Calculators › Descent calculator
Descent calculator
When to start down, how fast to descend, and the height to expect on a glidepath.
Plan a descent
Glidepath check
The height to expect at each distance from the threshold.
The sums
A descent path is a right-angled triangle. On a 3° path, each nautical mile covers 6,076 ft forward and tan(3°) × 6,076 = 318 ft down. From that:
- Distance = altitude to lose ÷ feet per NM
- Rate of descent = groundspeed × feet per NM ÷ 60
The rules of thumb
Pilots and controllers use two shortcuts for a 3° path, and the calculator shows both next to the exact answer:
- The 3× rule: thousands of feet to lose × 3 = miles needed. Losing 10,000 ft takes about 30 NM.
- 5 × groundspeed: at 140 kt, descend at about 700 ft/min.
Real descents need more room than the plain sum. Allow extra distance for slowing down, which is hard to do while descending, and remember that a tailwind raises your groundspeed.
Glidepath heights
An ILS glidepath is usually 3°, crossing the runway threshold at about 50 ft. The table gives the height at each distance, ignoring the small effect of the earth’s curve. The approach chart’s figures are the ones to use for a real approach, and they can differ slightly. The ILS guide explains how the beam works.
Quick answers
How do you calculate top of descent?
Multiply the thousands of feet to lose by 3 to get the miles needed. From FL350 down to 3,000 ft that’s 32 × 3 = 96 NM. The exact figure for a 3° path is 100.5 NM.
What rate of descent gives a 3° glidepath?
About 5 × your groundspeed. At 140 kt that’s about 700 ft/min, and the exact figure is 743 ft/min.
How many feet per mile is a 3° descent?
318 ft per nautical mile, usually rounded to 300 ft.
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For training and simulation only. These calculators are for learning and for games. Their results are automatic, and they don’t account for everything a real operation does. Never use them for real flight planning, navigation or air traffic control. Use your official charts, approved tools and procedures.
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