1000000000（十億、じゅうおく、one billion, one thousand million‚ one milliard, one yard）は、自然数であり、999999999の次の数で1000000001の前の数である。英語ではone billionあるいはbillionと表される。billionはbnと略されることもある。

標準形式では1 × 109と表記される。SI接頭語のギガは基礎単位の十億倍を表し、記号はGである。

十億年は、天文学や地質学でアイオーンと呼ばれることがある。

イギリス英語（アメリカ英語ではなく）では、billionはもっぱら1,000,000,000,000を表すが、もはや、ありふれたものではなく、この数十年で1,000,000,000という意味で使われるようになっている。

milliardという単語もまた、1,000,000,000を表す単語として使われ得る。英語ではめったに使われないが、この単語のバリエーションが他の言語でよく見られる。

インドの命数法では、100カロール（crore）または1アラブ（arab）として知られている。

1000000000は1000の立方数でもある。

大きさの感覚

時間

 109 seconds (1 gigasecond) equal 11,574 days, 1 hour, 46 minutes and 40 seconds (approximately 31.7 years, or 31 years, 8 months, 8 days).
 About 109 minutes ago, the Roman Empire was flourishing and Christianity was emerging. (109 minutes is roughly 1,901 years.)
 About 109 hours ago, modern human beings and their ancestors were living in the Stone Age (more precisely, the Middle Paleolithic). (109 hours is roughly 114,080 years.)
 About 109 days ago, Australopithecus, an ape-like creature related to an ancestor of modern humans, roamed the African savannas. (109 days is roughly  years.)
 About 109 months ago, dinosaurs walked the Earth during the late Cretaceous. (109 months is roughly  years.)
 About 109 years—a gigaannus—ago, the first multicellular eukaryotes appeared on Earth.
 About 109 decades ago, galaxies began to appear in the early Universe which was then 3.772 billion years old. (109 decades is exactly  years.)
 The universe is thought to be about 13.77 × 109 years old.

距離
 109 inches is , more than halfway around the world and thus sufficient to reach any point on the globe from any other point.
 109 metres (called a gigametre) is almost three times the distance from the Earth to the Moon.
 109 kilometres (called a terametre) is over six times the distance from the Earth to the Sun.

面積
 A billion square inches could make a square about one half mile on a side.
 A bolt of finely woven 1000-TC bed sheet linen with a billion thread crossings would have an area of , comparable to the floor area of a motel unit.

体積
 There are one billion cubic millimetres in a cubic metre, and a billion cubic metres in a cubic kilometre.
 A billion grains of table salt or granulated sugar would occupy a volume of about .
 A billion cubic inches would be a volume comparable to a large commercial building slightly larger than a typical supermarket.

質量
 Any object that weighs  would weigh about as much as 5,525 empty Boeing 747-400s.
 A cube of iron that weighs  would be  on each side.

製品
 As of July 2016, Apple has sold one billion iPhones. This makes the iPhone one of the most successful product lines in history, surpassing the PlayStation and the Rubik's Cube.
 As of July 2016, Facebook has 1.71 billion users.

自然
 A small mountain, slightly larger than Stone Mountain in Georgia, United States, would weigh (have a mass of) a billion tons.
 There are billions of worker ants in the largest ant colony in the world, which covers almost  of the Mediterranean coast.
 In 1804, the world population was one billion.

計算
A is a cube; B consists of 1000 cubes the size of cube A, C consists of 1000 cubes the size of cube B; and D consists of 1000 cubes the size of cube C. Thus there are  A-sized cubes in C; and 1,000,000,000 A-sized cubes in D.