Glide Ratio Calculator
Solve the glide ratio relation GR = L/D = HD / AL for any of its four variables — glide ratio, horizontal distance, altitude lost, or glide angle — with step-by-step math, a live glide-path schematic, a full linear sweep, and the complete flight-performance picture including sink rate, forward speed, flight time, best-glide speed and wind-penetration analysis.
The three equivalent definitions of glide ratio
Glide ratio can be computed three different ways and they always give the same number. First, geometrically: HD divided by AL, which is what you measure on a chart. Second, aerodynamically: lift divided by drag, because in a steady glide the lift balances most of the weight and the drag balances the component of weight along the flight path. Third, trigonometrically: 1 over tan θ, where θ is the glide angle below the horizontal. For a modern sailplane at GR = 50, the glide angle is only 1.15°, but the aircraft still descends 1,000 m for every 50 km of horizontal travel — a fact that matters enormously when planning a final glide with no margin.
Wind changes ground glide ratio, not air glide ratio
The glide ratio relative to the air is a fixed property of the aircraft for a given configuration — a sailplane with GR = 40 will always descend 1,000 m for every 40 km of air distance. But the glide ratio over the ground depends on the wind. Facing a 30 m/s headwind while flying at 100 km/h (27.8 m/s) forward airspeed, the ground speed drops to −2.2 m/s — you drift backwards and the glide ratio over the ground becomes negative. Facing the same wind downwind, the ground speed rises to 57.8 m/s and the effective glide ratio nearly doubles. This is why glider pilots fly fast into headwinds and slow in tailwinds; the "speed-to-fly" theory predicts the optimal airspeed for each wind condition.
Best glide speed vs minimum sink speed
A glider has two different optimum speeds. Best-glide speed (Vbg) maximises the distance for a given altitude loss — it is the airspeed at which the L/D ratio is highest, and it is what you fly after an engine failure to reach the furthest possible landing point. Minimum-sink speed (Vms) minimises the descent rate — it is what you fly in a thermal to climb as fast as possible while circling. These two speeds are always different: Vms is slower than Vbg by roughly 15–25% because the L/D peak is at a higher speed than the sink-rate minimum. Flying at Vbg gets you furthest; flying at Vms keeps you airborne longest. Every pilot should know both numbers for their aircraft.
Final glide and the glide slope you can afford
During the final glide back to an airfield, a pilot needs enough altitude to cover the remaining distance plus a safety margin. If the airfield is 30 km away and the sailplane's glide ratio is 40:1, the theoretical altitude needed is 30,000/40 = 750 m above the airfield — but pilots typically add 30–50% safety margin (call it 1,000 m) and then subtract any lift expected along the way. With a headwind the required altitude rises quickly: a 20 km/h headwind reduces the effective glide ratio from 40 to about 29, raising the required altitude from 750 m to over 1,000 m. This is why pilots compute "final glide" numbers continuously during a cross-country flight — the margin shrinks fast when you guess wrong.
How to Use This Calculator
The glide ratio relation links four quantities: GR, HD, AL, and θ. Pick which one you want the tool to solve, then fill in the others.
1. Choose what to solve for
Pick GR, HD, AL, or θ from the toggle. The calculator rearranges the relations GR = HD/AL and θ = arctan(AL/HD) automatically for whichever variable you choose.
2. Use realistic presets
Glide ratios from 2.5 (wingsuit) to 60 (modern racing sailplane), distances from 500 m to 500 km, altitudes from 100 m to 100 km, and angles from 1° to 25°.
3. Work in metres or kilometres
HD is entered in metres, AL in metres. The hints show the values in the other common units — km, ft, nautical miles — so you can sanity-check against a flight computer or a chart.
4. Read the result and verdict
The result card shows the answer plus a colour-coded verdict classifying the glide from a wingsuit through paragliders, hang gliders, light aircraft, sailplanes, and racing gliders up to the modern 60:1 class.
5. Check the schematic and charts
The schematic shows the glide path from a starting altitude to a landing point, with the altitude line (AL), the horizontal distance (HD), the glide angle arc, and an aircraft silhouette at the top. Three charts: GR vs AL, HD vs AL, and glide angle vs GR.
6. Switch to Advanced for depth
Advanced mode adds the arctangent glide-angle curve, a required-altitude bar chart, a sensitivity check, and companion flight-performance metrics including sink rate, flight time, ground speed with wind, ground glide ratio with wind, best-glide speed estimate, and the ratio of current to best L/D.
