Top of descent: estimate distance and vertical speed
A top-of-descent estimate connects the altitude to lose with a planned descent path and groundspeed. It gives a starting point for planning; it does not supply a clearance or account for every restriction, speed change, aircraft capability, or procedure.
Distance from altitude and descent gradient
Distance (NM) = altitude to lose (ft) ÷ descent gradient (ft/NM). Subtract the target altitude from the altitude at which the descent is planned to begin. For example, a 6,000 ft descent at 318 ft/NM takes about 18.9 NM. A 318 ft/NM gradient is close to a geometric 3° path.
Do not automatically use airport elevation as the target. A crossing restriction, an intermediate fix, or a published procedure may establish the altitude that matters first. The applicable chart and clearance determine the actual constraints.
Convert the path to a vertical speed
Vertical speed (FPM) = descent gradient (ft/NM) × groundspeed (kt) ÷ 60. At 120 kt groundspeed, 318 ft/NM corresponds to about 636 fpm. At 90 kt, that same ground-referenced path corresponds to about 477 fpm. Because groundspeed changes the relationship, update the estimate when the wind or planned speed changes.
The 3-to-1 and groundspeed-times-five shortcuts
The familiar 3-to-1 rule estimates roughly 3 NM for each 1,000 ft to lose. For 6,000 ft, it suggests about 18 NM. The geometric 3° value is about 318 ft/NM, so the two estimates are close in ordinary planning. The related groundspeed-times-five rule estimates the vertical speed for a 3° path: at 120 kt, 120 × 5 = 600 fpm. That is a convenient mental shortcut and runs slightly below exact 3° geometry, which is about 636 fpm at the same groundspeed.
Use rounded rules to cross-check arithmetic. If an altitude constraint, high descent rate, significant speed reduction, or changing wind matters, plan with the relevant procedure and aircraft guidance rather than relying on a single shortcut.
Losing 6,000 feet on a 3° path
Assume a planning exercise calls for losing 6,000 ft to a defined target at 120 kt groundspeed, with a nominal 3° descent path. The 3-to-1 estimate gives 18 NM; exact 318 ft/NM geometry gives about 18.9 NM. The corresponding vertical speed is around 636 fpm, while the groundspeed-times-five mental check gives 600 fpm. The small difference comes from rounding the geometry, not a second aircraft-performance model.
This calculation leaves out acceleration or deceleration, level segments, ATC restrictions, winds that vary with altitude, terrain, and the aircraft's descent capability. Those items can move the actual top of descent earlier or later.
Use published guidance and keep monitoring
The FAA Instrument Procedures Handbook describes the 3-to-1 distance estimate and groundspeed-times-five descent-rate rule. Instrument approach charts may publish a vertical descent angle, and aircraft manuals or avionics may provide additional descent guidance. Use current charts, clearances, aircraft procedures, and actual groundspeed; recalculate when conditions or constraints change.