Distance & Bearing Calculator

What is great-circle distance?

The By Air option on this tool reports the great-circle distance: the shortest possible path between two points measured along the surface of a sphere, rather than through it. Picture the Earth as a globe and stretch a piece of string taut between two pins — the string traces a great circle, the largest circle you can draw on a sphere's surface that passes through both points. That's fundamentally different from a driving distance, which follows roads, junctions and whatever detours the road network imposes, and is almost always longer than the great-circle figure. See Distance Between Two UK Postcodes for a worked example using postcodes directly.

The Haversine formula vs. driving distance

To calculate that distance, this tool uses the haversine formula — a standard piece of spherical trigonometry that takes two latitude/longitude pairs and returns the arc length between them along the surface of a sphere approximating the Earth. It's the same underlying maths used across navigation, aviation and mapping software wherever a fast, accurate straight-line distance is needed without the overhead of routing through an actual road network or flight path. A driving-distance API, by contrast, has to solve a much harder problem — real roads, junctions, one-way streets and traffic — so the two numbers answer different questions: haversine tells you how far apart two points are; a routing engine tells you how far you'd actually have to travel to get between them. The By Road options above (driving, walking, cycling) use a real routing engine rather than haversine, so they return an actual travel distance and time along real streets and paths — and the map draws that actual route, not just a straight line between the two points.

Why a straight line looks curved on a flat map

If you've ever noticed a flight path on an in-flight map display arc northward instead of running straight across a flat world map, that's the great circle showing itself. Standard web maps, including the one on this page, use a flat, rectangular projection to display a curved surface, and that projection distorts distance and direction the further you get from the equator. A straight line drawn on that flat projection is not the shortest real-world path — the genuinely shortest path, the great circle, would only look straight if drawn on an actual globe. The Earth also isn't a perfect sphere; it's very slightly flattened at the poles into what's technically an oblate spheroid. For most everyday distances, though, the sphere-based haversine formula is accurate to within a small fraction of a percent — more than close enough for comparing routes, checking delivery radii, or working out how far apart two addresses really are.