Straight-Line vs Driving Distance
London to Paris is 344 km in a straight line and about 460 km by road — both correct. Straight-line (great-circle) distance is the shortest path over the globe and what our distance pages show; road distance follows the network and runs 1.2–1.4× longer for most city pairs.
Two honest numbers
“How far is it?” has two defensible answers. The straight line — 'as the crow flies' — is the shortest path across Earth's surface between the two points, computed on the sphere (a great circle). The road distance is whatever the network offers: around estuaries, over passes, through interchanges. Neither is more true; they answer different questions, and the honest move — which our city distance pages make explicitly — is to label which one you're quoting.
Why flight paths look curved on a map
Flat maps lie a little by necessity: projecting a sphere onto a rectangle stretches the far north and south, so the genuinely shortest route between distant cities looks like a detour. Delhi–New York arcs near the pole not for scenery but because on the globe that is the straight line — stretch a string on any globe and it follows the flight path. The same effect at smaller scale is why our London–Paris line above bows gently rather than lying ruler-straight on the projected map.
The 1.3 rule of thumb
Transport researchers call the road-to-straight-line ratio the circuity or detour index, and it's remarkably consistent: 1.2–1.4 for most intercity pairs, with ~1.3 a solid default. Grid-connected plains sit near 1.2; mountains, fjords, and river crossings push past 1.5; and a ferry-less strait can make it infinite. So for a quick road estimate from any of our 1,000 distance pages: multiply by 1.3. For 344 km London–Paris that predicts ~447 km — pleasingly close to the real ~460 km via Calais.
When each number is the right one
Use straight-line when the rule or physics is point-to-point: delivery-radius and school-catchment eligibility (most 'within X km' rules mean the crow's version), antenna and drone range, running-crow comparisons, and the flight-time estimates on our pair pages. Use road distance for fuel, tolls, EV range and travel time — get it from any routing app. And for picking a meeting city between two places, neither: the geographic midpoint plus a real city nearby is exactly what the halfway-point tool computes.
Measure your own
The distance calculator gives the great-circle number and initial bearing between any two points — type places or click the map. Every city-pair page adds kilometers, miles and nautical miles, both bearings, the flight estimate and the time-zone gap. And if you're curious what a great circle looks like from your own position, drop a pin far away and watch the line bow: the sphere is always there under the map.