What This Tool Offers
Built around the glide ratio relation GR = L/D = HD/AL, the fundamental performance number for every unpowered or engine-off flight.
Four-way solving
Solve for glide ratio, horizontal distance, altitude lost, or glide angle. The calculator handles the trig relations automatically.
Live glide-path schematic
A drawn glide path from cruise altitude to touchdown with the altitude line, horizontal distance arrow, glide angle arc, and an aircraft silhouette at the top — updated in real time.
Glider-class verdict
Each result is classified from wingsuit through paragliders, hang gliders, light aircraft, training sailplanes, and high-performance racing gliders, with a coloured badge.
Multi-unit hints
HD and AL in metres, kilometres, feet and nautical miles; glide ratio as "X:1"; glide angle in degrees, radians and tangent.
Quad chart set
GR vs altitude lost, HD vs altitude lost, glide angle vs glide ratio, and a required-altitude comparison bar.
Reference tables
Typical glide ratios by aircraft, sink rate vs forward speed, distance achievable by altitude and glide ratio, and a full variable reference.
Reference Tables & Conversions
A distance converter and the flight-performance tables you need for real planning.
Distance unit converter
Typical Glide Ratios by Aircraft
Representative best-glide ratios for a range of unpowered and engine-off flight configurations. Actual values depend on weight, configuration, speed and conditions.
| Aircraft / object | Best glide ratio | Glide angle | Sink at best glide | Notes |
|---|---|---|---|---|
| Wingsuit (with parachute) | 2.5 – 3.5 | 16 – 22° | 50 – 90 km/h | Body-flight suit, high descent rate |
| Paper airplane (dart) | 3 – 5 | 11 – 18° | ~1–2 m/s at 5 m/s | Ideal small glider |
| Flying squirrel (natural) | 1.5 – 2.5 | 22 – 34° | Falls faster than it glides | Patagium only |
| Paraglider (EN-B) | 8 – 10 | 5.7 – 7.1° | 1.0 – 1.3 m/s | Ram-air canopy, no rigid structure |
| Hang glider (kingpost) | 10 – 13 | 4.4 – 5.7° | 0.8 – 1.1 m/s | Rigid wing, prone pilot |
| Hang glider (rigid) | 15 – 20 | 2.9 – 3.8° | 0.7 – 0.9 m/s | Modern topless wing |
| Cessna 172 (engine off, prop feathered) | 9 – 11 | 5.2 – 6.3° | 3.0 – 3.7 m/s | Best glide speed ≈ 65 KIAS |
| Piper PA-28 (engine off) | 10 – 12 | 4.8 – 5.7° | 3.0 – 3.5 m/s | Similar to Cessna |
| Boeing 737-800 (all engines out) | 15 – 18 | 3.2 – 3.8° | ~15 m/s | Best glide ≈ 220 KIAS at cruise weight |
| Airbus A320 (all engines out) | 15 – 18 | 3.2 – 3.8° | ~15 m/s | Similar to 737 |
| Training sailplane (ASK-21) | 34 – 38 | 1.5 – 1.7° | 0.65 – 0.75 m/s | Two-seat club standard |
| Standard-class sailplane (LS8) | 44 – 46 | 1.25 – 1.30° | 0.60 – 0.65 m/s | 15 m span, water ballast |
| 15-m racing class (Ventus) | 46 – 48 | 1.19 – 1.25° | 0.58 – 0.62 m/s | Competition standard |
| 18-m class (ASG-29) | 52 – 55 | 1.04 – 1.10° | 0.52 – 0.56 m/s | High-performance competition |
| 20-m class (ASG-29E, Arcus) | 55 – 58 | 0.99 – 1.04° | 0.48 – 0.52 m/s | Open-class adjacent |
| Open class (ASH-25, Eta) | 58 – 70 | 0.82 – 0.99° | 0.42 – 0.50 m/s | Custom-built, up to 30 m span |
| Space Shuttle (hypersonic) | 1 – 2 | 27 – 45° | Depends on speed | Steep reentry profile |
| Space Shuttle (final approach) | 4 – 4.5 | 12.5 – 14° | ~50 m/s | Steep "unpowered airliner" |
Distance Achievable from Altitude
Horizontal distance (km) for a range of glide ratios and starting altitudes.
