Usually #14, which goes with the typical 15 amp fused circuit.
If you insist, you can measure a straight unkinked part of the wire with a micrometer although I myself don't have wire diameter versus gauge handy at this moment.
(One morning on WRKO Radio) The tub's not clean! Let's remodel the bathroom.
(One afternoon on WRKO Radio) It's time for an oil change. I'm going to trade in the car for next year's model.