Occurred is one of those words that trips up writers consistently — not because its meaning is unclear but because its spelling requires applying a doubling rule that many writers apply inconsistently.
How to spell “occurred” correctly
Occurred is the past tense and past participle of occur. It is spelled with double r and double c: oc-c-u-r-r-e-d.
The double r follows a consistent rule of English orthography: when a verb ends in a single consonant preceded by a single stressed vowel, the final consonant is doubled before suffixes beginning with a vowel (-ed, -ing, -ence).
Occur ends in -r, the preceding vowel is u, and that vowel is stressed (the stress falls on the second syllable: oc-CUR). So when adding -ed or -ing, the r doubles:
occur → occurred, occurring, occurrence
The same rule applies to similar verbs: prefer → preferred, preferring; refer → referred, referring; deter → deterred, deterring; infer → inferred, inferring.
The double c is part of the base spelling of occur (not added by the rule) and appears across all forms: occur, occurred, occurring, occurrence.
Common misspellings
The most frequent errors are:
occured — single r, missing the doubling rule occurence — single r in the noun form occuring — single r in the present participle occured — applying the rule but misspelling the base
A useful mnemonic: occur has a double letter pair in its base form already (cc), and it doubles its final r when adding endings. Two pairs of doubled letters.
Meaning and usage
Occur means to happen, to take place, or to come to mind. In academic and professional writing, it is common in scientific and technical contexts because it is more neutral than happen and doesn’t imply an agent:
The reaction occurs at temperatures above 60°C.
Errors of this type occur when the input data is incomplete.
It occurred to the investigators that a confounding variable might be present.
The third use — occur to someone (to come to mind) — is distinct from the first two and takes a personal subject indirectly: it occurred to her means “she realized” or “the thought came to her.”
Occurred vs. happened
Occurred and happened are largely interchangeable, but occurred is somewhat more formal and often preferred in academic and technical writing. Happened implies an event in narrative time; occurred can describe both events and states or conditions:
A significant change occurred in the third quarter. (formal, academic)
Something happened in the third quarter. (more casual, implies narrative)
Trinka’s grammar checker catches spelling errors in commonly misspelled words like occurred, occurrence, and occurring, and distinguishes them from correct forms in context.
References
Garner, B. A. (2016). Garner’s Modern English Usage (4th ed.). Oxford University Press.
Merriam-Webster. (2023). Occur. https://www.merriam–webster.com/dictionary/occur