Plural of Series
“Series” is an invariant count noun: singular and plural have the same spelling. The useful grammar decision is therefore not which ending to add, but how to signal number in the sentence. Write “this series is” for one and “these series are” for more than one; do not create “serieses.”
The plural of “series” is “series.” The form does not change: one series, two series. Use the surrounding determiner, number, and verb to show whether you mean one or more than one.
Key details
Choose the right form
| Meaning / situation | Form / construction | Decision |
|---|---|---|
| one sequence or set | one series / this series is | Use singular context and singular agreement. |
| two or more sequences or sets | two series / these series are | Keep the same noun form and use plural context/agreement. |
| attempted plural ending | serieses | Avoid it; “series” is already the plural form. |
Quick tests
Plural-choice test
Do you mean more than one distinct series?
If yes: Keep “series” and make the surrounding grammar plural, such as “these series are.”
If no: Keep “series” and use singular context, such as “this series is.”
Caveat: The unchanged spelling means number must come from context; the page does not cover specialized subject-matter uses of the word.
Examples
This series is complete.
Singular context is shown by “this” and “is.”
These three series are complete.
Plural context is shown by the number, “these,” and “are,” while the noun stays unchanged.
Two new series begin next month.
The plural is still spelled “series.”
Further guidance
Core difference
Use “series” for both one and more than one. Change surrounding words—not the noun—to mark number.
Fast Test
Try replacing the noun phrase with “this one” or “these two.” If the sentence means more than one, keep “series” and use plural agreement.
Usage Boundary
Do not add an -es ending. This guide concerns noun number and agreement, not the specialized meanings of series in mathematics, broadcasting, or product naming.
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.
- SERIES | English meaning - Cambridge Dictionary (opens in a new tab)Cambridge University Press · Cambridge marks series as a count noun whose plural is also series, supporting an invariant singular/plural grammar task.