What is a billion? And other large numbers

This guide is one in a series on different aspects of statistical literacy. The others can be found in the House of Commons Library's Good Information Toolkit.

After 10 (‘tens’), 100 (‘hundreds’), and 1,000 (‘thousands’), English gives a new name to numbers 1,000 times bigger than the last. It is also common in English to put commas between every three digits in a large number, which corresponds to these names (one thousand is written 1,000, 1 million is written 1,000,000).

But until the 20th century, British English used a different naming system, which led to confusion over what number ‘a billion’ referred to.

What is a billion?

In official UK statistics, ‘a billion’ is used to denote 1 thousand million (1,000,000,000).

In historical British English, ‘a billion’ was used to denote 1 million million: (1,000,000,000,000). This definition of a billion is now known as the ‘long scale’, where each new term for a number above a million is 1 million times greater than the previous one.

By contrast, in American English, ‘a billion’ has long been used to refer to 1 thousand million. This definition of a billion is now known as the ‘short scale’, where each new term for a number above a million is 1 thousand times greater than the previous one.

The Oxford English Dictionary explains why UK and US usage of ‘billion’ differed until relatively recently:

[Using a billion to refer to a million million] reflects a system (now often referred to as ‘the long scale’) in which each successive member of the series of numerals ending in -illion … shows an increase of a million times, while [using a billion to refer to a thousand million] reflects a system (now often referred to as ‘the short scale’) in which each successive member of the same series shows an increase of a thousand times. The usage in French gradually shifted from the long to the short scale over the course of the 18th and early 19th centuries, when it was adopted as the standard use in North American English …

In French usage subsequently shifted back to the long scale in the 20th [century], a thousand million usually being expressed instead by milliard. By contrast, in British English [using a billion to refer to a thousand million] became progressively more common from the mid 20th [century], with the short scale officially adopted in publications by the UK Government from 1974. [Using a billion to refer to a thousand million] is currently also the usual sense in other varieties of English.

In 1974, the then Prime Minister, Harold Wilson, confirmed in a written reply that the meaning of ‘billion’ would become ‘a thousand million’, in conformity with international usage:

Mr Maxwell-Hyslop asked the Prime Minister whether he will make it the practice of his administration that when Ministers employ the word “billion” in any official speeches, documents, or answers to Parliamentary Questions, they will, to avoid confusion, only do so in its British meaning of 1 million million and not in the sense used in the United States of America, which uses the term “billion” to mean 1,000 million.

The Prime Minister: No. The word “billion” is now used internationally to mean 1,000 million and it would be confusing if British Ministers were to use it in any other sense. I accept that it could still be interpreted in this country as 1 million million and I shall ask my colleagues to ensure that, if they do use it, there should be no ambiguity as to its meaning.

What about other very large and very small numbers?

The modern form of the metric system is the Système International d’Unités (SI), or International System of Units. This gives the standard set of base units of measurement (of which there are seven) as well as the standard prefixes to denote the size of the unit.

The table below gives a selection of other very large numbers and very small numbers, their (short scale) name, SI prefixes and symbols, and scientific notations.

The format of scientific notations is 10n, where ‘n’ is the total number of zeros (for example, 109 is 1,000,000,000, or 1 billion). Where ‘n’ is negative, the number is less than one and ‘n’ refers to the number of decimal places.

How are these numbers used in practice?

If we are looking at energy, where the basic unit is a watt hour (Wh), then 1 trillion watt hours is known as a terawatt hour (TWh). This is equivalent to 1012, or 1,000,000,000,000 watt hours. For context, the UK’s total electricity consumption in 2023 was 317 TWh.

If we are looking at distance, where the basic unit is a metre (m), then a billionth of a metre is known as a nanometre (nm). This is equivalent to 10-9, or 0.000000001 metres. The width of a strand of human DNA is around 2.5 nm.

Significant increases or decreases in scale can quickly become hard to fully comprehend, as can their cumulative impact. The 1977 short film Powers of Ten illustrates the effect of changes in scale from extremely large to extremely small dimensions.

Footnotes