Exact rate and the times-five rule
For a constant path angle γ:
ft per NM = tan(γ) × 6,076.12 ft/NM
descent rate (fpm) = groundspeed (kt) × (ft per NM) / 60
A 3° path is 318.4 ft per nautical mile, so the exact rate is about 5.31 × groundspeed. At 120 kt that is 637 fpm. The memory item “GS × 5” gives 600 fpm — a convenient, slightly shallow target that will not quite hold a 3° slope. This page shows both, and the same identity used on the top of descent calculator.
Other gradients
Some charts and training material use 300 ft/NM (the HAT/300 cousin of a 3° VDP) as a round number; that is a 2.83° path, shallower than a 3° ILS. You can enter a published angle, a chosen vertical speed, or a gradient in ft/NM and convert among them.
Sources
FAA Instrument Flying Handbook (ILS glideslope, descent planning); FAA Instrument Procedures Handbook; AIM instrument approach procedures; standard 3° glide-path geometry (1 NM = 1,852 m). For starting the descent from cruise, use the top of descent calculator. For a non-precision VDP, use the visual descent point calculator.
Frequently asked questions
What descent rate do I need for a 3° glideslope at 120 kt?
About 637 feet per minute. The GS × 5 rule gives 600 fpm, which is about 37 fpm shallow of a true 3° path. Use 5.3 times groundspeed if you want a closer mental check.
Is this the same math as top of descent?
Yes — the same tan(angle) geometry. The top-of-descent page starts from altitude to lose and tells you when to start down. This page is the glideslope / VSI form: given the groundspeed and angle you are flying now, what rate to set.
Should I use indicated airspeed or groundspeed?
Groundspeed. The path angle is a geometry over the ground. A headwind reduces the required fpm; a tailwind increases it. Use the groundspeed you actually have, including from GPS.