Kaithi was the traditional script of the Bhojpuri and Magahi languages, and the popular script of the Awadhi and Maithili languages. These languages are now written largely in Devanagari. Kaithi was also used to write Urdu, which is now written in the Perso-Arabic script.

Several sources refer to Kaithi as the script of the Bihari language. This is misleading since there is no such language. Bihari is a subdivision of Eastern Indo-Aryan languages but the languages in this group are not spoken exclusively in the state of Bihar. Many of the languages classed as Bihari have been written in the Kaithi script at some point in their history.

Kaithi was used alongside Devanagari for writing Awadhi, which is spoken predominantly in Uttar Pradesh but also in Nepal, Madhya Pradesh, and Bihar. It is also the traditional script for Bhojpuri, which is spoken primarily in Uttar Pradesh and Bihar, as well as in Nepal. Bhojpuri-speaking ex-pat communities in Mauritius, Guyana, Trinidad, South Africa, Suriname and Fiji also use the Kaithi script. The Maithili language is spoken in Bihar and Nepal and was previously written in the Kaithi and the Mithila scripts, depending on the caste of the person writing. Kaithi was used for writing Bengali on the border of Bihar and Bengal, and for the Marwari language on the Western periphery in Rajasthan. Magahi, spoken in Bihar and West Bengal, was traditionally written in Kaithi. These languages began to be written in the Devanagari script in the early 1900s and Kaithi is now used rarely, if at all, and only in informal contexts such as personal communication.

Kaithi was also used in the law courts of Bihar for writing the Urdu language, which is now written in the Perso-Arabic script. The majority of extant legal documents from Bihar from the British period are in Urdu written in Kaithi.