bao nhiêu / mấy

Both mấy and bao nhiêu are interrogative quantifiers meaning 'how much' or 'how many', but they are not interchangeable.

1. Expected Quantity

• Use mấy when you expect a small answer, typically under 10. It carries an inherent assumption of smallness and is highly conversational. A classic example is asking about age: use mấy tuổi for children.
• Use bao nhiêu when you expect an answer of 10 or more, when the quantity is completely unknown, or when asking about uncountable nouns. It is standard in both everyday speech and formal writing. For age, use bao nhiêu tuổi for adults.

Note: The 'under 10' rule is fluid in real life. Native speakers often use mấy to sound casual when asking about a relatively small, indefinite group (e.g., asking a party at a restaurant Anh đi mấy người?), even if the answer might be 12 or 15.

2. Grammar Rules

• With countable nouns: Mấy acts syntactically like a number, so it requires a classifier for most objects (e.g., mấy cái bút). However, certain common nouns act as their own classifiers and do not need one (e.g., mấy người, mấy ngày, mấy năm). With bao nhiêu, the classifier is optional and frequently dropped (e.g., bao nhiêu bút or bao nhiêu sách).
• With uncountable nouns: Only bao nhiêu can be used (e.g., bao nhiêu tiền).

3. Time and Dates

• Clock Time vs. Duration: Vietnamese exclusively uses mấy giờ for 'what time is it?'. For duration, you can use either depending on the expected length: mấy for a small number (e.g., mấy tiếng = how many hours, mấy ngày = how many days) and bao nhiêu for larger or unknown durations.
• Dates: When asking for the day of the month, both ngày mấy and ngày bao nhiêu are standard and commonly used, since days go up to 31. For months, use mấy (tháng mấy) because there are only 12. For the year, you can use either bao nhiêu (năm bao nhiêu) or, very commonly in everyday speech, mấy (năm mấy, e.g., Em sinh năm mấy?).

4. Position in the Sentence
When asking for a quantity, these words go before the noun. However, when asking for a rank, date, or sequence, they go after the noun. Compare these minimal pairs:

• mấy ngày (how many days?) vs. ngày mấy (what date?)
• bao nhiêu năm (how many years?) vs. năm bao nhiêu (what year?)

5. Using Them in Statements
In declarative sentences, mấy functions as an indefinite quantifier meaning 'a few' or 'some' (e.g., Tôi có mấy quyển sách = I have a few books). Similarly, bao nhiêu can be used to mean 'so much' or 'so many' (e.g., biết bao nhiêu người = so many people), or in paired structures like có bao nhiêu, dùng bấy nhiêu (use as much as you have). Learners often mistake these statements for questions, so pay attention to the context!