Many English speakers stop mid-sentence when faced with mistakingly versus mistakenly.
Both look plausible because they stem from the familiar word mistake, but one form is the accepted standard in modern US English and the other is unusual or marked wrong in formal settings.
This confusion shows up often in emails, school writing, and even professional reports.
Knowing which form to use saves embarrassment and makes your writing clearer.
In this guide, you’ll get quick answers on correctness, feel confident about when and where to use each, get real examples you can copy, and learn how to fix common slips.
Quick Answer
Use mistakenly. It is the standard, widely accepted adverb meaning “in error or by mistake.” Mistakingly appears very rarely, is considered nonstandard, and is discouraged in formal writing.
Key Differences at a Glance
Here’s a simple way to see how the two spellings compare:
• Mistakenly: correct adverb, used in US English for errors
• Mistakingly: rare/nonstandard, not recommended
• Formal contexts accept only mistakenly
• Everyday informal speech may tolerate mistakenly only
Origin and Why Two Forms Exist
The adverb mistakenly comes from the adjective mistaken (the past participle of mistake). In English, many adverbs are formed by adding -ly to adjectives: quick → quickly, happy → happily, mistaken → mistakenly. This pattern is well entrenched in usage.
The form mistakingly would come from using the present participle mistaking plus -ly. That is not the productive pattern here, so it did not gain wide acceptance. Some dictionaries list mistakingly in very rare or historical contexts, but most modern guides and style manuals do not recommend it.
British vs American English
There is no regional spelling difference for this adverb. Both American and British English prefer mistakenly. Mistakingly is not standard in either variant.
Which One Should You Use?
• Casual messages: Always use mistakenly. For example:
- I mistakenly thought today was Monday.
• Social posts: Mistakenly sounds natural and clear.
- I mistakenly shared the wrong photo.
• Workplace emails: Stick with mistakenly to avoid reader confusion.
- The report was mistakenly submitted early.
• Formal/academic writing: Only mistakenly is accepted.
- Data was mistakenly interpreted in the initial draft.
Tip: If you want even simpler phrasing, by mistake or in error works well in all registers.
Common Mistakes (and Quick Fixes)
• Mistake: Writing I mistakingly sent the wrong file.
Fix: I mistakenly sent the wrong file.
• Mistake: Believing both forms are equally acceptable.
Fix: Always pick mistakenly in serious writing.
• Mistake: Using mistakingly because it looks logical.
Fix: Remember the rule: adverbs often come from adjectives, not present participles.
• Mistake: Thinking mistakenly is only British or only American.
Fix: It’s standard everywhere English is written today.
• Mistake: Replacing mistakenly with similar but incorrect adverbs.
Fix: Choose by mistake, incorrectly or in error to vary style.
Everyday Examples (Real Contexts)
Text message:
• ❌ I mistakingly thought we start at 5.
• ✔ I mistakenly thought we start at 5.
Social media:
• ✔ I mistakenly tagged the wrong friend in that post.
Parenting/child context:
• ✔ He mistakenly believed bedtime was later tonight.
Workplace/email note:
• ❌ Our system mistakingly assigned the wrong dates.
• ✔ Our system mistakenly assigned the wrong dates.
More professional rewrite:
• ✔ The project timeline was mistakenly entered into the database.
Comparison Table
| Context | Best Choice | Why |
| General writing | Mistakenly | Standard adverb, widely accepted. |
| Formal/academic | Mistakenly | Expected in essays/reports. |
| Emails/messages | Mistakenly | Clear and natural. |
| Conversation | Mistakenly | Most speakers use this. |
| Grammar learning | Mistakenly | Follows adjective + -ly rule. |
FAQs
Is mistakingly a real word?
Some references show it historically or in rare listings, but it’s not standard in modern English and is usually discouraged.
Which is grammatically correct?
Mistakenly is correct; mistakingly is considered nonstandard.
Can I ever use mistakingly?
Only in creative or informal contexts where you deliberately bend conventions. Otherwise, avoid it.
Does dictionary recognition differ?
Standard dictionaries list mistakenly. Rare/unabridged dictionaries might list mistakingly with limited usage notes.
Is mistakenly formal or informal?
It works in both; formal in essays, clear in everyday conversation.
Mini Quiz
- Which spelling is standard in modern English?
Answer: Mistakenly - Choose the correct phrase: She ___ thought the test was today.
Answer: mistakenly - True or false: Mistakingly is preferred in American business writing.
Answer: False - Can by mistake replace mistakenly in a sentence?
Answer: Yes - Which form fits professional reports?
Answer: Mistakenly
Conclusion
The confusion between mistakingly and mistakenly is understandable, but the correct choice is clear: mistakenly is the standard, widely accepted adverb in US English and beyond.
Use it confidently in emails, papers, and everyday writing to show you mean something happened in error or unintentionally.
Small spelling choices like this help your writing feel polished and professional.
