Fill-in vs. Fill In
“Fill-in” is a hyphenated noun for a temporary substitute and can also be an attributive adjective. “Fill in” is the two-word phrasal verb used for completing blanks, substituting for someone, or supplying missing information.
Use “fill-in” for a temporary substitute person or, in mainly US use, as an attributive adjective. Use “fill in” as the verb for completing information, substituting temporarily, or bringing someone up to date.
Key details
Choose the right form
| Meaning / situation | Form / construction | Decision |
|---|---|---|
| temporary substitute person | fill-in | Use the hyphenated noun. |
| temporary substitute before a noun | fill-in | Use the hyphenated attributive form where that regional usage applies. |
| complete information or substitute temporarily as a verb | fill in | Use the two-word phrasal verb. |
Quick tests
Meaning / construction test
Is the expression functioning as a verb that can change tense, such as “filled in” or “will fill in”?
If yes: Use the spaced verb “fill in.”
If no: Use “fill-in” when the documented noun or attributive modifier is intended.
Caveat: Cambridge distinguishes the hyphenated noun/adjective from the phrasal verb; this guide does not claim that every noun formed from a phrasal verb follows the same pattern.
Examples
Dana will fill in for the receptionist tomorrow.
The expression functions as a phrasal verb.
We hired a fill-in for the afternoon shift.
The hyphenated form is a noun meaning a temporary substitute.
Please fill in the missing date.
The two-word form is the verb for supplying information.
Further guidance
Core difference
Use the documented noun or modifier form when naming a thing, result, person, or concept; use the spaced form when the words function as a verb.
Fast Test
If the expression can take ordinary verb tense or inflection in the sentence, the spaced verb form is usually the relevant choice.
Variant Boundary
Cambridge distinguishes the hyphenated noun/adjective “fill-in” from the phrasal verb “fill in,” which has several senses.
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.
- FILL SOMETHING IN — English meaning (opens in a new tab)Cambridge Dictionary · Hyphenated noun/adjective “fill-in” and phrasal verb “fill in”