How many joules are in one megajoule?

One megajoule is equivalent to one million joules. The prefix "mega-" in the metric system denotes a factor of one million (10^6). Therefore, when converting from megajoules to joules, you multiply the number of megajoules by 1,000,000.

For instance, if you have 1 megajoule and you want to express it in joules, you would calculate:

1 megajoule × 1,000,000 joules/megajoule = 1,000,000 joules.

Hence, the correct answer is that one megajoule equals 1,000,000 joules.