100.5 ÷ 450 × 60 = 13.4 minutes (13 min 24 s), and the vertical speed is 318.4 × 450 ÷ 60 = 2,388 ft/min. The 3:1 rule gives 32 × 3 = 96 nm, and the ground speed × 5 rule gives 450 × 5 = 2,250 ft/min.
Now slow to 250 kt below 10,000 ft and allow 10 nm of level flight to slow down. Above 10,000 ft: 25,000 ÷ 318.4 = 78.5 nm. Below: 7,000 ÷ 318.4 = 22.0 nm, at 318.4 × 250 ÷ 60 = 1,327 ft/min. With the 10 nm slow-down, the top of descent is 78.5 + 10 + 22.0 = 110.5 nm out, and the whole descent takes about 17½ minutes.
Rules of thumb
- 3:1 rule: allow 3 nm for every 1,000 ft to lose. It is a slope of 333 ft per nm, a little steeper than 3° (3.14°).
- Vertical speed: ground speed × 5 gives roughly the rate for a 3° path. The exact factor is 5.3, so at 450 kt it is 2,388 rather than 2,250 ft/min.
- Slowing down: a common planning figure for jets is about 1 nm of level flight for every 10 kt you need to lose. Speed brakes or a steeper idle descent can shorten this.
- Wind: a headwind cuts your ground speed, so the same rate of descent gives a steeper path and the descent covers less ground. A tailwind does the opposite: start earlier.
- Metric: 3° is about 52 m per km of distance, and 1 m/s is 197 ft/min.
Speed limits below 10,000 ft
Many countries set 250 kt as the top indicated airspeed below 10,000 ft, including the United States and much of Europe under SERA for VFR and some airspace classes. Rules and exemptions differ by country and airspace, so check your local publication. In this calculator you can enter any altitude and speed.
Units
1 nautical mile = 1,852 m = 6,076 ft. 1 kt = 1.852 km/h = 1.151 mph. 1 m/s = 196.85 ft/min.
For planning and education. Follow ATC, your aircraft's flight management system and your operator's procedures.
Common questions
How do you calculate top of descent?
Divide the height you need to lose by the slope of your descent path. On a 3° path the slope is about 318 ft per nm, so losing 32,000 ft takes about 100.5 nm. Then add any distance needed to slow down, and any distance before the airport where you want to reach your target altitude.
What is the 3 to 1 rule for descent?
Allow 3 nm for every 1,000 ft you need to lose. That is a slope of 333 ft per nm, a little steeper than 3°. Losing 32,000 ft by the 3:1 rule takes 96 nm, compared with 100.5 nm on an exact 3° path. The calculator shows both.
What rate of descent do I need for a 3 degree path?
Multiply the slope by your ground speed and divide by 60. A 3° path is about 318 ft per nm, so at 450 kt you need about 2,388 ft per minute. A quick rule of thumb is ground speed times 5, which gives 2,250 ft per minute at the same speed.
How does wind affect top of descent?
A headwind cuts your ground speed, so the same rate of descent gives a steeper path and the descent covers less ground. A tailwind does the opposite, so you need to start down earlier. Enter the ground speed you expect in the descent to see the effect on the distance and rate.
How much distance do I need to slow down before descending?
A common planning figure for jets is about 1 nm of level flight for every 10 kt you need to lose. Speed brakes or a steeper idle descent can shorten it. In the worked example, allowing 10 nm to slow to 250 kt puts the top of descent about 110.5 nm out.
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