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.
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.
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.
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.
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.
Estimate only. Results reflect your inputs and standard formulas. Double-check important decisions independently.