EPSG:4326 vs EPSG:3857
The two most common coordinate systems in web mapping, and the single most common reason your data ends up in the wrong place. Here is the difference in plain English.
The short answer
EPSG:4326 stores positions as latitude and longitude in degrees on the curved surface of the Earth. EPSG:3857 stores positions as X and Y in metres on a flat map. They describe the same places using completely different numbers, so mixing them up shifts your data, sometimes by thousands of kilometres.
Rule of thumb: store and share data in EPSG:4326, draw it on a web map in EPSG:3857. Both use the WGS84 datum, so converting between them never changes the datum, only how the position is written.
Side by side
EPSG:4326 (WGS84)
- • Geographic (unprojected) system
- • Units: degrees
- • Latitude: -90° to +90°
- • Longitude: -180° to +180°
- • Example: 37.7749, -122.4194
- • Used by GPS, GeoJSON, KML, GPS exchange
EPSG:3857 (Web Mercator)
- • Projected (flattened) system
- • Units: metres
- • Y: roughly ±20,037,508
- • X: roughly ±20,037,508
- • Example: -13,627,361, 4,547,665
- • Used by Google Maps, OSM, Leaflet, Mapbox tiles
Both coordinates above point to the same spot in San Francisco. The numbers look nothing alike because one is measured in degrees and the other in metres.
Why the same place has two sets of numbers
EPSG:4326 describes a position as an angle: how far north or south of the equator (latitude) and how far east or west of the prime meridian (longitude). This is great for storing a location anywhere on Earth, but degrees are not a flat unit, so you cannot draw them directly on a screen without stretching the map.
EPSG:3857 takes those angles and projects them onto a flat square using the Web Mercator formula, giving an X and Y value in metres from the centre of the map. That flat grid is exactly what a screen needs, which is why almost every slippy web map renders in 3857.
Worth knowing: Web Mercator only covers latitudes between about -85.06° and +85.06°. The true poles stretch to infinity, so they are simply cut off. That is why you never see the very top or bottom of the world on a standard web map.
Common mistakes (and how to spot them)
Your data sits in the ocean near Africa
Coordinates collapsed to 0, 0 (known as Null Island). Usually the projection was not read and degrees were treated as metres.
Everything is squashed into a tiny dot
3857 metre values (millions) were loaded as if they were 4326 degrees (under 180), so the whole dataset shrinks to a point.
The map is rotated or mirrored
Latitude and longitude were swapped. EPSG:4326 officially lists latitude first, but GeoJSON and many tools expect longitude first. See latitude or longitude first?
Are they interchangeable?
No. You cannot rename a 4326 file to 3857 (or the reverse) and expect it to work. The actual numbers have to be transformed. The good news is that the transform is exact and reversible because both systems share the WGS84 datum, so no accuracy is lost.
Use the file reprojection tool to convert a whole GeoJSON, Shapefile or KML between the two, or the point converter to check a single coordinate.
Convert between EPSG:4326 and EPSG:3857
Two free tools that work entirely in your browser. No signup, no upload.
Frequently asked questions
Is EPSG:3857 the same as Web Mercator?
Yes. EPSG:3857 is the official code for the system commonly called Web Mercator or Pseudo-Mercator. You may also see the older codes EPSG:900913 and EPSG:3785 referring to the same thing.
Which should I use for a web map?
Most web map libraries display tiles in EPSG:3857 but accept your data in EPSG:4326 and convert on the fly. So you usually store data in 4326 and let the map handle the projection to 3857.
Why does my data appear off the coast of Africa?
That spot at 0, 0 is called Null Island. It almost always means the coordinate system was not recognised, so values were read as zero or as the wrong unit. Confirm whether your data is in degrees (4326) or metres (3857).
Do I lose accuracy converting between them?
No. Both share the WGS84 datum, so the conversion is mathematically exact and reversible. The only practical limit is that 3857 cannot represent latitudes beyond about ±85.06°.