Is there any online tool for converting coordinates between different EPSG?
This one's pretty handy: http://cs2cs.mygeodata.eu/
Also useful for batch conversions.
To do a one-off projection, I usually use the geometry service on our ArcGIS Server's REST endpoint.
You could also use ArcGIS Online's Rest Endpoint, which is available here.
Here is a sample which shows how you can project from 4326 to 3857.
Please note that you need to comply with their Terms & Conditions.
Two other handy tools for future reference:
- twcc.fr (the world woordinate converter)
- epsg.io, very useful for both transformation, retrieving available coordinate systems, ...
And to find which coordinate system might be suitable for your needs : georepository.com