**Mega** (symbol **M**) is a SI prefix in the SI system of units denoting a factor of 10^{6}, ie one million; 1,000,000.

For example, a megawatt is 1,000,000 watts.

The prefix is sometimes applied in non-standard ways:

- one megaton (a unit often used in measuring the power of nuclear weapons) is 1,000,000 tons. The
*metric*tonne is 1,000 kg, so it is in fact this unit that should be called the*megagram*. - in computing: