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top of descent calculator

Where to leave the cruise, how fast to come down, and whether that crossing restriction is actually achievable.

Not true airspeed.

3° is the standard profile.

A descent is a distance problem, so it runs on groundspeed: a tailwind pushes the top of descent further back, a headwind lets you leave it later.

Start down at

100.5 NM

before the target

Descent rate

2,388 fpm

318 ft per NM

Time

13.4 min

from top of descent

35,000 ft3,000 ft3:1 ruletop of descenttarget3.0°

Losing 32,000 ft. The 3:1 rule of thumb would say 96 NM, and groundspeed × 5 would say 2,250 fpm: close enough in the cockpit, and a useful check that you have not typed something wrong.

How the answer is worked out

Distance = altitude to lose ÷ (tan of the angle × 6,076.12 ft per NM). At three degrees that gradient is 318.4 ft per NM, which is where the 3:1 rule comes from: three miles buys you a thousand feet, near enough. Rate = groundspeed × 101.27 × tan of the angle, and at three degrees that reduces to roughly groundspeed × 5.3, which is why the mental version is groundspeed × 5. Notice the distance does not depend on speed but the rate does, so a descent planned at cruise speed goes wrong the moment you are asked to slow down.

For training and reference only: always fly the profile your aircraft, its performance and your clearance allow.

A descent is a distance problem before it is an altitude problem. You need to know how far out to start down, what rate that demands, and whether the aircraft can do it comfortably. This calculator answers all three, and works backwards from a crossing restriction when ATC gives you one.

Key takeaways

  • The 3:1 rule is three miles per thousand feet, and approximates a three degree path closely enough to plan with.
  • Groundspeed × 5 gives the rate of descent for a three degree path: 120 knots needs about 600 feet per minute.
  • Descents run on groundspeed, so a tailwind means starting down earlier.
  • Anything beyond roughly 1,000 feet per minute is a rushed descent in a light aircraft. Ask for track miles rather than diving.
  • Plan the descent before you need it. Late descents are where mistakes cluster.

Working out where to start down

Take the altitude you need to lose and divide it by the gradient you intend to fly. At the standard three degrees, you lose roughly 320 feet per nautical mile, which is where the 3:1 rule comes from: three miles buys you a thousand feet.

From 35,000 feet down to sea level, the rule gives 105 miles and the exact three degree geometry gives about 110. That difference does not matter in the cruise, which is the point of a rule of thumb: it is quick, it is close, and it is easy to check.

The rate that goes with it

Distance tells you where to start; rate tells you how to fly it. For a three degree path, multiply groundspeed by five. The exact relationship is groundspeed multiplied by 101.3 and by the tangent of the angle, which the calculator uses, but the mental version is what you will actually reach for descending through 8,000 feet with a controller talking.

Notice that the rate depends on speed while the distance does not. Slow down and the same geometry needs a gentler rate. Speed up and it needs a steeper one. This is why a descent planned at cruise speed goes wrong if you are then asked to slow down early.

Crossing restrictions, and when to say no

“Cross twenty miles east of the field at 5,000” is the same problem inverted: the distance is fixed and the rate is what falls out. The second tab does this, and tells you when the answer is unreasonable.

That judgement matters. A restriction needing 3,000 feet per minute is arithmetically fine and operationally poor, particularly in a piston aircraft where a fast descent at low power risks shock cooling. The professional response is to ask for track miles, an earlier descent, or to advise you are unable. All three are far easier said before you have started down than after.

Frequently asked questions

How do you calculate top of descent?

Divide the altitude you need to lose by the descent gradient. The standard rule of thumb is 3:1, three nautical miles for every thousand feet, so 30,000 feet to lose needs about 90 miles. This calculator does it exactly from your chosen flight path angle and groundspeed, and shows the rule of thumb alongside for comparison.

What is the 3 to 1 rule in aviation?

The 3:1 rule says you need three nautical miles of track distance for every thousand feet of altitude to lose. It approximates a three degree descent path, which is the standard profile for both instrument approaches and normal descents. It is deliberately simple so it can be done in your head.

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

Multiply your groundspeed in knots by five. At 120 knots groundspeed that is 600 feet per minute. The exact figure is groundspeed times 101.3 times the tangent of the angle, which for three degrees works out slightly higher than the rule of thumb, but times five is close enough to fly.

Does wind affect the top of descent?

Yes, through groundspeed. A descent is a distance problem, so a tailwind means you cover the ground faster and must start down earlier, while a headwind lets you leave it later. Always plan the descent on groundspeed rather than true airspeed.

What is a reasonable rate of descent?

For most light aircraft, 500 to 800 feet per minute is comfortable and keeps passengers and engines happy. Beyond about 1,000 feet per minute you are into a rushed descent, and in a piston aircraft rapid descents at low power risk shock cooling. If a restriction demands more, ask for track miles or an earlier descent instead.

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