How do you calculate top of descent?

Divide the altitude to lose by the height lost per nautical mile on your chosen path. On a 3-degree path that is 6,076 × tan(3°) = 318 ft per nautical mile, so 35,000 ft to lose needs about 110 nm. The rule of three, 3 nm per 1,000 ft, gives 105 nm because it is really a 3.14-degree path.

Why does the rule of three not match a 3-degree descent?

They are two different paths that happen to sound the same. Three nautical miles per 1,000 ft works out at 333 ft per nautical mile, which is an angle of 3.14 degrees. A true 3-degree path loses 318 ft per nautical mile. The rule of three is therefore slightly steeper and always gives a top of descent a few miles later than the geometry does. Over 35,000 ft the gap is about 5 nm.

What rate of descent do I need for a 3-degree path?

Roughly five times your groundspeed in knots, in feet per minute. At 300 kt that is 1,500 ft/min against a computed 1,592 ft/min, so the rule of thumb runs about 6 per cent shallow and you drift slightly high if you fly it exactly. Half the groundspeed times ten is the same sum said differently.

Should I use groundspeed or true airspeed?

Groundspeed, always. The descent path is fixed relative to the ground, so a tailwind stretches the distance you cover for the same rate of descent and a headwind shortens it. Using true airspeed in a 60-knot tailwind at 300 kt would put the top of descent about 20 nm early.

Why do I need extra miles beyond the calculated distance?

Because the calculation assumes a constant groundspeed and a clean aircraft flying straight to the level-off point. A real arrival slows down, which costs track miles, and speed reduction below about 250 kt in the terminal area typically needs a shallower segment or level flight. Adding 5 to 10 nm as a deceleration allowance is common practice, and the tool has an input for it.

Does this work for a non-precision or visual descent?

The geometry is the same for any straight path: it is trigonometry, not a procedure. What it does not do is respect step-down fixes, minimum sector altitudes, terrain or an approach's own vertical profile. It gives you the ideal path between two levels, which is a planning figure and a cross-check, never a substitute for the published procedure.

Descent geometry

Top of descent calculator.

Work out where to start down, the rate of descent it needs and the profile it flies, from the level you are leaving, the level you must be at and your groundspeed.

Top of descent

Enter the level you are leaving, the level you need to be at, and your groundspeed. Choose a path angle, or fix the rate of descent and let the tool work out the angle.

Descent mode

Cruise altitude (ft)Feet, not flight level: FL350 is 35000.

Level off at (ft)

Groundspeed (kt)Held constant through the descent.

Path angle (degrees)Three degrees is the standard approach and descent path.

Level or decel allowance (nm)Optional extra track miles for slowing down or a level segment.

Descent profile

Start down this far from the point you need to be level. The rules of thumb are shown next to the computed answer so you can see how close the mental maths gets.

Start down at

109.9 nm

Track distance to the level-off point

Rate of descent

1,592 ft/min

Rule of thumb: 5 x groundspeed = 1,500 ft/min

Time in the descent

22.0 min

Altitude to lose

35,000 ft

Path angle

3.00°

5.2% gradient, 318 ft per nm

Rule of three

105 nm

3 nm per 1000 ft, which is a 3.14 degree path

The dashed line is the cruise level. Where a deceleration allowance is set, the profile levels off early and the last miles are flown level at the target altitude, which is where a real arrival slows down. Distances are along the ground track, not slant range.

In the EASA ATPL exam

Descent geometry sits in ATPL General Navigation (subject 061) and Flight Planning and Monitoring (subject 033), and it turns up inside Performance questions as a hidden first step. Navigation asks it directly: given a level to lose and a groundspeed, find the distance or the rate. Flight Planning uses it inside longer questions about arrival planning and fuel to destination, where getting the descent point wrong changes the whole answer.

The examiner's favourite trick is mixing the units. Altitude comes in feet, distance in nautical miles, speed in knots and rate in feet per minute, and the conversion between them is the definition of the nautical mile: 6,076 ft. Any question that gives you three of the four is testing whether you can move between them without a calculator.

Exam-style example

You are at FL350 and need to be level at 3,000 ft at a waypoint. Groundspeed is 420 kt and you plan a 3-degree path. How far out do you start down, and at what rate?

Altitude to lose: 35,000 − 3,000 = 32,000 ft.
Distance: 32,000 ÷ 318 ft per nm = 100.6 nm. The rule of three gives 32 × 3 = 96 nm.
Rate: 420 × 5 = 2,100 ft/min from the rule of thumb, or 420 × tan(3°) × 101.27 = 2,229 ft/min exactly.
Time: 32,000 ÷ 2,229 = 14.4 minutes.

Start down about 101 nm from the waypoint at roughly 2,200 ft/min. In the exam the rule-of-three answer of 96 nm is usually the one in the options, so read what the question asks for: the geometric path or the mental-maths approximation.

Common trap: Flight levels are not feet, and the level you are at is not the altitude you have to lose. FL350 to 3,000 ft is 32,000 ft of descent, not 35,000. Sitting under that is the second trap: at 420 kt groundspeed the rule of three and the true 3-degree path differ by about 4.5 nm, which is nearly 40 seconds of flying, so the two answers are far enough apart that the exam can offer both.

