In linguistics, vowel length is the perceived or actual duration of a vowel sound when pronounced. Vowels perceived as shorter are often called short vowels and those perceived as longer called long vowels.

On one hand, many languages do not distinguish vowel length phonemically, meaning that vowel length alone does not change the meanings of words. However, the amount of time a vowel is uttered can change based on factors such as the phonetic characteristics of the sounds around it: the phonetic environment. An example is that vowels tend to be pronounced longer before a voiced consonant and shorter before a voiceless consonant in the standard accents of American and British English.

On the other hand, vowel length is indeed an important phonemic factor in certain languages, meaning vowel length can change word-meanings, for example in Arabic, Czech, Dravidian languages (such as Tamil), some Finno-Ugric languages (such as Finnish and Estonian), Japanese, Kyrgyz, Samoan, and Xhosa. Some languages in the past likely had the distinction even though their modern descendants do not, with an example being Latin versus its descendent Romance languages like Spanish and French. Length also plays a lesser phonetic role in Cantonese, unlike in other varieties of Chinese, which do not have phonemic vowel length distinctions.

Whether vowel length alone changes word-meanings in English depends on the particular dialect; it is able to do so in a few non-rhotic dialects, such as Australian English, Lunenburg English, New Zealand English, South African English, and possibly some (vernacular) English of Southern England. For instance, vowel length can distinguish park /paːk/ from puck /pak/ in Australian and New Zealand English, or bared /beːd/ from bed /bed/ in any of these dialects. Phonemic vowel length perhaps marginally occurs in a few rhotic dialects too, such as Scottish English and Northern Irish English (see Scottish vowel length rule).

Languages that do distinguish vowel length phonemically usually only distinguish between short vowels and long vowels. Very few languages distinguish three phonemic vowel lengths; some that do so are Estonian, Luiseño, and Mixe. However, languages with two vowel lengths may permit words in which two adjacent vowels are of the same quality: Japanese ほうおう, hōō, "phoenix", or Ancient Greek ἀάατος [a.áː.a.tos], "inviolable". Some languages that do not ordinarily have phonemic vowel length but permit vowel hiatus may similarly exhibit sequences of identical vowel phonemes that yield phonetically long vowels, such as Georgian გააადვილებ, gaaadvileb [ɡa.a.ad.vil.eb], "you will facilitate it".

Stress is often reinforced by allophonic vowel length, especially when it is lexical. For example, French long vowels are always in stressed syllables. Finnish, a language with two phonemic lengths, indicates the stress by adding allophonic length, which gives four distinctive lengths and five physical lengths: short and long stressed vowels, short and long unstressed vowels, and a half-long vowel, which is a short vowel found in a syllable immediately preceded by a stressed short vowel: i-so.

Among the languages with distinctive vowel length, there are some in which it may occur only in stressed syllables, such as in Alemannic German, Scottish Gaelic and Egyptian Arabic. In languages such as Czech, Finnish, some Irish dialects and Classical Latin, vowel length is distinctive also in unstressed syllables.

In some languages, vowel length is sometimes better analyzed as a sequence of two identical vowels. In Finnic languages, such as Finnish, the simplest example follows from consonant gradation: haka → haan. In some cases, it is caused by a following chroneme, which is etymologically a consonant: jää "ice" ← Proto-Uralic *jäŋe. In non-initial syllables, it is ambiguous if long vowels are vowel clusters; poems written in the Kalevala meter often syllabicate between the vowels, and an (etymologically original) intervocalic -h- is seen in that and some modern dialects (taivaan vs. taivahan "of the sky"). Morphological treatment of diphthongs is essentially similar to long vowels. Some old Finnish long vowels have developed into diphthongs, but successive layers of borrowing have introduced the same long vowels again so the diphthong and the long vowel now again contrast (nuotti "musical note" vs. nootti "diplomatic note").