Zoom level to map scale calculator

The numbers come from the standard Web Mercator formula for 256 pixel tiles on a spherical earth:156543.033928 m/px at zoom 0 on the equator, each zoom step halving it, and the cosine of the latitude shrinking it further away from the equator. That sphere is what Google Maps, OpenStreetMap, and Leaflet all use, but the earth is not a sphere and most screens show tiles at 2 or 3 device pixels per CSS pixel, so treat the scale ratio as an approximation, not a printed-map scale.