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 — not too early (wasting altitude) and not too late (arriving too high and too fast for a stabilised approach).

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 target vertical speed to set in the aircraft to maintain 3°.

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 descent 2–3 nm early is better than arriving high. ATC may give earlier descent clearance; accept it graciously and plan accordingly.