billionth

A billionth is the fraction 1 divided by a billion, equivalently 10−9. In decimal form it is 0.000000001.

In the short scale now commonly used in science and many English-speaking contexts, a billion equals 10^9, so a billionth is 10−9. In the historical long scale, once used in parts of Europe, a billion equaled 10^12, making a billionth 10−12. Modern technical usage typically adopts the short-scale value, while some general-language references may still reflect the long-scale convention.

The term is closely linked to SI prefixes. The prefix nano- denotes 10−9, so many everyday measurements reflect a billionth of a unit: a nanometer is one billionth of a meter, a nanosecond is one billionth of a second, and a nanoliter is one billionth of a liter.

Mathematically, a billionth can be expressed as the fraction 1/1,000,000,000, or as the decimal 0.000000001. As a percentage, it is 0.0000001%.

In science and engineering, billionths arise in contexts requiring very small scales, such as nanotechnology, semiconductor manufacturing, and measurements of concentration or time at micro- and nano-levels. The term is common in both theoretical discussions and practical descriptions, where precision to the nanometer or nanosecond is routinely specified.