Calipers are not the best way to measure something that small- a micrometer would be better. I'll assume you're in North America- AWG-land.
So from the diameter, the total cross-sectional area of the wire is about 0.57mm^2, so your wire is about AWG 20 (0.51mm^2)- as listed in the linked wire table. You can't expect a really accurate answer without an accurate measurement, and even then there will be some variations.
If you don't have the information that's typically printed on the wire you still don't have enough information to confidently use it in many applications- the voltage and temperature rating of the insulation, any safety approvals, and the gauge are all important, and sometimes other things are important too, such as the type of insulation used.