Top of Descent & Descent Rate Calculator

How it works

Distance = (altitude to lose ÷ 1,000) × 3 nm + buffer; rate (ft/min) = altitude ÷ (distance ÷ ground speed × 60) ≈ ground speed × 5 for a 3° path; time = altitude ÷ rate

Descending is planned backwards from the destination. The 3:1 rule — three nautical miles for every thousand feet — gives a comfortable three-degree path, the same angle as an ILS glideslope, and the mental shortcut of multiplying ground speed by five gives the vertical speed that flies it. A tailwind stretches the ground distance covered, so add distance or rate; a headwind does the opposite. Most pilots add a few miles to slow to approach speed and configure, and light aircraft pilots often prefer a fixed rate of around 500 ft/min to protect passengers’ ears and avoid shock-cooling engines, working out the start point from time instead. Flight-management computers do exactly this arithmetic when they compute a top-of-descent point.

Worked example

Descending from 8,500 to 1,500 ft (7,000 ft) at 140 knots: 3:1 says start 21 nm out plus 3 nm to slow down — 24 nm — at about 700 ft/min, taking 9 minutes; at a gentler 500 ft/min you would start 36 nm out.

Frequently asked questions