GSM means grams per square metre — how much cotton is packed into the towel. It is the closest thing towels have to a spec.
The ranges
**300-400 GSM** is a thin, fast-drying towel. It packs small and dries on a radiator overnight, which in a small room with poor ventilation is a real advantage. It does not feel luxurious.
**450-550 GSM** is the everyday range and the right default for halls. Absorbent enough, still dries in a day.
**600-700 GSM** is hotel weight. It feels excellent and it absorbs more water. In a room with no airer and no radiator it will still be damp the next morning, which is how towels start to smell.
**Above 700 GSM** is bath-sheet territory and impractical in a student room.
The practical advice
Two mid-weight towels beat one heavy one. You need a spare while the first is in the wash, and the laundry room in halls is rarely free when you want it. Check the pack includes at least two bath towels and, ideally, a face cloth — the cheapest thing to buy separately, and the one most often left out.