EPSG:4326 and EPSG:3857
EPSG:4326 is plain WGS 84 latitude and longitude in degrees — what GPS gives you. EPSG:3857 (also called Web Mercator, Pseudo-Mercator or Google Mercator) projects those degrees onto a square in metres, which is how Google Maps, Mapbox, OpenStreetMap and most tile services draw the world.
X = R × λ · Y = R × ln(tan(π/4 + φ/2)), with R = 6,378,137 m and angles in radians.
Paste X, Y in metres
Enter two large numbers such as -8238310.24, 4970071.58 and the converter recognises them as Web Mercator metres. X comes first. Valid values lie between about ±20,037,508 m — the edge of the square world map.
Do not measure in Web Mercator
Web Mercator stretches distances by 1 / cos(latitude): at 60° north a metre on the map is only half a metre on the ground. Use it for display and tile maths, not for distances or areas. For measurement use UTM or the distance tool, which works on the ellipsoid.
Limits
The projection is cut at about 85.051°N and S so the world fits a square. Points closer to the poles cannot be shown and are clamped.
Frequently asked questions
What is EPSG:3857?
EPSG:3857 is Web Mercator, the projected coordinate system used by most web maps. It gives X and Y in metres on a square map of the world.
How do I convert EPSG:3857 to latitude and longitude?
Paste the X and Y values. Longitude = X / 6378137 in radians; latitude = 2·atan(e^(Y/6378137)) − π/2. The converter does this and shows every other format too.
Is EPSG:3857 accurate for distances?
No. Its scale grows toward the poles, so distances and areas are exaggerated away from the equator.