Top of Descent Calculator
Cruise altitude, level off target, ground speed, and descent rate: how many miles out to start down, how long it takes, and how it compares to a 3° profile.
Fill ground speeds from winds aloft
Uses the NOAA winds and temps aloft forecast near an airport along your route: your TAS and true course become start and end ground speeds.
Quick cross checks: the 3 to 1 rule says about 21 NM; a 3° profile would take 22 NM at 637 ft/min (ground speed × 5). Distances are over the ground, so plan with ground speed, not airspeed, and start down earlier with a tailwind.
How the top of descent is calculated
- Altitude to lose = cruise altitude − level off altitude
- Time = altitude to lose ÷ descent rate
- Distance = ground speed × time
- 3° cross check: distance ≈ altitude to lose ÷ 318 ft per NM, flown at about ground speed × 5 ft/min
The classic mental math version is the 3 to 1 rule: three miles per thousand feet. This tool gives you the exact numbers for your rate and ground speed, and shows the 3° profile beside them so you can pick whichever the day calls for.
Plan to a fix, not the field
Measure the distance from where you need to be level: the 45° entry at pattern altitude, an IFR fix at its crossing altitude, or below a shelf of Class B. If you plan to the airport symbol you'll arrive overhead high, and a slam dunk descent close in is how unstabilized approaches start. Descending through unfamiliar terrain at night? Check the performance guide and your airport's page for obstacles and pattern details before you commit to a profile.
FAQ
What is the 3 to 1 rule for descent planning?
Multiply the altitude you need to lose, in thousands of feet, by 3 to get the distance in nautical miles. Losing 6,000 ft takes about 18 NM. It approximates a 3 degree path and works at typical GA speeds.
What descent rate should I plan?
500 ft/min is the common GA comfort target for passengers. For a 3 degree path, the rate that holds it is about ground speed × 5 (120 kt needs roughly 600 ft/min).
Why does the calculator use ground speed instead of airspeed?
The descent covers distance over the ground, and wind changes that. A tailwind means fewer minutes per mile, so the same vertical speed needs more distance: start down earlier.
When should I start my descent?
Start at the distance this calculator gives, measured from where you want to be level: the pattern 45 entry, an IAF crossing altitude, or an airspace shelf, not the airport itself.
The calculator is just the start
These same calculations power the SkyReady app — alongside an offline-first logbook, automatic FAA currency, proficiency scoring, and a one-tap Go/No-Go readiness brief. Free to log.