When we, geographers, gis developers and map geeks alike are using some data in Universal Transversal Mercator or with unprojected data, this question arises:

Which UTM zone are we talking about?

Well, there are some interesting calculations that are fast to do and will help us discover some things (like central meridian, utm zone, etc).

How to get the central meridian based on the UTM zone?

CM = 183 - 6 * Z
example
CM = 183 - 6 * 22
CM = 183 - 132
CM = 51

Heres the proof: http://spatialreference.org/ref/epsg/29192/ogcwkt/ . This is a projection suited for use in Brazil, describing the UTM zone 22S. Check the central meridian parameter: -51.

To find out the boundaries of that utm zone, add 3º to the central meridian and subtract 3º to it (-54 and -48).

Calculate central meridian based on longitude

CM = (6 * int(longitude/6)) + 3
CM = (6 * int(-48/6)) + 3
CM = (6 * -8) + 3
CM = -45

This one is useful, since from the central meridian you can easily find out the zone.

Calculate UTM zone from central meridian

UTMZ = 30 - abs(int(CM/6))
UTMZ = 30 - abs(int(-45/6))
UTMZ = 30 - abs(int(-7,5))
UTMZ = 23

I hope this calculations help everyone navigate throught the world of cartography and GIS data!