Descent Rate Calculator
Find the vertical speed to hold a 3° or any glide path, plus feet per mile, the VDP and the height to be at each mile out.
3° is the normal glide slope for ILS and most approaches. The angle is printed on the approach chart.
Use your true airspeed on final and the wind component along the runway. A headwind slows you over the ground, so you need less vertical speed.
For example from the final approach fix to the runway.
Or the touchdown zone elevation (TDZE) from the chart.
TCH: how high the path crosses the runway end. Often about 50 ft; enter 0 to aim at the threshold.
To time the VDP from the final approach fix.
Use when the DME is at or beyond the far end of the runway, so readings grow as you fly away from it.
How your descent rate is worked out
Turn ground speed into feet per minute, find the slope, then multiply.
- 1
Speed in ft per minute
120 kt × 101.27
= 12,152 ft/min - 2
Slope of the path
tan 3.0° = 0.0524
= 318 ft per nm - 3
Vertical speed
12,152 × 0.0524
= 637 ft/min - 4
Rule-of-thumb check
120 × 5 = 600
37 ft/min under
Read more: the formula, a worked example and the rules of thumb
On an approach the glide path is a fixed line over the ground, usually 3° below level. To stay on it, your vertical speed has to match how fast you cover the ground: the faster you go, the faster you need to come down. This calculator gives the exact vertical speed for any ground speed and angle, and the other numbers pilots use on final. To plan where to leave the cruise, use the top of descent calculator.
The formula
ground speed in ft/min = ground speed (kt) × 6,076 ÷ 60 = kt × 101.27
vertical speed (ft/min) = ground speed (kt) × 101.27 × tan(angle)
slope (ft per nm) = 6,076 × tan(angle) (3° ≈ 318 ft per nm)
gradient (%) = tan(angle) × 100
angle from height and distance = arctan(height to lose ÷ (distance in nm × 6,076))
VDP distance from threshold (nm) = (MDA − threshold elevation − TCH) ÷ slope
altitude on the path = threshold elevation + TCH + distance × slope
In metric, vertical speed (m/s) = ground speed (m/s) × tan(angle), and 1 m/s = 196.85 ft/min.
Worked example
At a ground speed of 120 kt you cover 120 × 101.27 = 12,152 ft every minute. On a 3° path, tan 3° = 0.05241, so the vertical speed is 12,152 × 0.05241 = 637 ft/min. The slope is 0.05241 × 6,076 = 318.4 ft per nm, a gradient of 5.24%. The ground speed × 5 rule gives 120 × 5 = 600 ft/min, 37 ft/min less than the exact figure.
Height and distance: to lose 1,500 ft in 5 nm you need 1,500 ÷ 5 = 300 ft per nm. That is arctan(300 ÷ 6,076) = 2.83°, and at 120 kt a rate of 300 × 120 ÷ 60 = 600 ft/min, taking 2.5 minutes.
VDP: with an MDA of 900 ft and a threshold at 400 ft, the height above the threshold is 900 − 400 = 500 ft. With a 50 ft crossing height the VDP is (500 − 50) ÷ 318.4 = 1.41 nm from the threshold, reached 1.41 ÷ 120 × 3,600 = 42 seconds before the runway. The rough rule of 500 ÷ 300 gives 1.67 nm.
Altitude check: 5 nm out you should be 50 + 5 × 318.4 = 1,642 ft above the threshold, an altitude of 400 + 1,642 = 2,042 ft. Going the other way, 2,000 ft is (2,000 − 400 − 50) ÷ 318.4 = 4.87 nm from the threshold.
Rules of thumb
- Ground speed × 5: a quick 3° rate. The exact factor is 5.3, so the rule reads a little low, by about 6%.
- 300 ft per nm: a quick 3° slope (exact 318). It is handy for altitude checks: 3 nm out ≈ 900–955 ft above the runway.
- Height ÷ 300: a quick VDP distance in nm from the MDA height above the touchdown zone.
- Each 0.5° extra: adds about 17% to the rate at 3°. A 3.5° path at 120 kt needs 743 ft/min.
- Headwind and tailwind: work out your ground speed first. A 10 kt change in ground speed changes the 3° rate by about 53 ft/min.
What the angles look like
| Angle | ft per nm | Gradient | At 120 kt |
|---|---|---|---|
| 2.5° | 265 | 4.37% | 531 ft/min |
| 3.0° | 318 | 5.24% | 637 ft/min |
| 3.5° | 372 | 6.12% | 743 ft/min |
| 4.0° | 425 | 6.99% | 850 ft/min |
| 5.5° | 585 | 9.63% | 1,170 ft/min |
Units
1 nautical mile = 1,852 m = 6,076 ft. 1 kt = 1.852 km/h = 1.151 mph = 101.27 ft/min. 1 m/s = 196.85 ft/min.
For training and planning. Fly the published approach, follow ATC and your aircraft's procedures, and use the VDP, angles and altitudes printed on the chart where they are given.
Common questions
How do you calculate the rate of descent for a 3 degree glide path?
Multiply your ground speed in knots by 101.27 to get feet per minute, then by the tangent of the angle (0.0524 for 3°). That works out to about ground speed × 5.3. At 120 kt the exact rate is 637 ft/min. The quick rule of ground speed × 5 gives 600 ft/min, a little shallow.
How many feet per nautical mile is a 3 degree glide slope?
A 3° path drops about 318 ft for every nautical mile flown (tan 3° × 6,076 ft). Pilots often round it to 300 ft per nm. As a gradient it is 5.24%, or about 52 m for every kilometre flown over the ground.
How do you calculate a visual descent point (VDP)?
Take the MDA height above the threshold and divide it by the slope of the path. With an MDA 500 ft above the threshold, a 3° path and a 50 ft threshold crossing height, the VDP is (500 − 50) ÷ 318 = 1.41 nm from the threshold. The rough rule of height ÷ 300 gives 1.67 nm.
How does wind affect the descent rate on an approach?
The glide path is fixed over the ground, so the rate you need follows your ground speed. Flying 130 kt with a 10 kt headwind gives 120 kt over the ground and 637 ft/min on a 3° path. With a 10 kt tailwind you cover the ground at 140 kt and need about 743 ft/min.
What height should I be at 5 miles out on a 3 degree glide path?
On a 3° path that crosses the threshold at 50 ft, you should be 50 + 5 × 318 = 1,642 ft above the threshold at 5 nm. Add the threshold elevation for the altitude on your altimeter: with a 400 ft runway that is about 2,042 ft. Use the altitude check mode for a full table.
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