倍【bèi】

倍 means "times", as in "three times as many". The number comes straight before it. The plainest frame is A 是 B 的 N 倍: "A is N times B".

两倍 is the usual way to say "twice as much"; 二倍 also appears in maths.

With 比: the difference

With 比, the number before 倍 measures the difference, not the total. 比 B 大一倍 says A is bigger than B by one B – so A is twice B.

增加了 or 增加到

The same split shows up with "increase". 增加了一倍 is "increased by one times" – doubled. 增加到三倍 is "increased to three times" – tripled.

Where it goes wrong

English "twice as big" leads learners to say 两倍 with 比. With 比, "twice" is 一倍.

• × 这个房间比那个大两倍 (meaning twice the size) → 这个房间比那个大一倍 / 这个房间是那个的两倍

倍 measures how much more, not how much less. "Half as expensive" takes 一半.

• × 这个比那个便宜两倍 → 这个的价格是那个的一半