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One was widely treated as prime into the early twentieth century. It was excluded so that every integer factors into primes in exactly one way, which is a decision about what makes the rest of arithmetic work rather than a discovery about the number.

The number of seconds in a lifetime of 32 years is roughly 1 billion. A billion seconds is 31 years and eight months, which is the easiest way to feel the gap between a million and a billion: a million seconds is eleven and a half days.

There are over 1 billion tonnes of maize harvested worldwide each year. Figure as of 2023. The largest cereal harvest of the three, though most of it is not eaten directly by people — the bulk goes to animal feed and to industrial uses including ethanol.

Being number one is the standard shorthand for market leadership, chart position and ranking. The convention that first is best is not universal: in some contexts a lower number denotes seniority and in others it denotes an entry-level grade.

The number of bits a boolean needs, and the number it usually occupies is 1 (worldwide, most languages). Almost every implementation gives a boolean a whole byte or more, because addressing a single bit costs more instructions than the seven bits save. Bit fields exist for the cases where the trade goes the other way.

There are 1 × 10²¹ bytes in a zettabyte (worldwide, decimal storage units). Storage vendors count in powers of ten and operating systems often count in powers of two, which is why a drive sold as one terabyte shows up as about 931 gigabytes.

There are 1 metre, defined since 1983 as the distance light travels in 1/299,792,458 of a second (worldwide, SI).

The number of bytes in a terabyte is 1 trillion. On the decimal definition standards bodies use. The binary equivalent is a tebibyte, which is about ten per cent larger.

The safety factor a structure is designed to exceed is greater than 1 (worldwide, structural engineering). A factor of 1.0 means the structure fails exactly at its design load. Real factors run from about 1.4 for well-understood loads in steel to 10 or more where the material is variable or the consequence of failure is severe.

There are more than 1 billion cars in use. Figure as of 2023. About 1.5 billion motor vehicles of all kinds, counting lorries and buses.

There are about 1 × 10⁷⁷ possible outputs of the SHA-256 hash function (worldwide, cryptography). Exactly 2²⁵⁶, which is close to the estimated number of atoms in the observable universe. That comparison is the entire security argument.

There are more than 1 billion bicycles in use. Figure as of 2023. Roughly twice the number of cars, and the most widely owned vehicle in the world.

The speed of sound in dry air is about 1 thousand kilometres per hour (worldwide, at 20 °C at sea level). 1,235 km/h, or 343 metres per second. It depends on temperature rather than on air pressure, which is why it is slower high up.

There are 1 horn on a rhinoceros of the Indian and Javan species (South and South-East Asia). African white and black rhinoceroses have two.

There are roughly 1 million Earths that would fit inside the Sun by volume (the Solar System). By volume, not by mass. Packed as spheres the figure is nearer 960,000, and the round million assumes they can be poured.

There are over 1 million described species of insect. More than half of all described species of any kind are insects, and beetles alone account for about 400,000.

The number of hydrogen atoms across the diameter of a human hair is roughly 1 × 10⁶. A hair is about 70 micrometres across and a hydrogen atom about a tenth of a nanometre.

The number of nanometres in a metre is 1 billion (worldwide, SI). Visible light spans about 380 to 700 of them, so the unit is the natural one for optics.

There are more than 1 million asteroids with catalogued orbits (the Solar System). Figure as of 2024. About 1.3 million, nearly all of them found by automated sky surveys since the 1990s.

There are roughly 1 × 10⁸⁰ atoms of ordinary matter (the observable universe). An order-of-magnitude estimate, not a count. It is derived from the measured density of ordinary matter and the size of the observable universe, and the exponent is the part worth trusting. Often used as the point where a number stops having a physical referent: a googol, 10¹⁰⁰, is larger than this by twenty orders of magnitude.

The diameter of the Sun is nearly 1 million kilometres (the Solar System). 1.39 million kilometres, about 109 times the Earth's diameter.

There are about 1 million nephrons in a human kidney (adults, per kidney). Counts vary roughly threefold between individuals, from about 200,000 to over 2 million, and the number is fixed before birth — lost nephrons are never replaced.

There are roughly 1 × 10³⁰ bacteria on Earth. An order-of-magnitude estimate from sampling soil, ocean and subsurface habitats and extrapolating. The exponent is the part worth trusting. More bacteria than there are stars in the observable universe, by a factor of billions.

The energy released by a lightning strike is about 1 billion joules. Strikes vary by more than an order of magnitude, so this is a central figure rather than a typical one. It is roughly the chemical energy in thirty litres of petrol, delivered in a few thousandths of a second.

There are roughly 1 × 10¹⁹ atoms in a grain of sand (worldwide, a half-millimetre quartz grain). An order-of-magnitude figure: grains vary in size by a factor of ten, and the count varies with the cube of that.

The density of fresh water is 1 thousand kilograms per cubic metre (worldwide, at 4 °C). Exactly 1,000 by the original definition of the kilogram, which was the mass of a litre of water. The modern definition comes from the Planck constant instead, and the round number survives as a very close approximation.

1 is the atomic number of hydrogen, the lightest and most abundant chemical element in the universe.

A 1-year anniversary is named for paper in the modern gift lists. From the modern gift lists rather than from any old tradition. The lists themselves disagree — American ones often give clocks — and both were substantially extended by the jewellery and gift trades in the twentieth century.

1 January is New Year's Day, observed in most countries using the Gregorian calendar; other calendars begin their year on different dates. 1 January became the start of the Roman year in 153 BCE and was fixed as the start of the Gregorian year by the 1582 reform. Adoption was uneven — Britain and its colonies kept 25 March as the legal start of the year until 1752 — and lunisolar calendars such as the Chinese, Islamic and Hebrew ones begin their years elsewhere entirely.

1 May is May Day / International Workers' Day, observed in widely in Europe and elsewhere as a labour holiday; separately as a spring festival in parts of Europe. Two distinct observances share the date. International Workers' Day commemorates the 1886 Haymarket affair in Chicago and is a public holiday across much of Europe, Latin America and Asia. Older European spring festivals on the same date are unrelated in origin. The United States, where the commemorated events took place, marks Labor Day in September instead.

In English the indefinite article 'a'/'an' descends from the Old English word for one, ān — the same root behind 'alone' (all-one) and 'atone' (at-one). The unrelated superlative 'number one', meaning best or most important, is a much later idiom.

In Sikhism, the Guru Granth Sahib opens with the numeral one, read as Ik Onkar. The scripture begins with a digit rather than a word. Writing the oneness of God as the numeral itself, before any language, is generally read as deliberate. This is a scholarly account rather than the tradition's own framing.

The number of the amendment protecting speech, religion, press, assembly and petition is 1 (United States). Its position is not a ranking: it was third in the original list of twelve sent to the states, and became first only because the two ahead of it were not ratified at the time.

The number of basis points that make one hundredth of a per cent is 1 (worldwide, financial markets). A basis point exists because 'a quarter of a per cent' and 'a quarter per cent' are ambiguous in speech and money moves on speech. Twenty-five basis points is unambiguous.

The rise in global mean surface temperature since the late nineteenth century is about 1 degree Celsius (global). The figure is an average over a warming that is far from even: land has warmed faster than ocean, and the Arctic faster than anywhere.

There are over 1 billion lightning flashes over the Earth each year. Something like forty a second, concentrated overwhelmingly over land and in the tropics. Lake Maracaibo in Venezuela has more than anywhere else on Earth.

1 is the multiplicative identity: multiplying any number by 1 leaves it unchanged. By modern convention it is classified as neither prime nor composite — a prime must have exactly two distinct positive divisors, and 1 has only one.