Top of Descent Calculator
Planning when to begin your descent is one of the most important elements of IFR and cross-country flight planning. Starting too late results in a rushed, steep descent; starting too early wastes fuel and altitude. This calculator computes the top of descent (TOD) distance in nautical miles from your current cruise altitude and target altitude at a chosen descent gradient. The standard 3-degree glidepath gradient of 318 ft/nm (equivalent to a 3:1 slope) is pre-filled and is the most common professional reference. You can also enter a custom gradient. The result tells you how many nautical miles from your target point you should begin the descent. Always add a buffer for speed reduction, ATC constraints, and wind.
Top of descent formula
Altitude to Lose (ft) = Current Altitude - Target Altitude
TOD Distance (nm) = Altitude to Lose / Descent Gradient (ft/nm)
The descent gradient in ft per nm is related to the descent angle by: ft/nm = 6,076.1 x tan(angle degrees). A 3-degree angle gives 6,076.1 x tan(3) = 318.3 ft/nm. A common pilot shortcut is to divide altitude to lose (in thousands of feet) by 1,000 then multiply by 3 to get nm (3 x altitude in thousands).
Common descent gradient reference values
- 3 degrees (standard instrument approach/jet descent): 318 ft/nm.
- 2.5 degrees: 265 ft/nm.
- 500 ft/min at 90 knots ground speed: approximately 333 ft/nm.
- 500 ft/min at 120 knots ground speed: 250 ft/nm.
- Always cross-check with your FMS or GPS distance to the fix when available.
Top of descent calculator: frequently asked questions
What is the top of descent?
The top of descent (TOD) is the geographic point at which an aircraft must begin its descent from cruise altitude to arrive at a lower altitude at a specific point, such as a fix or destination airport. Calculating TOD prevents arriving too high or too low.
What is a standard descent gradient?
The standard IFR descent gradient used in jet operations is 3 degrees, which corresponds to approximately 318 ft per nautical mile. A common rule of thumb is 3 times the altitude to lose in thousands of feet gives the distance in nautical miles (e.g., 10,000 ft to lose = 30 nm).
How do I calculate TOD distance?
TOD Distance (nm) = Altitude to Lose (ft) / Descent Gradient (ft/nm). For a 3-degree path: gradient = 318 ft/nm. To lose 15,000 ft at 318 ft/nm requires 47.2 nm. Add distance for speed deceleration and wind correction as needed.
How do I convert degrees to ft per nm?
Multiply the descent angle in degrees by 100.7 to get approximate feet per nautical mile. A 3-degree descent equals about 318 ft/nm (3 x 106.1). More precisely, ft/nm = 6076.1 x tan(angle in degrees).
Does wind affect top of descent?
Wind affects ground speed during descent, which changes how quickly you cover distance. A headwind means you cover fewer miles per minute, so you need to start descending later relative to time. This calculator computes distance needed; adjust for wind using your descent ground speed.
Official sources
- FAA Instrument Flying Handbook (FAA-H-8083-15B): faa.gov.
- FAA Aeronautical Information Manual (AIM), Chapter 5: faa.gov.
Reviewed by the CalculatorHub team, edited by James Graham, 15 June 2026. See our methodology.