Both “which number” and “what number” are correct English. They are not the same, though. Choose which number when the options are limited or already known. Choose what number when the options are open or unknown.
The quick test
Ask yourself: can the listener picture a set of choices?
- If yes, use which number.
- If no, use what number.
When to use “which number”
“Which” points to a choice from a group. The group can be a list, a card, a screen, or a set of numbers you just mentioned.
- Which number is bigger, 48 or 84?
- Which number on this list is a prime?
- Which number should I press to reach the front desk?
- I can’t tell which number is yours on this receipt.
In each sentence, the options are visible or understood. The speaker is asking the listener to pick one.
When to use “what number”
“What” asks about something open. There is no set of choices, or the set is so large that it doesn’t matter.
- What number am I thinking of?
- What number do you get when you add 19 and 23?
- What number should I call to reach your office?
- What number is your seat?
In the first two, the answer could be any number, so “which” would sound odd. Math questions often use “what number” for this reason: “What number times 6 equals 54?”
When both work
Sometimes the choices are not clear, and either word fits. The meaning shifts only slightly.
- What number should I call? (You haven’t given me any numbers yet.)
- Which number should I call? (You gave me two or three to choose from.)
The same applies to “What number bus goes downtown?” and “Which number bus goes downtown?” Both are natural. “Which” suggests the person is looking at a set of bus routes. “What” treats the question as fully open.
Quick comparison
| Which number | What number | |
| Choices | Limited or known | Open or unknown |
| Feels like | Picking from a group | Asking for any answer |
| Example | Which number is larger, 5 or 9? | What number comes after 99? |
Common mistakes
Using “which number” for an open math question.
Less natural: Which number do you get when you multiply 8 by 7?
Better: What number do you get when you multiply 8 by 7?
Using “what number” when the choices are right in front of you.
Less natural: What number of these two is correct, 14 or 41?
Better: Which number is correct, 14 or 41?
Adding “of” after “which” or “what.”
Say “which number” or “what number,” not “which number of.” If you mean a count, use “how many”: “How many students came?”
A note on “what number” and “what’s the number”
“What number” often appears in everyday questions about phone numbers, addresses, rooms, and tickets. Many people say “What’s your phone number?” because it is short and direct. “Which number is yours?” works only when you can point to a set of numbers.
Takeaway
Use which number when someone is choosing from a known group. Use what number when the answer can be anything. If you can’t decide, ask whether the listener can see the choices. If they can, say “which.”
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I write clear, practical English lessons for everyday use. On Lingotexting, I break down grammar, vocabulary, and word types into simple ideas you can apply quickly. My focus is accuracy, real examples, and helpful visuals, so learners build confidence, improve writing, and communicate naturally in school, work, and daily life.