King's College

Short answer: Write "The dog wagged its tail." Use its (no apostrophe) for possession; use it's (with apostrophe) only for "it is" or "it has."

Below: a quick test, the rule, many real-world wrong/right pairs, rewrites for different tones, a checklist, and nearby confusable words to fix in the same pass.

Quick answer: Which is correct?

"The dog wagged its tail." Use its (no apostrophe) when something belongs to or is associated with "it." Use it's only when you mean "it is" or "it has."

Quick test: Replace it's with "it is" or "it has." If the sentence still works, keep it's. If it breaks, use its.
Possession (the tail belongs to the dog) → its (no apostrophe).
Common trap: Many possessives use an apostrophe (John's), but its is the exception.

Core explanation: the rule in one line

its = possessive pronoun (like his, her, our). it's = contraction of it is or it has.

Test: Expand it's to "it is" or "it has." If that makes sense, it's is correct; otherwise use its.
Example possessive: The machine lost its calibration.
Example contraction: It's time to recalibrate the machine. (it is)

Grammar details and edge cases

its applies to animals, objects, organizations, and ideas. Use sex-specific pronouns (his/her) only when the subject's sex matters.

Never add an apostrophe to form the possessive of it - that's the frequent mistake.
Collective nouns: The team lost its chance (not it's).
Proper names still use an apostrophe: James's dog wagged its tail.
Contractions inside clauses: It's been a long week (it has).

Usage: Correct: The committee published its report.
Incorrect: The committee published it's report.
Usage: Correct: It's been updated twice this month. (it has)

Real usage and tone: formal vs casual

Formality affects whether you use contractions, not the choice between its and it's.

Formal: The organization revised its policy. (avoid contractions)
Neutral: It's important we verify its assumptions before sending.
Casual: It's so cute-its tail never stops wagging!

Usage: Formal: The device delivered its expected performance.
Casual: It's acting like its usual self.

Examples: wrong/right pairs across contexts

Read the incorrect sentence, then the correct replacement. These are templates you can paste into drafts.

Incorrect: It's collar was missing after the walk.
Correct: Its collar was missing after the walk.
Incorrect: The plant dropped it's leaves.
Correct: The plant dropped its leaves.
Incorrect: It's been modified to include it's own battery.
Correct: It's been modified to include its own battery.
Incorrect: The car lost it's hubcap.
Correct: The car lost its hubcap.
Incorrect: The software warns it's users.
Correct: The software warns its users.
Incorrect: The island changed it's coastline after the storm.
Correct: The island changed its coastline after the storm.

Work - Email: Incorrect: It's budget for Q3 is tight.
Correct: Its budget for Q3 is tight.
Work - Report: Incorrect: It's impact on revenue was significant.
Correct: Its impact on revenue was significant.
Work - Slide: Incorrect: It's timeline looks optimistic.
Correct: Its timeline looks optimistic.
School - Essay: Incorrect: It's role in the plot is ambiguous.
Correct: Its role in the plot is ambiguous.
School - Lab: Incorrect: The sample lost it's reactivity.
Correct: The sample lost its reactivity.
School - Caption: Incorrect: It's correlation with temperature was low.
Correct: Its correlation with temperature was low.
Casual - Text: Incorrect: I love my dog, it's tail is huge.
Correct: I love my dog, its tail is huge.
Casual - Social: Incorrect: Look at that duck-it's feathers are wild.
Correct: Look at that duck-its feathers are wild.
Casual - Caption: Incorrect: It's taste surprised me.
Correct: Its taste surprised me.
Tricky mix: Incorrect: It's been polished to improve it's shine.
Correct: It's been polished to improve its shine.

Try your own sentence

Test the whole sentence, not just the phrase. Context usually makes the correct choice obvious.

Rewrite help: swap tone without breaking the rule

Below are rewrites that preserve the possessive its when ownership is meant.

Work (concise): The dog wagged its tail, signaling a positive response to the new protocol.
Work (formal): The animal exhibited increased activity; its tail movement rose markedly after treatment.
School (analytic): The dog's tail wagged in response to the stimulus, supporting hypotheses about attachment; its frequency increased by 30%.
School (concise): The dog wagged its tail, consistent with prior studies on social reward.
Casual (vivid): The dog wagged its tail so hard it nearly collided with the couch-pure joy.
Casual (chatty): It's obvious the pup's happy-its tail hasn't stopped moving all morning!

Memory trick, checklist, and fast-edit habit

Simple habits catch most errors in a few seconds.

Mnemonic: Apostrophe = missing letter. If no letter is missing (possession), no apostrophe → its.
Two-second habit: Read the phrase aloud. If "it is" sounds wrong, switch to its.
Three-step checklist: 1) Expand it's → "it is"/"it has". 2) If sensible, keep it's; if not, change to its. 3) Re-scan similar words (your/you're, their/there).

Quick test example: "It's collar" → "It is collar" (fails) → use "Its collar."

Similar mistakes to fix at the same time

When you scan for it's/its, check these common confusables too.

your vs you're: Your idea vs You're right (you are).
their vs there vs they're: Their car (possession), there (place), they're = they are.
who's vs whose: Who's = who is; Whose shows possession.
let's vs lets: Let's = let us; lets = allows.
its vs his/her: For animals, use its unless sex-specific pronouns are needed.

Usage: Incorrect: Your going to love its flavor.
Correct: You're going to love its flavor.
Usage: Incorrect: Whos coat is this?
Correct: Whose coat is this?

Hyphenation and spacing - punctuation notes that don't change the rule

Hyphens, line breaks, and spacing can affect appearance but not the possessive vs contraction rule.

Do not add spaces inside contractions ("it 's" is wrong).
If a word is split across lines, rejoin it before applying the expansion test.
Hyphenated phrases keep the same rule: The staircase-its history is noted-remains correct (its = possessive).

Usage: Caption: The lighthouse-its beacon lit the shore-guided ships. (Correct)

FAQ

Is "The dog wagged it's tail" ever correct?

No. It's means "it is" or "it has." Since the tail belongs to the dog, use its without an apostrophe.

What's a fast mental test I can use while editing?

Replace it's with "it is" or "it has." If the sentence still makes sense, keep it's. If not, change to its.

Should I avoid it's in formal writing?

Avoid unnecessary contractions in formal writing, but using it's is fine when "it is" or "it has" fits. The possession rule for its still applies.

Are there exceptions where possession uses an apostrophe with "it"?

No. Standard English always uses its (no apostrophe) for the possessive pronoun.

Why do so many people get this wrong?

Because most possessives use an apostrophe (John's, the dog's) and contractions are common. Speed and habit lead to adding an apostrophe where it doesn't belong. The expansion test fixes this quickly.

Make the habit stick

Before sending or publishing, search for "it's" and run the expansion test. That single habit catches most mistakes.

If you prefer automated checks, use a grammar tool to flag its/it's errors and suggest rewrites you can paste directly into your document.