100

The temperature water boils at is 100 degrees Celsius (at one standard atmosphere). It is also a ceiling: food in boiling water cannot exceed it, which is why boiling browns nothing: the Maillard reaction needs about 140 degrees and frying and roasting reach it.

Turning 100 attracts formal recognition in several countries: the United Kingdom sends a card from the monarch on request, and the United States a greeting from the President. Japan long presented a silver cup to each new centenarian and switched to a cheaper one as numbers rose — a small illustration that the milestone is becoming less rare as life expectancy climbs.

A 'hundred' has not always meant 100. Germanic and Norse counting used a long hundred of 120 — twelve tens rather than ten tens — and the usage survived in medieval English and Scandinavian trade, where goods were reckoned by the long hundred well after decimal counting was established elsewhere. Distinguishing the two mattered commercially, which is why records sometimes specify which hundred is meant.

There are 100 legs a centipede is named for but rarely has (centipedes; the count is always an odd number of pairs). Species range from 30 to over 300 legs, and because the pairs are always odd in number no centipede has exactly 100.

There are 100 centimetres in a metre (worldwide, SI). The centimetre is not an SI base or preferred unit — SI favours steps of a thousand — but it survives in everyday use because a hundredth of a metre happens to be a convenient size for a human body.

There are roughly 100 billion stars in the Milky Way (the Milky Way). Estimates span a factor of four. Counted by mass and brightness rather than one at a time, which is why the range is so wide.

There are roughly 100 trillion synapses in an adult human brain (adults). Estimates run from 100 to 150 trillion. The count is inferred from the neuron count and an average number of connections, not measured directly.

The total length of the blood vessels in an adult human body is roughly 100 thousand kilometres (adults). A widely repeated figure derived from capillary density estimates rather than measured end to end. Mostly capillaries, which are about the width of a red blood cell. Enough to circle the Earth twice over.

The diameter of our galaxy is roughly 100 thousand light-years (the Milky Way). Light leaving one edge when the first humans appeared would still be crossing it today.

Fermium is the heaviest element producible in quantities that can be seen and handled. From element 101 onward, chemistry is done on a few atoms at a time before they decay.

100 degrees Celsius is the boiling point of water at standard atmospheric pressure — one of the two fixed reference points (with 0°C, the freezing point) that originally defined the Celsius temperature scale.

The frequency in the middle of the FM broadcast band is about 100 million hertz. The band runs from 87.5 to 108 MHz in most of the world. A wavelength of about three metres, which is why an FM aerial is the length it is.

HTTP 100 Continue lets a client ask whether a large request body is worth sending before sending it. It is one of the few status codes that a browser handles entirely without surfacing anything to the user.

There are over 100 billion transistors on a leading-edge processor die. Figure as of 2024. The first commercial microprocessor, the 4004 of 1971, had 2,300.

There are roughly 100 thousand commercial flights each day. Figure as of 2024. Around 10,000 aircraft are in the air at any moment during the busiest hours.

In English, "century" is the milestone term for 100 — used of ages and anniversaries, and in cricket for a batter reaching 100 runs in a single innings. The cricket sense is specific to Commonwealth cricket-playing countries; the general sense is not.

A hundredth birthday brings a congratulatory message from the monarch in the United Kingdom and several Commonwealth realms, and comparable recognition from heads of state elsewhere — a presidential greeting in the United States, a message from the Emperor in Japan. The practice has had to scale: the number of centenarians has risen sharply, and the UK office that sends the messages now handles thousands a year.

There are 100 yards between the goal lines on an American football field (the United States, NFL and college rules). Plus two end zones of ten yards each, making 120 in total. The yard is the unit the entire sport is scored and discussed in, which is why the game resisted metrication where others did not.

Reaching 100 runs is cricket's defining individual milestone, and the sport's structure makes it a genuine endurance feat rather than a scoring threshold — a batter can be dismissed at any point and does not get another attempt in that innings. Failing on 99 has its own recognised name.

The 100 metres is the shortest Olympic sprint and the event whose winner is conventionally called the fastest person in the world. Reaction time is measured from the blocks and anything under 0.1 seconds is ruled a false start, on the physiological grounds that no one can genuinely respond to a sound faster than that.

There are about 100 trillion US dollars of gross world product (worldwide, nominal terms). Figure as of 2022. Measured at purchasing power parity the figure is substantially higher, which is why two credible sources can differ by half without either being wrong.

100 is 10², which is why it anchors the decimal system's second place value and why proportions are expressed per hundred — 'per cent' is Latin per centum, 'by the hundred'. 100 divides evenly by 2, 4, 5, 10, 20, 25 and 50, so the most common fractions all convert to whole-number percentages.