Top of Descent Calculator (3-to-1 Rule)
When to start down: the 3-to-1 rule with ground-speed-aware descent rate — altitude to lose, miles required, and the VSI target that flies it.
The 3-to-1 cockpit version: altitude to lose in thousands × 3, plus a mile per ten knots of tailwind, plus a few for slowing down. The FPM half of the rule: ground speed × 5 gives the 3° rate.
Formula
⚠️ For flight planning and education only — verify with current charts, AFM and ATC clearances. Not for primary navigation.
When to start down: the 3-to-1 rule with ground-speed-aware descent rate — altitude to lose, miles required, and the VSI target that flies it.
About Top of Descent Calculator (3-to-1 Rule)
Arriving high is GA's signature arrival error, and the cure is one multiplication done ten minutes early: the 3-to-1 rule. This calculator runs the full version — altitude to lose against a chosen descent angle gives the start-down distance, ground speed converts the angle to a VSI target — exactly the computation an FMS performs for its TOD arrow, including the GS×5 shortcut that produces the 3° rate without a calculator.
How to use Top of Descent Calculator (3-to-1 Rule)
- 1Enter — sensible defaults are pre-filled so you see a worked result immediately.
- 2Read the live results: .
- 3Check the "With your numbers" line to see the formula distance = altitude ÷ (tan(angle)×6076); at 3°: alt/1000 × 3.1 nm; FPM = GS × angle-gradient/60 substituted step by step.
- 4Adjust inputs (or flip the unit toggle) until the scenario matches yours, then copy or share the result.
Why use Top of Descent Calculator (3-to-1 Rule)?
- ✓Instant, free and private — every calculation runs in your browser, nothing is uploaded
- ✓Built on the published formula distance = altitude ÷ (tan(angle)×6076); at 3°: alt/1000 × 3.1 nm; FPM = GS × angle-gradient/60 with sources cited on the page
- ✓The 3-to-1 cockpit version: altitude to lose in thousands × 3, plus a mile per ten knots of tailwind, plus a few for slowing down. The FPM half of the rule: ground speed × 5 gives the 3° rate.
- ✓Switch units, tweak any input and watch every result update live
Frequently asked questions
How does the 3-to-1 rule work mentally?+
Altitude to lose in thousands of feet, times three, gives the miles: losing 7,500 ft needs ~22 nm (7.5 × 3). Then the companion rule for the rate: ground speed times five gives the ft/min for 3° — 140 kt wants 700 fpm. Two multiplications and the descent profile is set; this tool just adds the precision and the non-3° options.
Why is 3 degrees the standard?+
It's the convergence point of several constraints: ILS glideslopes are built at 3° (obstacle clearance vs approach length economics), jet cabins descend comfortably at the resulting rates, idle-thrust descents in transport jets naturally fall near it, and passenger ears tolerate it. GA aircraft can fly steeper comfortably — but 3° as the default makes every arrival rehearse the approach geometry.
What adjustments does the basic rule need?+
Three classics: add a mile per 10 knots of tailwind (the wind extends your descent), add 1–2 miles per 10 knots of speed reduction needed at the bottom (slowing costs level or shallow segments), and start earlier for passenger comfort in unpressurized aircraft (300–500 fpm cabin rates mean shallower profiles — our cabin-comfort descent tool handles that version).
What do I do when ATC keeps me high past my TOD?+
Recompute, don't mourn: each mile past TOD steepens the required angle — the tool inverted. Past about 4°, normal-category options appear in order: slow down first (less ground covered per minute of descent), then drag (gear, approach flaps early), then request delaying vectors or a 360. The skill is noticing you're high while 'high' is still two adjectives away from 'unstable.'
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