UTM zone by city

UTM divides the world into 60 strips of longitude, each 6 degrees wide. The number is floor((longitude + 180) / 6) + 1, counting east from the antimeridian, so zone 1 starts at 180 degrees west and zone 60 ends back at it.

The EPSG code then follows from the hemisphere. On WGS 84 the northern zones are EPSG:326 followed by the two-digit zone, and the southern ones are EPSG:327 followed by it. Zone 43 north is EPSG:32643 and zone 43 south is EPSG:32743. They are different coordinate systems, not two views of one.

Each zone has its own central meridian, three degrees in from the western edge, where the scale factor is 0.9996. Scale is slightly too small there and slightly too large at the edges, which is the deliberate trade. On the central meridian that is one part in 2500, roughly 40 cm per kilometre; toward the edge the scale passes back through unity and reaches about one part in 1000 near the equator.

A zone is about 670 km wide at the equator and much narrower further north: at 52 degrees it is only around 410 km. Plenty of cities sit close enough to an edge that an ordinary project crosses it.

London is the clearest case in the table. It sits at 0.13 degrees west, which is about 9 km from the zone 30 and 31 boundary running down the prime meridian. Greenwich is in zone 31. A borough-wide survey that assumes zone 30 throughout is working across a zone edge, and eastings restart at each one.

Hyderabad is 52 km from the 44 and 43 boundary, and Delhi 77 km. Those are comfortable for a single site and tight for a corridor. The number is worth checking before the flight rather than after the processing.

The standard carries two exceptions, and a surprising number of online calculators implement only the formula and return the wrong zone for both.

Southwest Norway: between 56 and 64 degrees north, zone 32 is widened westward to start at 3 degrees east rather than 6, taking three degrees from zone 31 so the mainland coast stays in one zone. Bergen sits at 5.32 degrees east. The formula says zone 31; the correct answer is zone 32, EPSG:32632.

Svalbard: between 72 and 84 degrees north the odd zones are widened and zones 32, 34 and 36 are not used at all. Zone 31 covers 0 to 9 degrees east, zone 33 covers 9 to 21, zone 35 covers 21 to 33 and zone 37 covers 33 to 42. Ny-Alesund sits at 11.92 degrees east, so the formula says zone 32 - a zone that does not exist at that latitude - while the correct answer is zone 33.

The table below applies both exceptions, and the zone finder applies them to any point you give it.

Eastings restart in every zone, so two points in different zones can carry almost identical coordinates while being hundreds of kilometres apart. Nothing in a coordinate file warns you about this; the data simply folds over itself.

There are three honest ways out. Extend one zone past its nominal edge and accept the growing distortion, which is normal practice for a few tens of kilometres as long as it is written down. Split the deliverable at the boundary and hand over two datasets. Or use a national grid designed to span the whole country, which is exactly why the British National Grid, Lambert-93 and their equivalents exist.

What you must not do is let half the data sit in one zone and half in the other without saying so.

Every row here is a city centre, which is the right answer for the question people type and the wrong answer for a survey ten kilometres out of town. A city close to an edge may well have its suburbs on the other side.

Use the table to orient yourself, then put your actual coordinates into the zone finder. It takes a map click, typed coordinates or your current location and returns the zone, the EPSG code, the central meridian and the MGRS grid square for that exact point, exceptions included.