Top of descent calculator

What this calculator shows

Top of descent is the approximate point where a descent can begin to reach a lower target altitude. This calculator uses the common three-to-one planning rule, estimating three nautical miles of horizontal distance for each 1,000 feet of altitude to lose.

It also estimates time to cover that distance at the entered ground speed and the average vertical speed implied by those values. The result is a planning estimate, not a clearance or an instruction to begin a descent.

How to use the calculator

Enter current and target altitudes in feet MSL, then the planned ground speed in knots. The target altitude must be below the current altitude. The calculator mirrors these values in the page URL.

Use ground speed for the planned descent segment. The calculation assumes that speed remains steady from the top of descent to the target altitude.

How the calculation works

First, the calculator finds the altitude to lose. It then applies the three-to-one rule and derives time and average vertical speed:

altitude to lose = current altitude - target altitude

descent distance = altitude to lose / 1,000 x 3 NM

time minutes = distance / ground speed x 60

average descent rate = altitude to lose / time minutes

Example calculation

For a descent from 9,500 ft to 2,500 ft at 120 kt ground speed:

1. Altitude to lose: 9,500 - 2,500 = 7,000 ft.
2. Estimated distance: 7 x 3 = 21 NM.
3. Time at 120 kt: 21 / 120 x 60 = 10.5 min.
4. Average vertical speed: 7,000 / 10.5, or about 667 ft/min.

Assumptions and limitations

The three-to-one rule is a rule of thumb, not a procedure or a guaranteed descent profile. Actual descent planning depends on aircraft performance, speed changes, wind, terrain, airspace, traffic, ATC instructions, and required level-offs.

This result does not include deceleration, configuration changes, a level segment, or a buffer for constraints. Follow clearances, published procedures, instructor guidance, and the aircraft's POH or AFM. Do not use the estimate by itself to make an operational decision.

Sources and methodology

The calculator implements the three-to-one descent planning rule as an approximation and uses elementary distance, speed, time, and rate arithmetic. The FAA Pilot's Handbook of Aeronautical Knowledge and Airplane Flying Handbook provide broader flight planning and descent context. The current POH or AFM and applicable clearances take precedence.

Related tools and resources

• Time, speed, and distance calculator
• True airspeed calculator