Great Circle Distance Calculator
Show the math
What Your Result Means
- Great Circle (Shortest): The shortest distance between two points on the surface of a sphere, in miles. This is the path an aircraft would ideally follow (though real routes deviate for airspace, wind, and safety).
- Great Circle (km): The same distance in kilometers, for metric users and international flight planning.
- Flat Map Distance: A naïve Pythagorean distance using latitude/longitude as a flat grid with cosine correction for longitude. On short distances this is close; over long distances it diverges significantly from the true great circle.
- Difference: How much shorter the great circle is compared to the flat map approximation. Larger differences show where Earth's curvature matters most — typically on long east-west routes at high latitudes.
- Initial Bearing: The compass heading (0–360°, measured clockwise from north) at the origin point when departing along the great circle route. This bearing changes continuously along the path.
How This Calculator Works
You pick origin and destination airports from a searchable database of roughly 9,500 commercial airports worldwide — searchable by IATA code, ICAO code, name, or city. The tool reads the airport's published coordinates and applies the Haversine formula — which uses the law of haversines on a spherical Earth with radius 3,959 miles — to compute the central angle, then multiplies by the radius for distance. A flat-map distance is computed separately using cosine-corrected Euclidean math for comparison. The initial bearing is calculated using the atan2 forward-azimuth formula.
Quick Questions
How accurate is the Haversine formula?
For most practical purposes, Haversine is accurate to within about 0.3% of the true geodesic distance. The Earth is an oblate spheroid, not a perfect sphere, so the Vincenty formula or Karney's method gives slightly more precise results — but the difference is generally less than a few miles even on intercontinental routes.
Why does the flat-map distance differ from the great circle?
A flat map treats latitude and longitude as a rectangular grid, which works well over short distances but breaks down over long ones. Earth's curvature means the shortest path between two distant points curves toward the poles, making the great circle shorter than a straight line on a Mercator projection.
What is the initial bearing used for?
It's the compass heading you'd point at departure to follow the great circle route. On a great circle, the bearing changes continuously — you don't fly a constant heading. Navigators use initial bearing for departure planning and waypoint routing.
Do airlines actually fly great circle routes?
Airlines aim for great circle paths but deviate for jet stream winds, restricted airspace, ETOPS safety corridors, and ATC routing. The actual flight path is typically 5–15% longer than the theoretical great circle distance.
Sources
- Wikipedia — Haversine Formula (derivation and implementation)
- Movable Type — Calculate Distance, Bearing Between Lat/Long Points (bearing formula reference)
- OurAirports (public-domain airport database — IATA/ICAO codes and coordinates)
Method & review
Estimate only. Results reflect your inputs and standard formulas. Double-check important decisions independently.