Math formula for transforming from EPSG:4326 to EPSG:3857

Projection EPSG:3857 uses the method EPSG:1024 for Forward and Reverse calculations.

The method is fully documented at Geomatics Guidance Note No 7, part 2

There's an mathematical example at page 40 where you can use to cross-reference your code.

17/11/2020 EDIT: The link to the above documentation from times to times, gets broken as things evolve. If that happens please feel free to update it as necessary.