| Altitude lost | GR = 5 | GR = 10 | GR = 20 | GR = 40 | GR = 60 |
|---|
Variable Reference
| Symbol | Meaning | Typical range | Units |
|---|---|---|---|
| GR | Glide ratio (L/D) | 1 – 70 | dimensionless |
| HD | Horizontal distance | 10² – 10⁶ | m or km |
| AL | Altitude lost | 10 – 10⁵ | m |
| θ | Glide angle below horizontal | 0.5 – 45 | degrees |
| L/D | Lift-to-drag ratio | 1 – 70 | dimensionless |
| Vfwd | Forward airspeed (TAS) | 15 – 300 | m/s |
| Vsink | Sink rate = Vfwd / GR | 0.3 – 50 | m/s |
| Vground | Ground speed = Vfwd − Vwind | can be negative | m/s |
| T | Flight time = AL / Vsink | 10 – 10⁴ | s |
What Does GR = L/D = HD / AL Measure?
The glide ratio is the single most important performance number for any unpowered flight. It says how far an aircraft will travel horizontally for every metre it descends, and it is numerically equal to three different things that always coincide in a steady glide: the ratio of horizontal distance to altitude lost (the geometric definition), the ratio of lift to drag (the aerodynamic definition), and the reciprocal of the tangent of the glide angle (the trigonometric definition). A glide ratio of 40 means the aircraft travels 40 metres forward for every metre down, sinks at 1 metre per second when flying at 40 metres per second, and descends along a path 1.43° below the horizontal.
Glide Ratio Formula
The three equivalent definitions are:
Rearranged to solve for the other three unknowns:
AL = HD / GR
θ = arctan( AL / HD ) = arctan( 1 / GR )
The sink rate and flight time follow from the forward speed:
T = AL / Vsink = AL · GR / Vfwd
The effective ground glide ratio in the presence of wind is:
Glide Ratio Variables
| Symbol | Meaning | Notes |
|---|---|---|
| GR | Glide ratio | Dimensionless; equals L/D in steady unpowered flight |
| HD | Horizontal distance | Ground distance covered during the glide |
| AL | Altitude lost | Vertical height lost during the glide |
| θ | Glide angle below horizontal | θ = arctan(1/GR); for small angles θ(deg) ≈ 57.3/GR |
| L/D | Lift-to-drag ratio | Same numerical value as GR in a steady glide |
| Vfwd | Forward airspeed | Best-glide speed maximises GR |
How Each Variable Affects the Glide
The relation is exactly linear in HD and inverse in AL. Doubling the horizontal distance doubles the glide ratio for the same altitude loss; doubling the altitude lost halves the glide ratio for the same distance. The glide angle is a nonlinear function of GR — for large GR the angle is very small and changes very slowly (going from GR = 40 to GR = 80 halves the angle from 1.43° to 0.72°), while for small GR the angle changes rapidly (going from GR = 2 to GR = 1 changes the angle from 26.6° to 45°). This is why glider performance is often quoted in GR rather than in angle — the GR number is more discriminating for high-performance aircraft.
In practice the glide ratio of a given aircraft depends on its configuration and airspeed. A sailplane might achieve 46:1 at 100 km/h with full water ballast, but only 40:1 at 90 km/h without ballast, and only 30:1 with the undercarriage extended. An airliner in an all-engines-out glide has a ratio of about 17:1 at its best glide speed of 220 knots, but this drops to 10:1 if the windmilling engines add drag and the flaps are not retracted. The pilot's job after an engine failure is to fly at the correct best-glide speed and configure the aircraft to minimise drag — everything else follows from the GR number.
Glide Ratio Calculation Examples
GR = 15,000 / 1,000 = 15.0 : 1
Glide angle: arctan(1/15) = 3.81°
HD = 40 × 1,000 = 40,000 m = 40 km
AL = 30,000 / 40 = 750 m
tan θ = 500 / 10,000 = 0.05
θ = arctan(0.05) = 2.862° = 0.04996 rad
Distance: 10 × 2,000 = 20,000 m = 20 km
Sink rate: 33.4 / 10 = 3.34 m/s
Flight time: 2,000 / 3.34 = 599 s = 9.98 min
Air distance from 1,000 m: 40 km
Ground speed: 27.8 − 5.6 = 22.2 m/s
Flight time: 1,000 / (27.8/40) = 1,439 s = 24 min
Ground distance: 22.2 × 1,439 = 31,900 m ≈ 31.9 km
Effective ground GR: 31.9 / 1.0 = ≈ 32 : 1 (down from 40:1)
Distance: 17 × 11,000 = 187,000 m = 187 km
Sink rate: 113 / 17 = 6.65 m/s
Flight time: 11,000 / 6.65 = 1,654 s = 27.6 min
Frequently Asked Questions
In a steady unpowered glide, the aircraft is in equilibrium — no net force, no acceleration. The forces on the aircraft are lift (perpendicular to the flight path), drag (opposite to the flight path), and weight (vertically downward). Resolving forces along and perpendicular to the flight path: the component of weight along the path equals drag, and the component of weight perpendicular to the path equals lift. If the glide angle is θ below the horizontal, the weight along the path is W·sin θ and the weight perpendicular to the path is W·cos θ. So drag = W·sin θ and lift = W·cos θ. Dividing, L/D = cos θ / sin θ = 1/tan θ = GR. In other words, glide ratio and lift-to-drag ratio are exactly the same number in a steady unpowered glide — an aerodynamically efficient wing (high L/D) glides far, and a draggy wing (low L/D) glides poorly. This is why glider designers obsess over L/D.
