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.
Where 3 degrees and 318 feet come from
A 3° path is the gradient nearly every instrument approach, every PAPI and every VASI is built around, which is the reason to plan the en route descent to the same number: the profile you fly down from cruise then matches the one you will fly on final, and nothing has to be renegotiated close in. The arithmetic is one line. A nautical mile is 6,076 ft, and the tangent of 3° is 0.0524, so a 3° path loses 6,076 × 0.0524 ≈ 318 ft every mile. Round 318 to roughly a third of a thousand and you have the 3 to 1 rule: three miles for every thousand feet you need to lose.
A worked example
Cruising at 8,500 ft, heading for a field at 500 ft elevation with a 1,000 ft pattern, so you want to be level at 1,500 ft. That is 7,000 ft to lose.
- At a passenger friendly 500 ft/min: 7,000 ÷ 500 = 14 minutes. At 120 kt ground speed you cover 2 NM a minute, so start down 28 NM out.
- The 3 to 1 cross check: 7 × 3 = 21 NM. That is the 3° answer, and it is shorter because 3° at 120 kt is a 600 ft/min descent rather than 500.
- Now add a 20 kt tailwind. The descent still takes 14 minutes, but at 140 kt ground speed those minutes cover 33 NM. Plan from the 28 NM figure and you arrive 5 miles of altitude high.
Both answers are correct; they are answers to different questions. The 3° number is where to start if you want to match the approach gradient. The 500 ft/min number is where to start if you want the cabin comfortable. Pick one before you leave cruise rather than splitting the difference in the descent.
Descent rate for a 3° path
Ground speed × 5 is the rule of thumb; the exact figure is ground speed × 318 ÷ 60.
| Ground speed | Rule of thumb (× 5) | Exact 3° rate |
|---|---|---|
| 90 kt | 450 ft/min | 477 ft/min |
| 100 kt | 500 ft/min | 530 ft/min |
| 120 kt | 600 ft/min | 636 ft/min |
| 140 kt | 700 ft/min | 742 ft/min |
| 160 kt | 800 ft/min | 848 ft/min |
| 180 kt | 900 ft/min | 954 ft/min |
Distance at 3 to 1
| Altitude to lose | Distance | Time at 500 ft/min |
|---|---|---|
| 1,000 ft | 3 NM | 2 min |
| 2,000 ft | 6 NM | 4 min |
| 3,000 ft | 9 NM | 6 min |
| 5,000 ft | 15 NM | 10 min |
| 7,000 ft | 21 NM | 14 min |
| 10,000 ft | 30 NM | 20 min |
Add a mile or two per thousand if you will need to slow down as well as come down. Losing speed and losing height compete for the same energy, and an airplane that is on profile but 30 knots fast is not actually on profile.
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.
Two other constraints usually bite before the airport does. Airspace is one: a Class B or C shelf you must be under is a level off altitude in its own right, and it arrives well before the field. Terrain is the other, particularly at night or over rising ground, where the minimum safe altitude on the chart sets a floor that has nothing to do with your profile. Plan the descent in segments against whichever constraint comes first, then join them up.
Why the start point matters more than the rate
A descent begun in the right place can be flown at almost any comfortable rate. A descent begun late cannot be rescued without either a rate that makes the cabin unhappy or an arrival that is fast, high and close in — and a rushed, unstable approach is one of the more reliable precursors to a landing accident. That is the whole argument for working the number out in the cruise, while there is nothing else to do, rather than in the descent when there is.
Ground speed is the input most worth getting right, which is why this calculator asks for it rather than true airspeed. If you are working from forecast winds aloft rather than a live readout, the E6B wind triangle will turn them into a ground speed first.
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 x 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.
Why is a 3 degree path the standard?
It is the gradient almost every instrument approach and every visual glide slope is built around, so planning to it means the descent you fly en route matches the one you will fly on final. Three degrees is about 318 ft per nautical mile, which is where the 3 to 1 rule comes from.
How do I convert a descent gradient to a rate of descent?
Rate in feet per minute is roughly ground speed in knots multiplied by 5 for a 3 degree path. The exact figure is ground speed x 318 / 60. At 120 kt that is 636 ft/min, which is why the rule of thumb rounds to 600.
Does a tailwind change where I start down?
Yes, and it is the error that puts people high. A descent is a fixed number of minutes at a fixed rate, so a tailwind covers more ground in those minutes. Losing 7,000 ft at 500 ft/min takes 14 minutes whatever the wind; at 120 kt that is 28 NM and at 140 kt it is 33 NM.
What rate should I use with passengers or an unpressurized cabin?
Around 500 ft/min is the usual comfort figure, and it is about ear clearing rather than the airplane. Anything past roughly 1,000 ft/min gets uncomfortable for anyone with a cold, and children clear pressure more slowly than adults. Starting down earlier at a gentler rate is nearly always the better trade.
How do I plan a descent with a level segment or a step down?
Work it backwards in pieces. Take the last constraint first, find the distance for that segment, then treat the top of that segment as the new level off altitude for the segment before it. Adding the distances gives the point to leave cruise.