Earth radius 6371 km; educational approximation.
Great-circle (orthodromic) distance on a sphere with Earth radius R = 6371 km using the haversine formula. Latitudes must be within ±90° and longitudes within ±180°; inputs are treated as decimal degrees.
This calculator uses plain decimal numbers. If a result looks wrong, check whether you used a comma instead of a dot (or vice versa) where the field expects one.