Best-glide speed (Vbg, also called best-L/D speed) is the airspeed at which the aircraft achieves its maximum glide ratio. It is the speed you fly after an engine failure to reach the furthest possible landing point, and it is the single most important number in the emergency checklist. For most light aircraft, Vbg is around 65–75 KIAS; for a Cessna 172 it is 65 KIAS; for a Piper PA-28 it is 76 KIAS; for an airliner it is around 220 KIAS at cruise weight. Flying faster than Vbg increases drag and steepens the glide; flying slower increases the angle of attack, also increasing drag and steepening the glide. Only at Vbg is the glide ratio maximised. The speed is usually published as a red radial line on the airspeed indicator, and every pilot should memorise it for their type.
Surprisingly, no — the best-glide RATIO is essentially independent of weight for a given aircraft configuration. A heavier sailplane has the same L/D as a lighter one, because the extra weight increases both the lift required and the induced drag, but the ratio between them stays the same. What weight does change is the best-glide SPEED: a heavier aircraft must fly faster to generate the required lift at the optimum angle of attack, so Vbg increases as the square root of the weight. A sailplane loaded with 200 kg of water ballast will fly its best glide about 15% faster than the same sailplane empty, and it will descend correspondingly faster, but it will still cover the same horizontal distance from the same altitude. This is why competition pilots use water ballast — they get a faster cruise between thermals without sacrificing glide ratio. The exception is when the extra weight pushes the wing into a different drag regime (very high wing loading causing shock stall, for instance), in which case the L/D can drop.
Wind changes the glide ratio over the ground but not the glide ratio through the air. The air glide ratio is a fixed aerodynamic property of the aircraft at a given speed; the ground glide ratio depends on the wind component along the flight path. With a headwind of Vh, the ground speed is (Vfwd − Vh), and the effective ground glide ratio becomes GR·(Vfwd − Vh)/Vfwd. With a tailwind, the effective ratio increases. If the headwind exceeds the forward speed, the ground glide ratio becomes negative and the aircraft drifts backwards over the ground while still gliding forwards through the air — a state familiar to every paraglider pilot on a windy day. The "speed-to-fly" theory, developed by Paul MacCready, tells the pilot what speed to fly to maximise cross-country speed in any combination of wind and expected lift; the optimal speed rises with headwind and falls with tailwind.
Modern airliners have a best glide ratio of about 15:1 to 18:1 with all engines out and the propellers or fan blades windmilling. A Boeing 737-800 achieves around 17:1 at its best-glide speed of approximately 220 KIAS at cruise weight, which means that from 11,000 m (36,000 ft) it can glide about 187 km — roughly 100 nautical miles. An Airbus A320 is similar. This is actually quite good compared to a light aircraft, partly because airliners have very high aspect-ratio wings and fly at high Reynolds numbers, and partly because they are descending at a very high forward speed (a 17:1 glide at 220 KIAS gives a sink rate of about 6.6 m/s, which is fast). The famous "Gimli Glider" incident in 1983, when an Air Canada 767 ran out of fuel at 12,500 m and glided to a landing at Gimli, demonstrated that the 17:1 figure is achievable in practice — the aircraft covered about 100 km before touching down.
Best-glide speed maximises the horizontal distance covered per unit altitude lost, while minimum-sink speed minimises the vertical descent rate. Best-glide speed is always faster than minimum-sink speed by about 15–25% for typical aircraft. If you are trying to reach a landing point far away, fly at best-glide speed — you will cover the maximum distance before running out of altitude. If you are trying to stay in the air as long as possible (for example, in a thermal or waiting for a rescue), fly at minimum-sink speed — you will descend slowly and have more time. For a training sailplane like an ASK-21, best glide is about 90 km/h and minimum sink is about 75 km/h. Both numbers are published in the flight manual, and both are useful in different situations. Glider pilots switch between them constantly depending on what they are trying to achieve.
