The instrument panel
Top of Descent Calculator
Altitude to lose and groundspeed in — distance to start down and the descent rate for a 3° path out.
START DESCENT
- DESCENT RATE · 3° PATH
- —
- TIME IN DESCENT
- —
For training and reference · verify with official sources and your POH before flight · no tool on this site renders a go / no-go judgment
Why 3:1 works
A 3° slope drops about 318 ft per NM; call it three miles per thousand feet and the arithmetic fits in your head. The rate that holds the slope is groundspeed × 5 — five times 120 kt is 600 fpm, which is also a comfortable, ear-friendly rate for passengers in an unpressurized airplane.
Descending into a high-elevation field? Remember the airport sits at its density altitude, not its chart elevation, when it is time for the go-around performance you briefed.
Frequently asked questions
What is the 3:1 rule?
Three miles of ground track per thousand feet of altitude — the geometry of a roughly 3° descent path, the same slope as an ILS glideslope. Losing 10,000 ft takes about 30 NM; add a mile per 10 kt of speed you plan to bleed off.
What descent rate holds a 3° path?
Groundspeed × 5. At 120 kt over the ground that is about 600 fpm; at 150 kt, 750 fpm. It scales automatically with tailwinds — more groundspeed needs more rate for the same slope.
Does the rule change with wind?
The rule already uses groundspeed, so wind is built in — but it changes en route as winds shift. A strong tailwind on descent means starting down earlier and carrying a higher rate. Recheck the numbers when the groundspeed readout changes materially.