What this calculator does
Descent planning is one subtraction and one division, and the reason it is worth doing properly is that the arithmetic runs away from you quickly at altitude. Coming down 32,000 feet at three degrees needs just over a hundred nautical miles, which is further out than most people guess and a long way beyond where it starts to feel urgent.
The three-times rule is the mental version: altitude to lose in thousands of feet, multiplied by three, gives the miles. It gives 96 against the 100.5 the geometry produces, which is close enough to fly and deliberately a little tight.
The formula
A descent angle becomes a gradient in feet per nautical mile through tan(angle) × 6076.12, since there are 6,076.12 feet in a nautical mile. Dividing the altitude to lose by that gradient gives the distance. The rate of descent follows from how fast you are covering those miles: gradient × groundspeed ÷ 60 converts feet per mile into feet per minute.
| Term | Meaning |
|---|---|
| Descent angle | The flight path angle. Three degrees is the standard approach slope and a common cruise descent target. |
| Gradient | Feet lost per nautical mile. Three degrees is about 318 ft/nm. |
| Three-times rule | Thousands of feet to lose, times three, gives miles. A mental approximation of a three degree path. |
| Rate of descent | Feet per minute, which depends on groundspeed as well as on the angle. |
The inputs explained
| Field | What to enter |
|---|---|
| Cruise altitude (ft) | Current or planned cruise altitude. |
| Altitude to be level at (ft) | The altitude you need to be level at, which may be a crossing restriction rather than the airfield elevation. |
| Groundspeed in the descent (kt) | Groundspeed during the descent. It is rarely constant, so use a sensible average. |
| Descent angle (°) | Descent angle in degrees. Three is standard; two to two and a half is gentler and starts further out. |
When to use it
Planning a cruise descent
Enter cruise and the altitude you must be level at. The distance is where to start, and the rate of descent tells you whether that is comfortable or whether you need to begin earlier.
Meeting a crossing restriction
Set the target to the restriction altitude rather than the field elevation. Crossing restrictions are where descents go wrong, because the planning was done against the destination instead.
Checking the mental rule
Compare the three-times figure against the geometry. It runs about five per cent tight at three degrees, which is the right direction for a rule of thumb to err in.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
How far out should you start down?
Only the cruise altitude changes.
| Cruise altitude | Start down this far out | Rate of descent needed | By the three-times rule |
|---|---|---|---|
| 10,000 ft | 22.0 nm | 1,486 fpm | 21.0 nm |
| 20,000 ft | 53.4 nm | 1,486 fpm | 51.0 nm |
| 30,000 ft | 84.8 nm | 1,486 fpm | 81.0 nm |
| 35,000 ft | 100.5 nm | 1,486 fpm | 96.0 nm |
| 41,000 ft | 119.3 nm | 1,486 fpm | 114.0 nm |
What does the descent angle change?
The altitude to lose is fixed and only the angle changes.
| Descent angle | Start down this far out | Rate of descent needed | Descent gradient |
|---|---|---|---|
| 1.5° | 201.1 nm | 743 fpm | 159 ft/nm |
| 2° | 150.8 nm | 990 fpm | 212 ft/nm |
| 2.5° | 120.6 nm | 1,238 fpm | 265 ft/nm |
| 3° | 100.5 nm | 1,486 fpm | 318 ft/nm |
| 4° | 75.3 nm | 1,983 fpm | 425 ft/nm |
Questions
Why is the three-times rule three?
Because a three degree path is about 318 feet per nautical mile, and dividing a thousand feet by that gives roughly 3.1 miles. Rounding to three keeps the mental arithmetic easy and leaves you slightly high, which is the safer error.
Does the rate of descent depend on altitude?
No. It depends on the angle and the groundspeed only. What changes with altitude is the distance, and the fact that true airspeed and therefore groundspeed are usually higher up high.
Should I use groundspeed or airspeed?
Groundspeed, because the distance over the ground is what you are planning. A tailwind in the descent pushes the top of descent further out and raises the rate of descent needed.
How do I handle a speed reduction in the descent?
Allow extra distance. Slowing down takes track miles that this calculation does not include, and a descent planned exactly will usually leave you high once you start decelerating.
Is this good enough for real planning?
As a sanity check, yes. It is still-air geometry and does not know about wind, ATC, published crossing altitudes or the aircraft, so treat it as the starting point rather than the plan.
For the climb side, see climb gradient. For the wind that moves your groundspeed there is wind correction angle, and fuel burn endurance and range covers how far the fuel goes.