Yes, and the variation is what defines the performance envelope of a glider. Every aircraft has a specific best-glide speed at which the L/D is maximised, and the glide ratio falls off on either side. Flying too slowly increases the induced drag (because the wing must operate at a higher angle of attack to generate the required lift) and the glide ratio drops. Flying too fast increases the parasite drag (because drag rises with the square of speed) and the glide ratio drops again. The full relationship between sink rate and airspeed is called the "speed polar" or "sink polar", and it is measured by the manufacturer during flight testing. Modern gliders publish their polar so that pilots can compute optimal speeds for cross-country flying using a flight computer or a MacCready ring.
The basic calculation is AL = HD / GR — divide the remaining distance by the glide ratio. If the airfield is 30 km away and the glider has a glide ratio of 40:1, the theoretical altitude needed is 30,000/40 = 750 m above the airfield. But no competent pilot flies on that number alone. Standard practice is to add a safety margin of 30–50% (so 1,000–1,150 m instead of 750 m), subtract any altitude expected from lift along the route, and then adjust for wind. A 20 km/h headwind reduces the effective glide ratio from 40 to about 32, so the same 30 km final glide would require 940 m instead of 750 m. Glider pilots use flight computers that continuously recompute the final-glide altitude as they fly, using the current wind, the expected sink, and the McReady speed ring. Pilots who ignore these computations occasionally land out in a field — or worse, in a tree.
Variables
| Symbol | Quantity | Unit |
|---|---|---|
| GR | Glide ratio | |
| x | Horizontal distance | m |
| h | Altitude lost | m |
What it means
The glide ratio (also called lift‑to‑drag ratio, L/D) is a key performance metric for aircraft, gliders, and birds. It is defined as the ratio of the horizontal distance an object can travel to the vertical distance it descends in still air. Mathematically, it equals the lift divided by the drag (L/D). A higher glide ratio means better aerodynamic efficiency; for example, a modern glider may have L/D ≈ 60:1, while a typical small aircraft has about 10:1. The glide ratio determines the maximum range achievable without engine power. It is also used in aviation to predict the aircraft’s sink rate and to plan emergency landings. The formula connects to the aircraft’s polar curves and is influenced by wing design, aspect ratio, and airspeed. Understanding glide ratio is essential for pilots and aircraft designers to optimise performance and safety. In nature, it explains the soaring capabilities of birds and the design of efficient wing profiles.
Worked example
Glide Ratio – Two Examples
Real‑World| Parameter | Value |
|---|---|
| Horizontal | 15,000 m |
| Vertical | 1,000 m |
| Parameter | Value |
|---|---|
| Horizontal | 8,000 m |
| Vertical | 500 m |
Common mistakes
- Lift and drag: L/D is the ratio of lift to drag – higher is better. Do not confuse glide ratio with descent angle.
- Units: Glide ratio is dimensionless; the horizontal distance and altitude lost must be in the same units.
- Best glide speed: The maximum glide ratio occurs at a specific airspeed; flying faster or slower reduces the ratio.
- Wind effects: Glide ratio over ground is affected by headwind/tailwind; the air‑mass glide ratio (L/D) remains the same.
- Aircraft configuration: Glide ratio changes with flaps, gear, and engine settings – use the clean configuration for best glide.
Applications
The glide ratio, defined as the ratio of lift to drag (L/D) or horizontal distance per unit altitude loss, is a key aerodynamic performance metric for aircraft and gliders. It indicates how efficiently an aircraft can convert potential energy into horizontal motion without engine power. A higher glide ratio means greater range for a given altitude. This parameter is crucial in aircraft design for optimising wing and airfoil shapes, as well as for mission planning in emergency scenarios (e.g., engine‑out landings). Glide ratio is also used in the design of unpowered vehicles such as hang gliders, paragliders, and even birds. Engineers and pilots use it to calculate the maximum glide distance and to determine optimal descent profiles. Understanding glide ratio helps in improving fuel efficiency and extending the operational range of aircraft.
- Aircraft performance analysis and design optimisation
- Glider and sailplane design
- Emergency landing planning (engine‑out situations)
- Design of high‑efficiency wings and airfoils
- Bird flight and biomimetic design studies
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