Both. They are the same word written two ways in the Roman alphabet, and neither is a mistake. Here is why the two spellings exist — and what the related words mean.
In Devanagari the word is कुण्डली. There is no ambiguity there — a single spelling, used across Hindi, Marathi, Nepali and Sanskrit. The variation appears only when the word is written in the Roman alphabet, because English has no standard way to transliterate Indian languages. Formal academic transliteration (IAST) gives kuṇḍalī, complete with diacritics almost nobody types. Strip the diacritics and you get kundali. Follow Hindi pronunciation instead and you get kundli.
The reason is a well-documented feature of Hindi called schwa deletion. Devanagari consonants carry an inherent short “a” vowel, but in Hindi that vowel is routinely dropped in certain positions. It is the same process that turns Rāma into Rām and kamala into kamal. In कुण्डली, the short “a” sitting between the ड and the ल disappears in ordinary Hindi speech, so the word is pronounced kund-lee — and people write what they say.
Languages that do not delete the schwa keep the vowel audible, so their speakers naturally write it in. That is why kundali is the more common spelling in Marathi, Gujarati and Nepali contexts, and in writing that stays close to Sanskrit. Neither community is spelling the word wrong; they are transcribing two different, equally real pronunciations of the same word.
कुण्डली derives from the Sanskrit kuṇḍala — a ring or a coil. The name describes the shape: a Vedic birth chart was traditionally drawn as a circular or enclosed diagram, mapping where the planets stood at the moment of birth. The North Indian chart is drawn as a diamond within a square and the South Indian as a rectangular grid, but the older circular sense survives in the name.
Janam kundali / janam kundli — janam means birth, so this is simply the birth chart. Often shortened to just kundli.
Janam patri, janampatri, janam patrika — the written horoscope document, traditionally a long scroll an astrologer prepared by hand and the family kept for life. Same contents, different word for the artefact.
Kundali milan / kundli milan — the comparison of two charts before marriage, usually scored through the eight-fold Ashtakoota system out of 36 points. English sources call it horoscope matching or guna milan.
Jathagam (ஜாதகம்) — the Tamil word for the same birth chart, from the Sanskrit jātaka. Jatakam is the Telugu and Malayalam form, and jataka the Kannada. If you have seen a South Indian horoscope described as a jathagam, it is a kundali in a different language and a different chart layout.
Kundalini — worth separating, because the spellings look close. Kundalini is a concept in yoga and tantra referring to coiled energy at the base of the spine. It shares the same kuṇḍala root — the idea of coiling — but has nothing to do with birth charts.
Whichever your family uses. If you are searching online, both spellings will find the same material, and any astrologer or site worth using will understand either. We spell it kundli across this site simply because that is the more common spelling in Hindi-language usage and in search — not because kundali is in any way incorrect.
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Create your kundliMore reading: What is a kundli? · How to read your kundli · The 27 nakshatras