Depending on the exact coating, you can use the density of the material to figure out the thickness.
Zinc has a density of 7.14 g/cm3 so you can first convert to grams per cubic metre using this page:
https://www.convertunits.com/from/7.14+g/cc/to/grams/cubic+meter
So the density of Zinc is 7,140,000 g/m^3.
If you have a 1 meter thickness, then you would cover 7,140,000 grams per square metre.
Next convert 1 meter to microns:
https://www.convertunits.com/from/1+meter/to/micron
So 1 meter = 1,000,000 microns.
That means a coating with a thickness of 1,000,000 microns gives you 7,140,000 g/m^2 coverage. So then a 1 micron thickness translates to 7.14 g/m^2.
Conversely, for every 1 gram per square metre of coating, you get 0.14 microns of thickness:
1 gram per square metre = 0.14 microns thick
2 grams per square metre = 0.28 microns
3 grams per square metre = 0.42 microns
4 grams per square metre = 0.56 microns
and so on...
In general, you divide the grams per square metre by the density of the substance in g/cm3 to get the thickness in microns.
Measure the diameter by micrometer, strip the zinc using inhibited hydrochloric acid, then remeasure. A more accurate method involving weighing of a measured length is given in ASTM A90/A90M (which also gives the stripping solution).
Galvanised coatings:
They say the coating mass is usually between 100 g/m2 to 175 g/m2, so if we assume pure Zinc with a density of 7.14 g/cm2, then the thickness would vary from 14 microns to 24.5 microns.