Numbers in Vietnamese from không to một tỷ: sound changes and ordinals

Mười or mươi, một or mốt: four numbers that change

Ten has two shapes, and it is the first of the numbers in Vietnamese to change. Standing alone or opening a teen it is mười, with the low falling huyền tone: mười, mười một, mười chín. From twenty on, where ten is multiplied, it loses the tone mark and becomes mươi: hai mươi, twenty, ba mươi, thirty. The difference is one diacritic, and it carries real information, since mười hai is twelve and hai mươi is twenty.

One turns into mốt after the multiples of ten from twenty to ninety: hai mươi mốt, 21, chín mươi mốt, 91. Eleven stays mười một, because the rule starts at twenty-one, and 101 stays một trăm linh một. Wikipedia's article on Vietnamese numerals and the CLDR spell-out rules agree on this boundary.

Five becomes lăm in the units place of any number above ten: mười lăm, 15, hai mươi lăm, 25. The change avoids a clash with năm, year, which sounds the same as five. Some northern speakers say nhăm instead of lăm. A five after a zero keeps its plain form, so 105 is một trăm linh năm.

Four is the softest rule. Bốn is the counting word, but after twenty in the units place the Sino-Vietnamese tư is common: hai mươi tư, 24. Wikipedia notes that colloquial speech often uses bốn there too, and CLDR's spell-out prints tư. Fourteen is usually mười bốn; mười tư belongs to literary style.

Linh or lẻ, and không trăm: numbers with zeros inside

A zero in the tens place needs a word, or 105 would sound like 15. Northern Vietnamese uses linh, the Sino-Vietnamese zero: một trăm linh một, 101. Southern and central speech says lẻ, odd or extra: một trăm lẻ một. Both are standard in their regions; CLDR's spell-out rules use lẻ.

An empty hundreds place is read as không trăm, zero hundred. The year 2005 is hai nghìn không trăm linh năm in the north, and 1001 is một nghìn không trăm linh một. English speakers tend to drop the không trăm, but in Vietnamese the hundreds position is always read out once a number passes a thousand.

Không itself is the everyday zero and also the word for no and not. In physics texts and casual speech the loanword zêrô also appears, according to Wikipedia.

Trăm, nghìn or ngàn, triệu and tỷ

A hundred is trăm, normally một trăm. A thousand has two words that split the country: nghìn is the standard in the north, ngàn the word of the south, and both are understood everywhere. A million is triệu and a billion, a thousand million, is tỷ. The CLDR spell-out rules that software uses take the northern nghìn and spell 2026 as hai nghìn không trăm hai mươi sáu.

Large numbers in Vietnamese are grouped in thousands, as in English, and not in units of ten thousand as in Chinese, Japanese and Korean. The number 123,123,123 is read một trăm hai mươi ba triệu một trăm hai mươi ba nghìn một trăm hai mươi ba. Older Sino-Vietnamese units such as vạn, ten thousand, survive in fixed phrases and, Wikipedia notes, in counting bricks.

Written figures swap the two marks English uses. The CLDR number symbols for Vietnamese set the comma as the decimal separator and the full stop as the thousands separator, so 1.000.000 is one million and 3,5 is three and a half. Read aloud, the comma is phẩy: ba phẩy năm.

Using numbers in Vietnamese with classifiers

A number never sits directly on a noun. Between them comes a classifier that sorts the noun by kind: hai con mèo, two cats, uses con for animals, and ba cuốn sách, three books, uses cuốn for volumes. The days of the week in Vietnamese are the same ordinals as the table above, thứ hai to thứ bảy, so Wednesday is thứ tư.

Ten has a colloquial partner, chục. In Wikipedia's example, ten eggs are một chục quả trứng as often as mười quả trứng, and twenty can be hai chục. Twelve has a loanword of its own, một tá, a dozen. For a comparison with the counting systems of neighbouring languages, the digits page sets one to ten in several hundred languages side by side.