Small m or big M?

The abbreviation m is used as a suffix after a number, and the SI prefix M is used before a unit of measure. The expressions 50m tonnes and 50 Mt are thus equivalent, but the latter is always preferred. The suffix m (abbreviation) and prefix M (SI symbol) are not interchangeable.

The suffixes are all lower case: k for thousand, m for million, and b for billion—for example, 300k, $75m, and 2.2b. There is no space between the number and the suffix (50 m is 50 metres not 50 million). The SI prefixes can be lower or upper case: m for milli-, k for kilo-, M for mega-, G for giga-, T for tera-, etc.—for example, mL, kW, Mt, GPa, and TJ.

In particular, never use a numerical suffix in place of an SI prefix. The unit of tonne (t) is an SI unit, and so the correct SI symbol for billion tonnes is Gt, never Bt nor bt. If you don’t want to use the SI expression gigatonnes, then just spell out billion tonnes and don’t abbreviate it.

The SI prefixes are also commonly used as prefixes to non-SI units, such as Mtpa (same as the SI-compliant Mt/yr), Ma (geological age), and Moz (gold reserves). But only use with commonly used abbreviations and always define on first use.

For a complete list, see Formatting>Abbreviations and symbols in the Doxical Style Guide for Miners and Geoscientists.