I am pretty sure that when native speakers talk about speed, they say "速さ", "広さ" for width, "大きさ" for size, etc. Am I correct about this? If I am, then:

Why not say "遅さ" instead of "速さ", "狭さ" instead of "広さ"? I suspect that when you convert comparable adjectives to nouns, you should use the greater of the two. However, surely this must just be a convention, right? So,
"新幹線の速さは何ですか?" // wrong?
"新幹線の遅さは何ですか?" // wrong?

what about:
"snailの速さは何ですか?" // wrong?
"snailの遅さは何ですか?" // wrong?

And finally, are there cases, if there are any, where it's natural to use "遅さ", "狭さ", "小ささ", etc.?


  • 2
    I think the asymmetry in the choice of 速さ・遅さ (and so on) is related to markedness.
    – user1478
    Commented Dec 4, 2013 at 16:16

3 Answers 3


As you say, it is a convention to use the more "positive" word in Japanese. However, there are situations where we native speakers opt to use the "other" word. I could think of two conditions that make using the other word sound natural or even better.

1 Common knowledge:

This is when a thing's slowness, smallness, lightness, etc. is commonly known to the general public and the speaker decides that it can be used as a premise of a conversation. The slow speed of a snail (遅さ) is a prime example of this.

2 Contextual/Situational:

If you have to talk about the slow speed of a snail to someone who has never seen or heard of a snail, you would need to first explain how slow it moves. After explaining it, you can logically use 遅さ instead of 速さ.

Another example would be how "slowly" a Shinkansen train which is well-known for its high speed could run on a snowy day. It can actually run more slowly than a bicycle in certain places on a snowy day. In this case, you first explain the weather condition and you can use 遅さ with no problems or unnaturalness to the native ears.

Finally, allow me to correct your Japanese. You cannot say 新幹線の速さは何ですか as that clearly is a direct translation from English. You would say 新幹線の速さはどれぐらいですか、新幹線のスピードは何キロくらいですか, etc. Since we are discussing 速さ/遅さ, I probably should not be using the word スピード even though it is the more natural word choice for us.


Be careful regarding nouns vs. adjectives.

In your case, 速さ = speed is a noun, while 速い = fast and 遅い = slow are adjectives.

  1. This is basically same as in English.

    In English, you do not say What is fast of 新幹線? or What is slow of 新幹線?. Instead, you say What is the speed of 新幹線? = 新幹線の速さはなんですか?

    This is correct sentence, but not perfect. (It sounds weird.)

  2. The speed of a 新幹線 is not stable (while, e.g., the maximum speed of a 新幹線 is stable), so it is more common to ask about the speed using an adjective: How fast is 新幹線?, How slow is 新幹線?

    But 新幹線 is obviously a fast vehicle, and you want to ask about fastness. So the best sentence is How fast is 新幹線? = 新幹線はどれくらい速いですか?

You can apply this same strategy to the snail sentence:

snailの速さは何ですか? is not wrong, but not perfect. You want to ask slowness of a snail because it is a slow animal, so the best sentence is

"How slow is a snail?"

  • That's an interesting question. I don't know exact answer but here is my guess. Actually, there is a traditional form of 速さ, that is "速度" = "degree of speed" They are exactly the same meaning, but 速さ is more casual. However, Japanese does not have a word like "遅度". Thus 速さ was transformed from "速度".(But I don't know why Japanese does not have "遅度")
    – ponyoky
    Commented Dec 5, 2013 at 1:38

Unless I've missed something, it is because it sounds as odd in Japanese as it does in English. "新幹線の速さは何ですか?" - what is the speed of the bullet train? "新幹線の遅さは何ですか?" - what is the slowness of the bullet train?

Similarly, 広い is more like 'spacious' or 'extent'. 狭い is 'narrow'. If you attempt the higher JLPT levels (well, 3 & above) choosing the specific word is vital.

  • 1
    We NEVER say 新幹線の速さは何ですか? You guys are "translating" way too much.
    – user4032
    Commented Dec 4, 2013 at 14:21
  • @TokyoNagoya yes I realise that but I didn't think it helpful in answering the question to correct that Japanese as well. kinyo, I'm sorry but I am still struggling to understand what it is that you are asking. Perhaps somebody else will be able to help further. But perhaps for 小ささ: その車の小ささは、駐車に関していえば、実用面での利点である The car's small size is a practical advantage when parking. Commented Dec 4, 2013 at 15:49

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