Start with a question:
その鞄の軽＿＿に驚いた。
A. い B. く C. さ D. み
The answer is C. さ. "I was amazed at that bag's lightness" -- 軽さ turns the adjective "light" into the noun "lightness."
What さ Does
Adjectives describe states (高い, 軽い, 静かだ). But sometimes you need to talk about that state itself as a thing -- that's when you use さ:
| Adjective | + さ = Noun | Meaning |
|---|---|---|
| 高い (tall/expensive) | 高さ | height |
| 長い (long) | 長さ | length |
| 軽い (light) | 軽さ | lightness |
| 優しい (kind) | 優しさ | kindness (the quality) |
| 楽しい (fun) | 楽しさ | enjoyability |
Conjugation Rules
い-adjectives: Drop い, add さ
高い → 高さ
| Original | さ form | Example |
|---|---|---|
| 深い | 深さ | 海の深さは何メートルですか。 → How deep is the ocean? |
| 広い | 広さ | この部屋の広さは20畳です。 → This room is 20 tatami mats. |
| 美味しい | 美味しさ | この料理の美味しさは忘れられない。 → I can't forget how delicious this dish was. |
な-adjectives: Stem + さ
静か**(だ)** → 静かさ
| Original | さ form | Example |
|---|---|---|
| 静かだ | 静かさ | 夜の静かさが好きだ。 → I like the quietness of the night. |
| 便利だ | 便利さ | スマホの便利さに慣れた。 → I got used to the convenience of smartphones. |
| 大切だ | 大切さ | 健康の大切さを知った。 → I learned the importance of health. |
さ vs み
Japanese has another nominalizing suffix: み. Here's how they differ:
| さ | み | |
|---|---|---|
| Nature | Objective measurement/degree | Subjective feeling/sensation |
| Example | 高さ (height -- measurable) | 痛み (pain -- only you feel it) |
| Range | Works with almost all adjectives | Only certain adjectives |
| Feel | Like measuring with a ruler | Like feeling with your body |
Common み forms: 痛み (pain), 悲しみ (sadness), 楽しみ (anticipation/joy), 温かみ (warmth).
When in doubt, choose さ -- it almost always works.
Usage Scenarios
Measurements and Data
-
このビルの高さは200メートルです。 → This building is 200 meters tall.
-
荷物の重さを量ってください。 → Please weigh the luggage.
Marveling at Qualities
-
その鞄の軽さに驚いた。 → I was amazed at that bag's lightness.
-
母の優しさを思い出した。 → I remembered my mother's kindness.
Comparing Degrees
- 東京と大阪の暑さはどちらがひどいですか。 → Which is worse, Tokyo's heat or Osaka's?
Self-Test
Q1. Nominalize 「美しい」.
Show answer
美しさ. い-adjective: drop い, add さ = beauty (as a quality).
Q2. What does 「この街の静かさが気に入っている」 mean?
Show answer
"I like the quietness of this town." 静か (な-adjective stem) + さ = quietness (as a quality).
Q3. What's the difference between 「楽しさ」 and 「楽しみ」?
Show answer
- 楽しさ = the degree of fun (objective: "the enjoyability of this event is high")
- 楽しみ = anticipation/pleasure (subjective: "I'm looking forward to it" -- 楽しみにしている)
さ leans objective/measurable; み leans subjective/felt.
Summary
- い-adjective: drop い + さ = noun (高い → 高さ)
- な-adjective: stem + さ = noun (静かだ → 静かさ)
- さ turns adjectives from "describing a state" into "a discussable thing"
- さ is objective/measurable; み is subjective/felt
- When in doubt, use さ