Glide Ratio Calculator
Work out how far a glider can glide from a given height, and what a headwind does to the distance.
What it does
A glider has no engine, so the only number that decides how far it can go is the glide ratio. The FAA Glider Flying Handbook defines it plainly: the number of feet a glider travels horizontally in still air for every foot of altitude lost. A ratio of 30 to 1 means 30 feet forward for every foot down, and the same handbook points out that the maximum glide distance is the same as the maximum lift to drag ratio of the sailplane.
The second half of the definition is the one that catches people out. Glide ratio is also airspeed divided by sink rate, with both in the same units, so the ratio and the speed you fly it at describe the same glide. Flying faster does not give a better ratio, it gives a different point on the polar with a higher sink rate.
Distance is then simple. Multiply the height you have to lose by the ratio and you have the ground covered, as long as the height and the distance use the same length units. The tricky part is that the height has to be the height above the landing area rather than the height above sea level, and a sensible pilot keeps an arrival reserve in hand rather than gliding to the deck.
Wind changes the answer. Flying the same airspeed into a headwind, the ground speed is the airspeed minus the headwind, so the ground distance is cut by that same fraction. The FAA explains this in the glider performance chapter, where a 20 knot headwind moves the best glide point along the polar, which is a different and more refined calculation than the one on this page.
The tool covers the everyday case: a known height, a known best glide ratio, the speed that gives it and a headwind. It assumes steady air with no lift or sink, level ground at the end of the glide and no other traffic to think about.
How to use
- Pick imperial or metric units.
- Enter the height of the glider above the landing area, not above sea level.
- Enter the glide ratio of the sailplane.
- Enter the airspeed you will fly the glide at.
- Enter the headwind component along the track.
- Add an arrival reserve if you want to keep height in hand, then read the distance and the time.
Formula
- G
- Glide ratio, the horizontal distance for each unit of height lost.
- H
- Height available to lose above the landing area.
- V
- Glide airspeed.
- W
- Headwind component along the direction of travel.
- Sink rate
- Descent rate, which equals the airspeed divided by the glide ratio.
Height and distance must be in the same length units, so the tool converts feet to metres to nautical miles. The headwind correction holds the airspeed and the sink rate fixed, which is what a pilot does when the glide is worked from a fixed polar point. Finding the longest range against a wind means reworking the polar tangent, which the FAA describes separately and this page does not attempt.
Worked example
Example: 1,500 ft above the field with a 30:1 glide at 60 kt
Defaults: imperial units, 1,500 ft of height, a glide ratio of 30, 60 kt glide speed, a 15 kt headwind and no arrival reserve.
- Height to lose: 1,500 ft with no reserve.
- Distance in still air: 30 x 1,500 = 45,000 ft.
- In nautical miles: 45,000 x 0.3048 / 1,852 = 7.41 NM.
- Headwind factor: (60 - 15) / 60 = 0.75.
- Distance with the headwind: 7.41 x 0.75 = 5.55 NM.
- Ground glide ratio: 30 x 0.75 = 22.5:1, which is a quarter less range.
- Sink rate: 60 kt is 6,078 ft per minute, divided by 30 gives 203 ft per minute, so the glide takes 7.4 minutes.
- The tool shows 7.41 NM, 5.55 NM, 22.5:1, 7.4 min and 203 ft/min.
Reference table
| Glide ratio | Distance | Time to glide down at 60 kt |
|---|---|---|
| 20:1 | 4.94 NM | 4.9 min |
| 25:1 | 6.17 NM | 6.2 min |
| 30:1 | 7.41 NM | 7.4 min |
| 35:1 | 8.64 NM | 6.4 min |
| 40:1 | 9.87 NM | 5.5 min |
Limits
- Assumes steady air, so it does not account for lift, sink or thermal cores along the track.
- Holds the airspeed and the sink rate fixed, which is not how the longest range against a wind is found.
- Uses one glide ratio, while the best glide and the minimum sink are different points on the polar.
- Ignores the wind gradient and any valley lift or sink on the way down.
Common mistakes
- Using height above sea level instead of height above the landing area, which inflates the range.
- Mixing units, for example a glide ratio in feet with a height in metres.
- Forgetting that the headwind reduces ground distance but not the time to descend.
- Leaving out the arrival reserve, which is the margin between a planned landing and a real one.
- Assuming a tailwind doubles the range, when the distance still falls because the sink rate rises with speed.
Try it yourself
Quick checks. Answers sit tucked under each question.
How far can you glide from 2,000 ft at a 25:1 glide ratio?
Distance = 25 x 2,000 = 50,000 ft, which is 50,000 x 0.3048 / 1,852 = 8.23 NM.
Does a 20 kt headwind at 60 kt cut the range by a third?
The factor is (60 - 20) / 60 = 0.667, so the range falls by a third. From 1,500 ft at 30:1 the distance goes from 7.41 NM to 4.94 NM.
What height do you need to glide 10 NM at a 30:1 ratio?
Height = 10 x 1,852 / (0.3048 x 30) = 2,025 ft, so you need a little over 2,000 feet of height above the landing area.
Questions
What does a glide ratio of 30 to 1 mean?
The glider travels 30 units forward for every unit of height it loses. From 1,000 ft of height, a 30:1 glide covers 30,000 ft of ground.
Is best glide the same as minimum sink?
No. Best glide is the highest lift to drag ratio and gives the longest distance. Minimum sink is the slowest descent and gives the longest time in the air. They come from different points on the speed polar.
How much does a headwind hurt?
At a fixed airspeed the ground distance falls by the headwind divided by the airspeed. A 15 kt headwind at 60 kt glide speed cuts the range by a quarter.
Can I use this for a powered glider or a hang glider?
Yes, as long as you use the glide ratio at the speed you will actually fly. Sailplanes are usually quoted at their best glide speed, and hang gliders at a much lower speed with a far worse ratio.
Does the temperature of the air matter?
It changes the true airspeed and the sink rate for a given indicated speed, but the ratio itself stays roughly the same, so the distance in still air holds up.
What about flying in lift on the way down?
That is exactly what a good glide is for. A ridge lift band or a thermal on the route adds height, and the extra height buys more distance. The calculator on this page assumes no lift at all.
Sources
- FAA Glider Flying Handbook: Chapter 3, aerodynamics of flight
- FAA Glider Flying Handbook: Chapter 5, glider performance
- FAA Glider Flying Handbook, complete edition FAA-H-8083-13A