Rate of descent and glideslope calculator

What this calculator shows

This calculator estimates the vertical speed needed to follow a straight descent path at a chosen angle and groundspeed. It also estimates the distance and time to lose the altitude entered. The result is a geometric reference, not a clearance or a procedure for a particular approach.

Groundspeed matters because the airplane covers more horizontal distance each minute as groundspeed increases. To stay on the same descent angle, it must also lose more altitude each minute. A three-degree path at 120 knots therefore requires about twice the vertical speed of the same path at 60 knots.

How to use the calculator

Enter groundspeed in knots, a descent angle in degrees, and the altitude to lose in feet. Results update as the values change, and the URL retains the selected values for sharing or review.

Groundspeed. Enter expected or observed speed over the ground, not indicated airspeed. Wind changes groundspeed and therefore changes the vertical speed needed to maintain a fixed path.

Descent angle. Three degrees is a common example for a stabilized approach path, but it is not universal. Use the angle specified by the published procedure or the training exercise.

Altitude to lose. Enter the vertical difference in feet between the current point and the target point. This tool does not determine a safe descent start point or account for intermediate restrictions.

How the calculation works

The vertical distance per nautical mile is the tangent of the descent angle multiplied by the number of feet in a nautical mile. The vertical speed is that gradient multiplied by groundspeed in nautical miles per hour and divided by 60 minutes per hour.

feet per minute = groundspeed × 6,076.12 × tan(angle) ÷ 60

For a three-degree path, the gradient is about 318 feet per nautical mile. At 120 knots, the required vertical speed is about 637 feet per minute.

Time is estimated by dividing altitude to lose by vertical speed. Distance is then groundspeed multiplied by that time. The calculator rounds the displayed vertical speed to a whole foot per minute and other outputs to one decimal place.

Example calculation

A training example uses 120 knots groundspeed, a three-degree path, and 6,000 feet to lose.

1. The path gradient is approximately 6,076.12 × tan(3°), or 318 ft/NM.
2. The required rate is approximately 120 × 318 ÷ 60, or 637 ft/min.
3. Losing 6,000 feet at that constant rate takes about 9.4 minutes.
4. At 120 knots, that time covers about 18.8 NM.

The estimates change if groundspeed, angle, or the altitude difference changes.

When this estimate is useful

During instrument training, a student can compare a planned descent rate with the groundspeed expected on an approach. In visual training, the gradient can help explain how a stable descent path relates to distance from a runway or another reference point. A CFI can also use different groundspeeds to demonstrate why one fixed vertical speed does not preserve one fixed angle in changing wind.

For example, if groundspeed increases while the airplane remains on a three-degree path, the vertical speed must increase too. If the vertical speed stays unchanged, the path becomes shallower. This relationship is useful for understanding the indications on a vertical path display, but the display and published procedure govern the actual approach.

Common planning errors

Using indicated airspeed in place of groundspeed is a frequent source of error. Indicated airspeed describes motion through the air, while this geometry depends on distance covered over the ground. Wind can make those values meaningfully different.

Another mistake is treating the computed distance as a recommended descent point. The result includes only the altitude loss entered and the selected angle. It has no awareness of terrain, restrictions, traffic, or the published approach profile. Add the applicable constraints and use approved procedures when planning a real flight.

Assumptions and limitations

The calculation assumes a constant groundspeed and a straight path with a constant angle. It does not model acceleration, wind changes, aircraft response, terrain, step-down fixes, approach constraints, or required obstacle clearance. Real descent planning must include all published altitude and speed restrictions.

The displayed descent rate is a mathematical target only. Use the published approach, aircraft procedures, instructor guidance, and current operating information. A glidepath indication or vertical navigation setup is not interchangeable with this generic calculation.

Sources and methodology

The calculation uses right-triangle geometry and the defined length of a nautical mile. The FAA Pilot's Handbook of Aeronautical Knowledge provides flight-planning and navigation context. Published procedure charts and aircraft guidance remain controlling for a specific approach.

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