ATPL General Navigation guide True airspeed calculator E6B flight computer

How top of descent is calculated

The definition, the formula, a worked example and how it shows up in the ATPL exams.

The geometry, in one paragraph

A descent path is a right-angled triangle. The altitude to lose is the vertical side, the ground distance is the horizontal side, and the path angle is the angle between the hypotenuse and the ground. So the ground distance is the altitude to lose divided by the tangent of the angle, once the feet have been turned into nautical miles.

One nautical mile is 1,852 metres by definition, which is 6,076.1 ft. Multiply that by the tangent of the angle and you get the height lost per nautical mile: 318 ft for 3 degrees, 212 ft for 2 degrees, 425 ft for 4 degrees. Divide the altitude to lose by that figure and you have the distance.

Why pilots use the rule of three instead

Nobody computes a tangent in the cruise. The rule of three says three nautical miles for every 1,000 ft to lose: 30,000 ft to lose, 90 nm to run. It is quick, it is close, and it is deliberately a shade steep, which is the safe direction to be wrong in because it puts you slightly high and a slightly high aircraft can always descend faster.

The tool shows both figures side by side so you can see the size of the gap. At 10,000 ft to lose it is 1.4 nm. At 35,000 ft it is 5 nm. Neither matters much when you have a descent profile in front of you, and both matter in an exam that offers you two answers four miles apart.

The rate of descent, and the five-times rule

Once the angle is fixed, the rate follows from the groundspeed: the faster you go along the path, the faster you come down it. In feet per minute, the rate is groundspeed × tan(angle) × 101.27. For 3 degrees that reduces to about 5.3 times groundspeed, which is where the mental rule of five times groundspeed comes from.

The rule is about 6 per cent shallow, so flying it exactly leaves you very slightly above the path. That is the intended direction of the error. Fixing the rate instead of the angle is the other way round to work the problem, and this tool does both: give it a rate of descent and it tells you the angle and the distance that rate implies.

What this does not model

Groundspeed is held constant. A real descent decelerates, and a real arrival adds track miles for speed control, so the distance is a planning figure, not a clearance. The deceleration allowance input is there to add those miles back explicitly rather than pretending they do not exist.

It also has no idea about terrain, minimum sector altitudes, step-down fixes, an approach procedure's own profile, or an aircraft's idle-thrust performance. A jet's idle-path descent is computed by the flight management system against real aircraft data and real wind; this is the geometry that sits underneath it.

Good to know

Background and tips for getting the most out of this calculator.

Groundspeed, not airspeed

The path is fixed to the ground, so a tailwind stretches the distance covered for a given rate of descent. Feed the calculator groundspeed and the wind is already in the answer.

Two ways round the problem

Fix the angle and read the rate, or fix the rate and read the angle. Both give the same distance for the same path, and comparing them is the fastest way to see whether a rate you were given is realistic.

Add miles for slowing down

A real arrival needs track miles to decelerate. The allowance input adds them to the top of descent without touching the geometry, so you can see the two parts separately.

Frequently asked questions

Tap to read the full background and answers.

How do you calculate top of descent?: Divide the altitude to lose by the height lost per nautical mile on your chosen path. On a 3-degree path that is 6,076 × tan(3°) = 318 ft per nautical mile, so 35,000 ft to lose needs about 110 nm. The rule of three, 3 nm per 1,000 ft, gives 105 nm because it is really a 3.14-degree path.
Why does the rule of three not match a 3-degree descent?: They are two different paths that happen to sound the same. Three nautical miles per 1,000 ft works out at 333 ft per nautical mile, which is an angle of 3.14 degrees. A true 3-degree path loses 318 ft per nautical mile. The rule of three is therefore slightly steeper and always gives a top of descent a few miles later than the geometry does. Over 35,000 ft the gap is about 5 nm.
What rate of descent do I need for a 3-degree path?: Roughly five times your groundspeed in knots, in feet per minute. At 300 kt that is 1,500 ft/min against a computed 1,592 ft/min, so the rule of thumb runs about 6 per cent shallow and you drift slightly high if you fly it exactly. Half the groundspeed times ten is the same sum said differently.
Should I use groundspeed or true airspeed?: Groundspeed, always. The descent path is fixed relative to the ground, so a tailwind stretches the distance you cover for the same rate of descent and a headwind shortens it. Using true airspeed in a 60-knot tailwind at 300 kt would put the top of descent about 20 nm early.
Why do I need extra miles beyond the calculated distance?: Because the calculation assumes a constant groundspeed and a clean aircraft flying straight to the level-off point. A real arrival slows down, which costs track miles, and speed reduction below about 250 kt in the terminal area typically needs a shallower segment or level flight. Adding 5 to 10 nm as a deceleration allowance is common practice, and the tool has an input for it.
Does this work for a non-precision or visual descent?: The geometry is the same for any straight path: it is trigonometry, not a procedure. What it does not do is respect step-down fixes, minimum sector altitudes, terrain or an approach's own vertical profile. It gives you the ideal path between two levels, which is a planning figure and a cross-check, never a substitute for the published procedure.

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