How is top-of-descent distance calculated?

The descent angle is converted to a gradient in feet per nautical mile using the tangent of the angle times 6076, the number of feet in a nautical mile. Dividing the altitude to lose by that gradient gives the distance before the target to begin descending.

How does this differ from the 3:1 rule?

The 3:1 rule is a fixed shortcut for a 3-degree path that loses 300 feet per mile. This tool accepts any angle, so you can plan steeper or shallower descents and see the exact gradient and vertical speed for that angle.

What vertical speed should I fly?

The tool shows the vertical speed in feet per minute for your groundspeed and angle, computed as groundspeed times 101.27 times the tangent of the angle. Holding that vertical speed keeps you on the planned path to the target.

Should I use true airspeed or groundspeed?

Use groundspeed, because the descent distance and time depend on how fast you cover the ground. A tailwind increases groundspeed, moving the top of descent farther out and raising the vertical speed needed for the same angle.

Why add margin to the calculated distance?

The calculation assumes a constant-speed descent at the chosen angle. If you must slow down or configure flaps and gear before the target, begin a few miles early so deceleration does not flatten your path and overshoot the altitude.

What is the Top of Descent (TOD) Calculator?

Solve for top-of-descent distance, vertical speed, and time from cruise altitude, target altitude, groundspeed, and your chosen descent angle. Airline and IFR general-aviation pilots use it for precise descent planning. Runs in your browser. It runs free in your browser on Gera Tools, with nothing uploaded.

Top of Descent (TOD) Calculator

Name: Top of Descent (TOD) Calculator
Creator: Gera Tools
License: https://creativecommons.org/licenses/by/4.0/

Knowing exactly where to start down lets you fly an efficient, idle descent that arrives level at a crossing restriction or approach altitude. This calculator solves for the top-of-descent point, vertical speed, and time for any descent angle you choose.

How it works

The descent angle sets a gradient, and the distance and vertical speed follow:

altitude to lose   = cruise altitude − target altitude
gradient (ft/nm)   = tan(angle) × 6076.12
TOD distance (nm)  = altitude to lose / gradient
vertical speed     = groundspeed × 101.27 × tan(angle)   ft/min
time to descend    = TOD distance / groundspeed × 60      minutes

A shallower angle pushes the top of descent farther out and lowers the vertical speed; a steeper angle does the opposite.

Worked example

Descending from 33,000 ft to 4,000 ft at 3 degrees with a 420 knot groundspeed:

Altitude to lose: 29,000 ft
Gradient: tan(3°) × 6076.12 ≈ 318 ft per nautical mile
TOD distance: 29,000 / 318 ≈ 91 nm before the target
Vertical speed: 420 × 101.27 × tan(3°) ≈ 2,240 ft/min
Time to descend: 91 / 420 × 60 ≈ 13 minutes

Start a little earlier if you must decelerate before the target. Use groundspeed rather than airspeed so a headwind or tailwind is correctly reflected in both distance and vertical speed.

Choosing the right descent angle

The standard commercial descent path is 3 degrees, which matches a typical ILS glidepath and produces a comfortable, engine-near-idle descent. However, the appropriate angle depends on several factors:

Clearances. ATC-assigned crossing restrictions may force a steeper angle if you receive a late descent clearance. Conversely, a very early clearance on a long descent may allow an unusually shallow 2-degree path.
Aircraft performance. Business jets and GA aircraft may fly slightly steeper or shallower paths than heavy transport-category aircraft. Check your aircraft’s normal procedures for the recommended descent gradient.
Energy management. A 3-degree descent at cruise speed generates significant airspeed buildup. Plan for an idle descent with speed brake availability, and factor in when you need to slow to speed restrictions — typically 250 KIAS below 10,000 ft. These deceleration segments flatten the effective path, so the calculated TOD distance should have a few miles of margin built in.

The 3:1 rule vs this calculator

The 3:1 rule (three nautical miles per thousand feet of altitude) is a quick mental shortcut pilots use without a calculator. It is a reasonable approximation for a 3-degree path at typical jet groundspeeds. However, it can understate the required distance when groundspeed is high or when the path is shallower than 3 degrees. This calculator uses the full trigonometric formula, which accounts for any angle and is accurate across the full range of groundspeeds and descent angles — making it more useful for non-standard profiles or precise planning.

Common mistakes

Using indicated airspeed instead of groundspeed. Vertical speed and distance depend on groundspeed over the ground, not airspeed through the air. At cruise altitudes, the two can differ by 80 knots or more.
Ignoring deceleration. The calculator assumes a constant-speed, constant-angle descent. In practice, slowing from cruise Mach to clean descent speed and then to 250 KIAS below 10,000 ft each add distance to the actual top of descent. Add 3 to 7 nautical miles of buffer depending on aircraft type.
Not accounting for winds. Input the forecast groundspeed for the descent phase, not the cruise groundspeed, if winds change significantly with altitude.