by who or by whom: What's the difference & usage? 2026

Many English learners and even native speakers often stumble over the choice between “by who” and “by whom.” While it may seem like a minor detail, using the wrong form can make your writing or speech appear unpolished. The confusion arises mainly because English grammar has evolved over time, and modern usage sometimes blurs traditional rules.

Understanding the difference between “by who” and “by whom” is important for clear communication. In British English, maintaining formal grammatical rules is slightly more common than in American English, where casual speech sometimes bends the rules. However, knowing the correct usage helps you write confidently in emails, essays, reports, and even social media posts.

In this article, we’ll explore the correct form, highlight common mistakes, compare UK and US preferences, and provide practical tips and examples. By the end, you’ll never hesitate about which one to use again.

Quick Answer

“By whom” is grammatically correct when referring to the object of a verb or preposition. “By who” is considered incorrect in formal writing but often appears in casual speech.

Example:

Correct: This book was written by whom?
Incorrect: This book was written by who?

Simply put, if you can replace the word with “him” or her”, use whom. If you would say “single person”, use who.

Meaning of the Correct Form

Understanding “By Whom”

“By whom” is used when the pronoun is the object of a preposition. In English, “by” is a preposition, and when a pronoun follows a preposition, standard grammar rules require the object form whom, not who.

Examples in UK and US English:

The new policy was introduced by whom? (UK & US)

Notice that “by whom” fits naturally when the sentence could be rephrased using “him,” “her,” or “them”:

The letter was signed by him/her/them. → Correct.

In addition, formal writing and professional contexts like academic papers, legal documents, or official correspondence almost always prefer “by whom.”

Meaning of the Incorrect Form

What About “By Who”?

“By who” is technically incorrect in formal grammar because “who” is the subject form of the pronoun. However, many people use it casually, especially in spoken American English.

Examples:

This book was written by who? ❌ (formal)
By who are you inviting to the party? ❌ (should be “by whom”)

Why does it appear so often? Native speakers sometimes prioritize ease of speaking over strict grammar rules, especially in informal settings. Over time, phrases like “by who” have become somewhat acceptable in everyday conversation, though they remain incorrect in formal writing.

Keyword Comparison / Key Differences

Feature	By Whom	By Who
Grammar	Correct object form	Incorrect (subject form)
Formality	Formal, professional, academic	Informal, casual speech
Replacement Test	Replace with him/her/them	Would replace with single person→ incorrect
UK Preference	Strong preference for “by whom”	Rarely used in formal contexts
US Preference	Preferred in writing, but “by who” is common in speech	Accepted in casual conversation, avoided in formal writing
Example	The meeting was led *by whom*?	The meeting was led *by who*?

Key Takeaway: If you are writing an essay, report, or professional email, always use “by whom.” In casual conversation, “by who” is common, especially in American English.

Common Mistakes People Make

Here are some practical examples of common mistakes with explanations:

Mixing Subject and Object Forms
- ❌ By who was this decision made?
- ✅ By whom was this decision made?
Using “who” after a preposition
- ❌ To who did you give the book?
- ✅ To whom did you give the book?
Overusing “by who” in emails
- ❌ Please confirm by who will attend the meeting.
- ✅ Please confirm by whom will attend the meeting.
Ignoring context
- Sometimes people say “by who” even when it is clearly formal.
- ✅ Correct: The award was given by whom?
Confusion with contractions and casual speech
- ❌ Who’s going to handle this? By who?
- ✅ Who’s going to handle this? By whom?

Correct Usage in Sentences

Here are real-life examples for both UK and US readers:

Formal Writing

By whom was the project funded?
The evidence was provided by whom?

Informal Spoken English (US)

By who are you texting?
Who sent this email? By who?

Mixed Examples for Clarity

By whom should I address the letter? ✅
By who should I address the letter? ❌

Tip: If unsure, mentally replace with “him/her/them.” If it fits, use whom.

Tips to Remember the Correct Usage

“Him/Her/Them” Test
- If you can replace the pronoun with him/her/them, use whom.
- If you can replace with he/she/they, use who.
Formal vs Informal Context
- Use whom for writing, emails, reports.
- Casual speech can tolerate who, especially in American English.
Preposition Clue
- Prepositions like by, to, for, with → usually require whom.
Rephrase Trick
- Turn the sentence around:
  - Original: By whom was the speech delivered?
  - Rephrased: Who delivered the speech? ✅
Mnemonic:
- W-H-O-M = Him/Her/Object Measured
- This reminds you that “whom” is the object.

FAQs

1. Can I say “by who” in emails?

In informal emails to friends or colleagues, it’s sometimes used in US English. In professional or formal emails, always use by whom.

2. Is “by whom” used in spoken English?

Less frequently in casual speech, but more common in formal discussions, legal language, and official announcements.

3. Why do people say “by who” if it’s wrong?

It’s easier to say and many speakers don’t consciously apply grammar rules in casual conversation.

4. Can British English tolerate “by who”?

Not in formal writing. In casual conversation, it may appear, but traditional grammar still prefers by whom.

5. How do I know when to use “whom”?

Use the him/her/them test. If you can replace the pronoun with an object pronoun, it’s whom.

6. Are there any exceptions to the “him/her/them” test?

Rarely. Poetic or conversational English sometimes bends the rules, but in standard usage, the test works almost always.

7. Is “by whom” outdated?

Not at all! While modern English allows flexibility in casual speech, “by whom” remains standard in formal writing and professional contexts.

Conclusion

Choosing between “by who” and “by whom” may seem tricky at first, but the rules are straightforward:

Use by whom for formal, professional, and written English.
Use by who only in casual spoken English, primarily in the US.

Remember the him/her/them replacement trick, check your prepositions, and consider your audience. By mastering this simple rule, your English writing and speaking will sound polished and confident.