Top of Descent Calculator
Wind changes the descent rate, not the distance — and the 3:1 rule is really a 3.14 degree path, not the 3 degrees it is named after.
The rate rule against the trigonometry
Wind changes the rate, not the distance
What a fixed rate does to the path
Wind changes the rate, not the distance
This is the conflation worth clearing up. The 3:1 distance is fixed by geometry — a 40 kt tailwind doesn't move your top of descent at all. But it demands 33% more rate to fly the same path, because you cross the same ground in less time. The distance is a question about the air mass you're descending through; the rate is a question about how fast you're crossing it.
Which means you can hold the path constant or the rate constant, never both. A fixed 500 fpm is a 3.53° path at 80 kt and 1.41° at 200 — the same rate needing 27 nm from 10,000 ft in one case and 67 in the other. Holding the path is usually right, because the path is what clears terrain and meets a crossing restriction; the rate is merely how you achieve it.
And the 3:1 rule is a 3.14° path, not a 3° one. A thousand feet over three miles works out at 3.140° — 4.7% steeper than the glidepath everyone associates it with. A true 3° path loses 318 ft per mile, so a thousand feet actually needs 3.14 miles. Close enough to fly, and not the angle it's named after. (That it lands so near π is a coincidence, but a memorable one.)
The "half your groundspeed, add a zero" rate rule is 5.8% low for a 3° path. The exact multiplier is 5.307, not 5 — and because it's a pure ratio, that error is identical at every groundspeed. Being low means descending shallow, so you arrive high: the recoverable error, since drag is available and altitude isn't.
- Slowing down costs distance the rule doesn't count. 250 to 120 kt takes 5.6 nm on its own, which is why airline practice is 3× the altitude plus about 10 miles.
- Descent planning is a distance problem wearing an altitude problem's clothes. The altitude is fixed by the clearance and the rate by the aeroplane — the only thing you control is where you start, decided once, minutes before anything visible happens.
- Getting it wrong late is expensive precisely because the geometry has already committed. Excess altitude costs options, and the options get worse the longer you leave it.
- Over about 1,000 fpm close to the ground is worth noticing, not flying. If the arithmetic demands it low down, the honest reading is that the descent was started late.
How to use
- Decide whether you are holding the path or the rate — you cannot hold both.
- Work the distance from geometry, then the rate from your groundspeed.
- Add distance for any speed reduction; the 3:1 rule does not include it.
- Decide early — the geometry commits long before anything looks wrong.
Frequently asked questions
What is the 3 to 1 descent rule?
Multiply the altitude to lose in thousands of feet by three to get the distance in nautical miles — so ten thousand feet needs thirty miles. It is the standard mental shortcut for deciding where to start down, it works in any aircraft, and it takes no calculation beyond multiplying by three.
Is the 3 to 1 rule the same as a 3 degree glidepath?
Not quite, and the difference is worth knowing. A thousand feet over three nautical miles is 3.140 degrees, which is 4.7 per cent steeper than a true three-degree path. A genuine three-degree path loses 318 feet per mile, so a thousand feet actually needs 3.14 miles rather than three. Close enough to fly, and not the angle it is named after.
Does a headwind or tailwind change where I start descending?
No, and this is the thing most often confused. The distance is fixed by geometry — the path angle and the altitude decide it, and the wind appears nowhere in that calculation. What the wind changes is the RATE: a forty knot tailwind demands about a third more feet per minute to fly the same path, because you cross the same ground in less time.
How do I calculate the rate of descent for a glidepath?
Multiply groundspeed in knots by 5.307 for a three-degree path, or more generally by 101.3 times the tangent of the path angle. A hundred and twenty knots on a three-degree path needs 637 feet per minute. The rate depends on groundspeed rather than airspeed, which is why the number changes with the wind while the distance does not.
How accurate is the "half your groundspeed, add a zero" rule?
It is 5.8 per cent low for a three-degree path — the exact multiplier is 5.307 rather than 5. Because both figures are proportional to groundspeed, that error is identical at every speed rather than growing. Being low means descending shallow, so you arrive high, which is the recoverable error: drag is available and altitude is not.
Should I hold a constant rate or a constant path?
The path, in almost every case. It is the path that clears terrain and meets a crossing restriction, while the rate is merely how you achieve it. Holding a constant rate instead makes the path vary with groundspeed — five hundred feet a minute is a 3.53 degree path at eighty knots and 1.41 degrees at two hundred, which is the difference between needing 27 miles and 67.
Can I hold both the rate and the path constant?
No, and that is not a limitation of any particular aircraft. Rate, path and groundspeed are one relationship with three names, so fixing any two determines the third. If the groundspeed changes — which it does constantly, with wind and with deceleration — then either the rate or the path must move. Choosing which one is the actual decision.
Does slowing down need extra distance?
Yes, and the 3 to 1 rule does not include it. Going from 250 knots to 120 at a comfortable rate takes about 108 seconds and covers 5.6 nautical miles while barely descending. That is why airline practice is three times the altitude PLUS roughly ten miles, rather than three times alone — the extra covers the deceleration and the approach configuration.
What should I do if I am high on the descent?
Decide early, because the options get worse with every mile. In rough order of cost: increase the rate of descent, add drag, then ask for track miles or a delaying turn. The reason to decide early is geometric rather than procedural — the excess altitude that needs a modest rate increase twenty miles out needs an unreasonable one at five.
Is there a maximum sensible rate of descent?
Not a hard limit, but more than about a thousand feet a minute close to the ground is worth noticing rather than flying. Ground proximity warning systems care about it, passengers feel it, and it removes the margin to arrest a descent if something appears. If the arithmetic demands a high rate low down, the honest reading is that the descent was started late.
How do I work out the descent gradient in feet per mile?
Multiply 6,076 by the tangent of the path angle. A three-degree path is 318 feet per mile, and the 3 to 1 rule works out at 333. Published approach charts often give the gradient this way rather than as an angle, because feet per mile compares directly against a distance-to-run readout without any trigonometry in the cockpit.
Why is descent planning so unforgiving of a late decision?
Because it is a distance problem wearing an altitude problem clothes. The altitude is fixed by the clearance and the achievable rate is fixed by the aircraft, so the only genuinely free variable is where you start — and that decision is made once, several minutes before anything visible happens. By the time being high looks like a problem, the geometry has already committed.
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