Both “foci” and “focuses” are accepted plurals of “focus.” Do not assume the Latin-derived “foci” makes “focuses” incorrect; dictionaries record both forms.
ReviewedEvidence1 sourceSectionEnglish Usage
Quick answer
Both “foci” and “focuses” are accepted plurals of “focus.”
Key details
Confusion Familycompeting plural forms
Rule Familynumber and classical plurals
Termsfocus, foci, focuses
Variant ScopeMajor dictionaries record both “foci” and “focuses” as plurals of “focus.”
Further guidance
Core difference
Both “foci” and “focuses” are accepted plurals of “focus.”
Fast Test
Choose the plural that fits the discipline and house style, then use it consistently.
Usage Boundary
Do not assume the Latin-derived “foci” makes “focuses” incorrect; dictionaries record both forms.
Sources and evidence
Sources are shown with the role they play in this guide. Historical or style-sensitive claims are kept within the evidence boundary described above.
“Dogmas” is the ordinary English plural of “dogma,” while the classical plural “dogmata” is also recorded. The choice is therefore a genuine competing-plural question rather than a simple spelling variant.
Both “formulas” and “formulae” are accepted plurals of “formula.” Do not present “formulae” as the only correct plural or treat “formulas” as an error: both are dictionary-recorded forms.
“Forum” has two dictionary-recorded plurals: “forums” and “fora.” The regular plural is common in general use, while the Latin-derived plural survives especially in more formal or specialized contexts.
Cambridge records two plurals of “referendum”: regular “referendums” and formal “referenda.” The page keeps both forms valid and treats register/style as the choice factor.
