What Is Web Mercator? The Projection Behind Web Maps

Web Mercator is a variant of the Mercator projection that most online maps use to draw the world on a square grid of tiles. It treats the Earth as a sphere for its calculations, which keeps the math fast and simple. Its popularity comes from convenience: it keeps local shapes and angles correct, which makes streets look right at any zoom level.

Mercator in brief

The original Mercator projection dates from the 16th century and was designed for sea navigation. Its useful property is that a line of constant compass bearing appears as a straight line on the map. To achieve this it keeps angles true locally, but stretches distances and areas more and more as latitude increases. The poles cannot be shown at all, because they would be infinitely far away.

What the "web" version changes

Web Mercator applies the Mercator formulas using a sphere with the Earth's radius, while the underlying coordinates (latitude and longitude on WGS 84) are defined on an ellipsoid. This shortcut gives slight differences from true Mercator, but it is fast to compute and is simple to tile. It became the standard for web maps because of that simplicity.

Another practical consequence: the projection cuts off at about 85 degrees north and south, so that the world fits a square. Anything beyond that is not displayed.

Why it suits web maps

• Local shapes are preserved, so a street corner looks like a street corner at high zoom.
• North is always up, and the grid lines are straight and square.
• It tiles neatly. The whole map divides into a regular pyramid of squares, as explained in how map tiles work and map zoom levels.
• It is widely supported by mapping libraries, so layers from different sources line up.

What it is not good for

• Comparing areas: countries at high latitudes look much larger than they really are.
• Measuring distance or area directly on the projected map: the scale changes with latitude. Use geographic formulas or a projection suited to the region instead.
• Polar regions: these are poorly represented or cut off.

Coordinates you read and coordinates drawn

The latitude and longitude you copy from a map are not Web Mercator values; they are geographic coordinates. The projection is only applied internally when drawing. Some tools expose projected metres in an EPSG:3857 system, which is a different set of numbers that looks nothing like degrees. If a value is far outside the range of -180 to 180 (often in the hundreds of thousands or millions), you are looking at projected metres, not latitude and longitude.

For the broader concept, see what a map projection is.