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@clacke

There are some calculators online, but the equation isn't too bad as long as you're using a spheroid and not a geoid.

Great Circle Distance

d = r⋅arccos(sin(ϕ1)⋅sin(ϕ2)+cos(ϕ1)⋅cos(ϕ2)⋅cos(Δλ))

where r is the radius of the sphere, ϕ1 and ϕ2 are the latitudes of the two points, and Δλ is the difference in their longitudes.