Top of Descent Calculator

The top of descent calculator works out how far before your target altitude to start coming down. Enter the current altitude, the altitude to reach and your ground speed in knots. The result gives the altitude to lose, the distance a descent needs, the rate of descent in feet per minute and the time in the descent.

Add a distance to a fix and the calculator shows when to start down and how far to fly before you do. If the fix is too close, it shows the steeper angle or faster rate needed from where you are. Choose a descent angle or a rate of descent, whichever suits your operation. The default angle is 3 degrees, the normal glide path of an ILS. The 3 to 1 rule and the ground speed times 5 rule appear beside the exact figures, so you can compare them.

How to use the top of descent calculator

1. Enter the Current altitude (ft), the altitude you fly now.
2. Enter the Altitude to reach (ft), the level you want to descend to, for example 3,000 ft.
3. Enter the Ground speed (kt) from your navigation display or flight plan. Wind changes this number, so update it when the wind changes.
4. Enter the Distance to the fix (optional, nm) if you have a fix to meet. Enter 0 to see only the distance the descent needs.
5. Choose how to plan the descent. Use the descent angle, which defaults to 3.0 degrees, or the rate of descent (ft/min).
6. Read the distance, the rate, the descent angle and the time in the descent. If you entered a fix distance, read the point to start descending and the distance to fly before you start down.
7. Read any warning. An angle above 3.5 degrees triggers a message that the angle is steeper than the 3 degree ILS glide path. Open the steps and the two tables to see your numbers worked out.

The preset buttons load four cases: 10,000 to 3,000 ft at 3 degrees, reaching 2,500 ft at a fix 30 nm away, a 500 ft/min descent, and a jet from 35,000 ft. Copy link saves your inputs in the address, Print / save result makes a sheet, and Reset restores the example.

How do you calculate top of descent?

Divide the altitude to lose by the tangent of the descent angle, then by the feet in a nautical mile, 6,076.1 ft. For 10,000 ft down to 3,000 ft at 3 degrees you lose 7,000 ft. Tan 3 degrees is 0.05241, so the distance is 7,000 / (0.05241 x 6,076) = 22.0 nm. At 120 kt the descent takes 22.0 / 120 x 60 = 11.0 minutes.

What is the 3 to 1 rule for descent?

The 3 to 1 rule says to start down 3 nm before the target altitude for each 1,000 ft to lose. It works as a quick estimate without a calculator. At 3 degrees the exact figure is 3.14 nm per 1,000 ft, so the rule slightly underestimates. For 10,000 ft the rule gives 30 nm and the exact figure is 31.4 nm.

What is the rate of descent for a 3 degree path?

At 3 degrees the rate is about 5.31 ft/min for each knot of ground speed. At 100 kt that is 531 ft/min, while the ground speed times 5 rule gives 500 ft/min. At 120 kt the rate is 637 ft/min. If you enter a rate instead, the tool works out the angle. A rate of 500 ft/min at 100 kt gives an angle of 2.83 degrees.

Why use ground speed, not airspeed, for the descent rate?

Ground speed sets how far you travel over the ground each minute, and the descent angle compares height lost with that distance. A wind that changes ground speed changes the rate needed, but it does not change the distance for a given angle. A jet from 35,000 ft to 10,000 ft at 3 degrees and 420 kt loses 25,000 ft over 78.5 nm, needs 2,229 ft/min and takes 11 minutes. The ground speed times 5 rule gives 2,100 ft/min there, so the rule falls further short as speed rises.

Sources

The formulas use tangent geometry. One nautical mile is 6,076.1 ft. The 3 degree glide path comes from FAA Aeronautical Information Manual paragraph 1-1-9, which says the ILS glide path projection angle is normally adjusted to 3 degrees above horizontal. Checked October 2026.

Questions from readers

How far out should I start my descent?

Start down at the distance the calculator shows for your altitude loss and angle. For 10,000 ft to 3,000 ft at 3 degrees and 120 kt, that is 22.0 nm before the target level.

What descent rate gives a 3 degree glide path at 100 knots?

The exact rate is 531 ft/min. The ground speed times 5 rule gives 500 ft/min, which is a little low.

Does wind change the top of descent?

Wind changes your ground speed, so it changes the rate of descent needed. For a given angle, the distance to descend does not change.

Can I use the 3 to 1 rule in a jet?

Yes, as a first estimate. The tables show the exact distance next to the 3 to 1 figure from 1,000 to 30,000 ft, and at 3 degrees the 3 to 1 figure runs about 5 percent short.

What if I am already too high?

The tool shows the descent angle and rate needed from your current position. An angle above 3.5 degrees triggers a warning that it is steeper than the 3 degree ILS glide path.

Does this replace the aircraft manual?

No. Check the aircraft manual and your operator’s procedures for the descent profile and limits that apply to your aircraft.

Related tools

• Wind navigation tools: use it to find the other calculators for wind, speed and heading in one place.
• E6B flight computer: use it to convert a gradient between feet per nautical mile, percent and degrees.
• Wind correction angle calculator: use it to find the ground speed that feeds the descent planning.
• True airspeed calculator: use it to convert indicated speed to true airspeed at altitude.
• Density altitude calculator: use it to check how hot and high conditions affect performance.
• Crosswind calculator: use it to split the wind into crosswind and headwind components on the runway.
• Flight time calculator: use it to work out the time for the whole leg after the descent.
• Zulu time converter: use it to convert the times on your plan to UTC.