Which word do you think expresses a stronger degree or extent of something, "pretty" or "quite"?

Or how about "quite" or "fairly"?

"Rather or pretty?"

After consulting some dictionaries, they tell me that the following sentences would be ordered from strongest to weakest:

It's extremely expensive.
It's quite expensive.
It's pretty [rather / fairly] expensive.

I suppose I could agree with that, though I'd be open to hearing other arguments.

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When putting phrases like these into Japanese, we can run into some trouble, simply because dictionary entries can be a bit misleading.

For example, here are dictionary entries for the words けっこう and かなり as they relate to describing the extent or degree of something:

けっこう
quite; reasonably; fairly; tolerably

かなり
considerably; fairly; quite

As you can see, dictionaries don't help very much when trying to figure out the differing nuances of words.

Looking at those entries above, it may be hard to come to the (correct) conclusion that かなり is stronger than けっこう.

To put it another way, we can say that けっこう is a softener, like the words "pretty," "fairly," or "rather."

かなり, on the other hand, adds force/strength to a statement, like the words "quite" or "considerably."

An example:

今日はけっこうあったかいね。
きょう　は　けっこう　あったかい　ね。
It’s pretty warm today, huh?
Literally: “today + quite / pretty + warm + ね.”
Note: あったかい is a spoken abbreviation of the word あたたかい.

かなりあったかい would be warmer than けっこうあったかい.

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Now here's an example with かなり：

サンフランシスコの家賃はかなり高い。
サンフランシスコ　の　やちん　は　かなり　たかい。
Rent is quite expensive in San Francisco.
Literally: “San Francisco + の + rent + は + quite / rather + high / expensive.”

かなり高い is more expensive than けっこう高い.

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Personally, I use the word けっこう more often than I use かなり. They are both commonly used words, though.

Here's one way in which I might find myself using けっこう as a softener (i.e. meaning something like "pretty"):

A:
ワンダーウーマンどうだった？
ワンダーウーマン　どう　だった？
How was Wonder Woman?
Literally: “Wonder Woman + how + was?”

B:
けっこう面白かったよ。
けっこう　おもしろかった　よ。
It was pretty good.
Literally: “quite / pretty + was interesting / was fun + よ.”

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On the other hand, here is a situation in which we could use かなり：

Your friend is talking about how she was so nervous on her first day of work. Sympathizing with her, you mention how everyone gets quite nervous on their first day...

仕事の初日はかなり緊張するよね。
しごと　の　しょにち　は　かなり　きんちょう　する　よね。
Yeah, you get quite nervous on your first day of work, huh? // Yeah, the first day of work at a new job is quite nerve-racking, isn't it?
Literally: “work / job + の + first day + は + quite / rather + nervousness / tension + do + よね.”

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Multiple Meanings of けっこう

The word けっこう can also be used to decline offers.

In these situations, it means something like "I'm fine" or "That's all right."

When writing けっこう in these situations, we would typically use the kanji, which is 結構 (けっこう):

A:
コーヒーはいかがですか。
コーヒー　は　いかが　です　か。
Would you like some coffee?
Literally: “coffee + は + how (formal) + です + か.”

B:
いいえ、結構です。
いいえ、　けっこう　です。
No, I’m fine.
Literally: “no, + (I’m) fine / sufficient + です.”

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One more, and then we'll be done:

A:
もう一ついかがですか？
もう　ひとつ　いかが　です　か？
Would you like another one?
Literally: “another + how + です + か?”

B:
いいえ、もう結構です。
いいえ、　もう　けっこう　です。
No, I’ve had enough.
Literally: “no, + already + (I’m) fine / sufficient + です.”

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Well, I hope this lesson was かなり easy to understand.

If not, well, uh... sorry. ^^

Hope you're enjoying your studies!