How do you calculate how many volts are used to light a light bulb?

Normally that would be however many volts your house runs on. The US/Canada standard is 120 volts at 60 hertz, while most of Europe runs on 240 volts at 50 hertz. Keep in mind though there are all types of light bulbs that run on all voltages from 12 to 240. It should say the voltage on the bulb though? but if you live in North America and you are asking about a normal house-type bulb, your quest will most likely end with 120.