I needed to calculate a lot of distances between the points for my project, so I went ahead and tried to optimize the code, I have found here. On average in different browsers my new implementation runs 2 times faster than the most upvoted answer.

function distance(lat1, lon1, lat2, lon2) {
  const r = 6371; // km
  const p = Math.PI / 180;

  const a = 0.5 - Math.cos((lat2 - lat1) * p) / 2
                + Math.cos(lat1 * p) * Math.cos(lat2 * p) *
                  (1 - Math.cos((lon2 - lon1) * p)) / 2;

  return 2 * r * Math.asin(Math.sqrt(a));
}

You can play with my jsPerf and see the results here.

Recently I needed to do the same in python, so here is a python implementation:

from math import cos, asin, sqrt, pi

def distance(lat1, lon1, lat2, lon2):
    r = 6371 # km
    p = pi / 180

    a = 0.5 - cos((lat2-lat1)*p)/2 + cos(lat1*p) * cos(lat2*p) * (1-cos((lon2-lon1)*p))/2
    return 2 * r * asin(sqrt(a))

And for the sake of completeness: Haversine on Wikipedia.

The below function gives distance between two geocoordinates in miles

Copycreate function [dbo].[fnCalcDistanceMiles] (@Lat1 decimal(8,4), @Long1 decimal(8,4), @Lat2 decimal(8,4), @Long2 decimal(8,4))
returns decimal (8,4) as
begin
declare @d decimal(28,10)
-- Convert to radians
set @Lat1 = @Lat1 / 57.2958
set @Long1 = @Long1 / 57.2958
set @Lat2 = @Lat2 / 57.2958
set @Long2 = @Long2 / 57.2958
-- Calc distance
set @d = (Sin(@Lat1) * Sin(@Lat2)) + (Cos(@Lat1) * Cos(@Lat2) * Cos(@Long2 - @Long1))
-- Convert to miles
if @d <> 0
begin
set @d = 3958.75 * Atan(Sqrt(1 - power(@d, 2)) / @d);
end
return @d
end 

The below function gives distance between two geocoordinates in kilometres

CopyCREATE FUNCTION dbo.fnCalcDistanceKM(@lat1 FLOAT, @lat2 FLOAT, @lon1 FLOAT, @lon2 FLOAT)
RETURNS FLOAT 
AS
BEGIN

    RETURN ACOS(SIN(PI()*@lat1/180.0)*SIN(PI()*@lat2/180.0)+COS(PI()*@lat1/180.0)*COS(PI()*@lat2/180.0)*COS(PI()*@lon2/180.0-PI()*@lon1/180.0))*6371
END

The below function gives distance between two geocoordinates in kilometres using Geography data type which was introduced in sql server 2008

CopyDECLARE @g geography;
DECLARE @h geography;
SET @g = geography::STGeomFromText('LINESTRING(-122.360 47.656, -122.343 47.656)', 4326);
SET @h = geography::STGeomFromText('POINT(-122.34900 47.65100)', 4326);
SELECT @g.STDistance(@h);

Usage:

Copyselect [dbo].fnCalcDistanceKM

Reference: Ref1,Ref2