Functions: ぐらい attaches to the word it measures and does three related jobs.
1. Approximation — the everyday one. It follows the quantity: 30分ぐらい (about thirty minutes), どのぐらい (about how much, how long), これぐらい (about this much).
2. A minimum, often with a dismissive edge — "at least that much", "only that much": 挨拶ぐらいしなさい (at least say hello), これぐらいできる (I can manage this much, at least).
3. A yardstick for degree — X is so extreme that Y: 涙が出るぐらい嬉しい (so happy I could cry). Here ぐらい does not mean "about" at all; it names a measure of how much.
Variant: くらい is the same word and the same meanings. In modern usage the two are interchangeable, with a mild tendency toward ぐらい after numbers and くらい after これ, それ, and どの.
Similar words
- 約 is the formal equivalent of function 1, but it goes **before** the number, not after: 約30分, never 30分約. It belongs to writing and announcements; ぐらい to speech.
- 頃 (as ごろ) marks an approximate **point** in time, ぐらい an approximate **span**. 3時ごろ is around three o'clock; 3時間ぐらい is about three hours. Mixing these up is one of the commonest learner slips with this word.
- ほど covers the same ground but is more formal and more written. It is also the form required in negative comparisons (〜ほど〜ない), where ぐらい does not work.
Register: neutral and very common in speech. In writing and formal announcements, 約 or ほど usually replaces it.