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Descent Rate Calculator

Enter your groundspeed and glidepath angle to find the descent rate.

Result

637 ft/min
318 ft/NM
1.57 min

How it is calculated

The tool turns a groundspeed and a glidepath angle into a vertical speed. Groundspeed is in knots, the angle is in degrees, and if the angle is left empty it defaults to 3. One knot is one nautical mile, 6,076 ft, per hour, so a knot covers 101.269 ft per minute (6076 ÷ 60). Formulas: rate = groundspeed × 101.269 × tan(angle) in ft/min, loss per NM = 6076 × tan(angle) in ft/NM, and time per 1000 ft = 1000 ÷ rate in minutes. Groundspeed and angle must be above zero, and the angle must be below 90 degrees. Use groundspeed, not indicated airspeed; a headwind lowers groundspeed and therefore the rate you need.

Worked example

On a standard 3 degree glidepath at 120 knots groundspeed: tan(3°) is about 0.0524. Descent rate: 120 × 101.269 × 0.0524 = 637 ft/min. Altitude lost per nautical mile: 6076 × 0.0524 = 318 ft/NM. Time to descend 1000 ft: 1000 ÷ 636.9 = 1.57 min. Raising groundspeed to 140 knots on the same angle lifts the rate to 743 ft/min, but the 318 ft/NM stays the same because it depends only on the angle.

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Common mistakes

  • Entering indicated or true airspeed rather than groundspeed. With a tailwind or headwind the wrong speed gives the wrong vertical speed target.
  • Entering the speed in km/h or mph. The factor 101.269 is built for knots.
  • Typing the glidepath as a percent gradient (5%) rather than degrees. The tool takes degrees, so 3 means 3°, not 3%.
  • Applying one fixed rate for the whole approach. If the groundspeed changes as you slow down, the rate needed to hold the same angle changes with it.

About this calculator

An aircraft's descent rate — measured in feet per minute — is the vertical speed a pilot must fly to stay on a chosen glidepath angle at a given groundspeed. The relationship is straightforward trigonometry: descent rate ≈ groundspeed × tan(glidepath angle), commonly simplified with rules of thumb like the “3-in-1” rule pilots use for a standard 3-degree approach. This calculator computes the exact descent rate and altitude lost per nautical mile from your groundspeed and glidepath angle.

This figure is essential during instrument and visual approach planning: pilots cross-check their vertical speed indicator against the computed target rate to confirm they're tracking the glidepath correctly, whether flying an ILS, a visual approach, or a non-precision approach with a defined descent angle. Flight instructors also use the calculation to teach stabilized-approach technique, since flying too fast or too slow a descent rate relative to the required glidepath is a common cause of unstable approaches.

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Comments (1)

  • Editorial team

    The rate scales with groundspeed while the altitude lost per nautical mile does not. Slowing from 120 knots to 90 knots on a 3 degree path drops the rate to 478 ft/min, yet the path still loses 318 ft/NM. Use it to plan a descent, then compare against what your vertical speed indicator actually shows, because wind changes groundspeed all the way down. Never fly from this alone; the approach plate rules. Which glidepath angle does your home airfield use?

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