Descent Rate Formula: 3° Path, FPM & Groundspeed
A three-degree descent path is common in approach and descent planning. The faster the airplane moves over the ground, the faster it must descend in feet per minute to remain on the same geometric path.
Exact 3 degree descent rate formula
The geometric relationship is:
With groundspeed in knots, the result is feet per minute. The constants simplify to approximately:
At 120 knots groundspeed, an exact three-degree path is about 637 fpm. At 150 knots it is about 796 fpm.
The groundspeed × 5 rule
The familiar cockpit shortcut is:
At 120 knots, that gives 600 fpm instead of the exact geometric value near 637 fpm. At 150 knots, the shortcut gives 750 fpm instead of about 796 fpm. The shortcut is intentionally easy to calculate mentally and is useful as a starting estimate.
3 degree descent rate chart
| Groundspeed | Exact 3° rate | GS × 5 |
|---|---|---|
| 80 kt | 425 fpm | 400 fpm |
| 90 kt | 478 fpm | 450 fpm |
| 100 kt | 531 fpm | 500 fpm |
| 110 kt | 584 fpm | 550 fpm |
| 120 kt | 637 fpm | 600 fpm |
| 130 kt | 690 fpm | 650 fpm |
| 140 kt | 743 fpm | 700 fpm |
| 150 kt | 796 fpm | 750 fpm |
| 160 kt | 849 fpm | 800 fpm |
| 180 kt | 955 fpm | 900 fpm |
For the full 60–200 knot table, see the PilotDesk 3° descent-rate chart.
Use groundspeed, not airspeed
The vertical speed required for a geometric path depends on groundspeed. A headwind lowers groundspeed and therefore lowers the required descent rate. A tailwind raises groundspeed and raises the required rate. If groundspeed changes significantly during an approach or descent, the target vertical speed changes with it.
The 3 to 1 descent rule
A second common planning shortcut estimates the horizontal distance needed to lose altitude:
If you need to lose 6,000 feet, a rough three-degree descent begins about 18 NM before the target point. If you need to lose 3,000 feet, think roughly 9 NM. This is a planning estimate, not authorization to descend and not a substitute for published restrictions.
Why the 3 to 1 rule works
A true three-degree geometric path covers a little more than 3 NM horizontally for each 1,000 feet of vertical change. The 3:1 shortcut is close enough to make quick descent planning easy and pairs naturally with the groundspeed × 5 vertical-speed estimate.
Descent rate on a 3° path
Suppose you are 12,000 feet and want to be at 3,000 feet at a fix. You need to lose 9,000 feet. The 3:1 rule suggests starting about 27 NM before the fix. If groundspeed is 140 knots, the quick rate is roughly 700 fpm; the exact 3° geometric rate is about 743 fpm.
This does not account for level segments, speed changes, ATC restrictions, configuration changes, or a required deceleration before the fix. Those items may require beginning the descent earlier.
3 degree path vs. published procedure
A calculated three-degree path is only geometry. On an actual instrument approach, use the published procedure, vertical guidance, altitude restrictions, and applicable stabilized-approach criteria. A calculator should not be used to replace a glideslope, glidepath, VDA, or charted procedure requirement.
3 degree descent questions
What vertical speed should I use at 100 knots groundspeed?
About 531 fpm for exact 3° geometry, or 500 fpm using the common groundspeed × 5 shortcut.
What about 120 knots?
About 637 fpm exact, with a quick-rule estimate of 600 fpm.
Does a headwind change the descent rate?
Yes. A headwind reduces groundspeed, so a lower feet-per-minute descent rate is needed to remain on the same geometric path.
Is the 3:1 rule exact?
No. It is a cockpit planning shortcut. Use published procedures and guidance when they control the descent.
Related PilotDesk tools
3° Descent Rate Calculator · 3° Descent Rate Chart · Top of Descent Calculator · FPM to Gradient Calculator · Flight Planning Guide
References
The formula is standard flight-path geometry. FAA instrument guidance discusses vertical descent angles, groundspeed, and vertical speed. See current FAA instrument publications and PilotDesk Sources & Methods.