Top of Descent Calculator

What is Top of Descent (TOD)?

The Top of Descent is the geographic point at which you begin descending from cruise altitude to reach your target altitude (typically circuit height or approach altitude) at the right moment. Too early wastes altitude and burns fuel down low. Too late leaves you arriving high and fast, which is where stabilised approaches go to die.

The standard 3-degree descent profile

Commercial aviation uses a standard 3° instrument approach glide path for most arrivals. The 3° glide path has a convenient rule of thumb:

Altitude to lose (ft) ÷ 1,000 × 3 = Distance from destination (nm)

For example, descending from 8,000 ft to circuit height of 1,000 ft means losing 7,000 ft. TOD = 7,000 ÷ 1,000 × 3 = 21 nm from destination.

Required rate of descent (ROD)

For a 3° descent at a given ground speed:

ROD (ft/min) ≈ Ground Speed (knots) × 5

For example, at 120 knots ground speed: ROD = 600 ft/min. At 180 knots: ROD = 900 ft/min.

This is the vertical speed most pilots dial in, and it is close enough to three degrees for the mental arithmetic it is meant for.

The two rules of thumb do not quite agree, and it is worth knowing why

Both rules above are approximations of the same 3° path, and they round in opposite directions.

Three nautical miles per 1,000 ft is a gradient of 1,000 ÷ (3 × 6,076), which works out to a path of 3.14°, slightly steeper than three degrees. Ground speed × 5 gives a rate that flies a path of about 2.83°, slightly shallower.

Put them together on one descent and they disagree. Losing 7,000 ft at 120 knots, the distance rule puts TOD 21 nm out, and 21 nm at 120 knots takes 10.5 minutes. But 7,000 ft at the rule-of-thumb 600 ft/min takes 11.7 minutes, so you would still be 700 ft high crossing the field. The rate that actually flies 21 nm is 667 ft/min, which is ground speed × 5.6.

The calculator below reports both: the planning distance from the 3 nm rule, and the rate of descent that genuinely gets you down in that distance. Use ground speed × 5 as the mental check in the cockpit and expect it to need a couple of extra miles.

Practical application

IFR pilots plan TOD before departure. The calculation accounts for:

• Cruise altitude and target altitude
• Ground speed (factoring in expected wind at cruise)
• Approach or traffic pattern altitude at destination

Add a small buffer. Starting the descent 2 or 3 nm early is much better than arriving high. ATC may give earlier descent clearance; accept it graciously and plan accordingly.