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Top of Descent Calculator

Enter the altitude to lose, descent ratio and groundspeed to find your top-of-descent point.

How it is calculated

The distance needed to descend is TOD (NM) = altitude to lose (ft) ÷ 1000 × ratio. Altitude to lose is your current altitude minus the altitude you want at the target point; ratio is the nautical miles flown per 1000 ft lost (3 for the 3:1 rule).

If you also give a groundspeed in knots, the tool finds the time to cover that distance, time (min) = TOD ÷ groundspeed × 60, and the rate of descent ROD (ft/min) = altitude to lose ÷ time. This reduces to groundspeed × 1000 ÷ (60 × ratio), so it depends on groundspeed and ratio, not on the altitude. Without a groundspeed (or with zero), the rate line asks you to enter one and only the distance is shown. Altitude to lose and ratio must both be above zero, otherwise there is no result. Groundspeed is used as given, so wind is not corrected, and this is a rule of thumb, not a flight-planning tool.

Worked example

You are at 24,000 ft and want to be at sea level at the destination, so you must lose 24,000 ft. Use the 3:1 ratio and a groundspeed of 300 kt.

TOD = 24,000 ÷ 1000 × 3 = 72.0 NM before the destination. Time to fly 72 NM at 300 kt = 72 ÷ 300 × 60 = 14.4 minutes. Rate of descent = 24,000 ÷ 14.4 ≈ 1,667 ft/min.

Try this example →

Common mistakes

  • Entering the cruise altitude rather than the altitude to lose. If you want to be at 3,000 ft at the fix, enter 21,000, not 24,000.
  • Entering the ratio as degrees or feet per nautical mile. It is nautical miles per 1000 ft, so the 3:1 rule is 3.
  • Using true or indicated airspeed instead of groundspeed. A headwind lowers groundspeed and the required rate of descent; a tailwind raises it.
  • Expecting more altitude to give a higher rate of descent. At the same ratio and groundspeed the rate is the same; only the distance grows.
  • Using the result without allowing for speed reductions, restrictions or ATC clearances on the way down.

About this calculator

The top of descent (TOD) is the point in a flight where a pilot must begin descending to reach the target altitude smoothly by the time they arrive at their destination or a fix. It's commonly estimated with the 3:1 rule: distance in nautical miles = (altitude to lose in feet ÷ 1000) × 3 — so descending from 35,000 ft to sea level needs roughly 105 NM of descent distance.

Paired with a chosen groundspeed, the same numbers give the required rate of descent in feet per minute needed to fly that 3-degree-ish glide path. This rule-of-thumb math is standard practice for pilots doing quick mental flight planning and cross-checking what a flight management system (FMS) calculates automatically, especially in general aviation cockpits without an FMS.

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Comments (1)

  • Editorial team

    The rate shown is the average descent you need to hold from the moment you start down, so treat it as a cross-check against your vertical speed indicator, not a figure to fly in steps. At the 3:1 ratio it works out to about five and a half times the groundspeed in knots: 280 knots gives roughly 1,556 ft/min. Which groundspeed do you plan the descent with, and does it change the ratio you choose?

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