# When to use 一倍 vs 两倍

I would like to have a clarification on this: I know that 倍 means "-times, -fold", but I don't understand why 一倍 seems to mean "twice as (fast)" in the following sentence:

while we have to use 两倍 in the following sentences:

8是4的两倍。 X贵两倍。

I don't know how to tag this question so feel free to change it.

-

If some words that mean "is" (e.g 是/相当于 ) are used, then it's literal times.

Your speed is twice as mine.

Normally you will not say "one times" in this scenario. 你的速度是我的一倍 (Your speed is one times as mine) sounds strange in both English and Chinese. You can just say "你的速度和我的一样快"/ “我们俩一样快" (You are as fast as me).

If other "comparing" words that means "more than"(e.g. A B 大/高/快/etc.) show, then it's literal times + 1. Followed one has the same meaning of previous one.

You are one time faster than me.

-

Understand that you have an accpeted answer already and it's correct. I am adding this in hope to make it simple to understand:

They are the same. So when you are saying 快一倍, it means `100% faster`, or `twice the speed`, which means 200% of the origin speed - 两倍速度. The confusion comes in because when 一倍 is used, we `ADD` the value by itself, while 两倍 we `MULTIPLE` the value by 2. And we all know that A + A (一倍) = 2A (两倍)

-
This is exactly what I deduced from the accepted answer. That's a nice way of putting it though. – pg-robban Aug 28 '14 at 6:22

Tuttle's Learners

fold, time

Zhège xuéxiào de xuésheng rénshù bǐ wǒmen xuéxiào duō yí bèi. (= Zhège xuéxiào de xuésheng rénshù shì wǒmen xuéxiào de liǎng bèi.) The student number of this school is twice as big as that of our school. (—> The student population of this school is twice of ours.)

Oxford

② [indicating difference in quantity]

The output of rice is three times that of wheat in this area

but(!) - still from Oxford:

Your salary is twice that of mine.

Basically you've really got to look at the context to figure out the meaning!

Your sentence above `开车最少比骑自行车快一倍。` the 快 here has the same effect as 多 so it's really saying it's "one time as fast" which is really "twice as fast"...

Hope that makes sense

-
Regarding the last paragraph, 比骑自行车快一倍 is not saying it's "one time as fast" but "one time faster", which is legitimately equivalent to "twice as fast". – NS.X. Aug 26 '14 at 17:37

I think the more correct way to use 一倍/兩倍 is answered by user3306356 and Caff Huang.

But, in most contexts, 快一倍 and 快兩倍 have just same meaning. Because if we use 快兩倍 as "x * 3", the meaning will be too vague to understand.

If we really have to say something larger than two times of a value, such as "A = B * 3", a more practical way will be "A是B的3倍" instead of "A比B高兩倍".

-