Japanese has a single, beautifully regular tool for turning an adjective into a noun: the suffix 〜さ. 高い ("high / tall") becomes 高さ ("height"); 便利 ("convenient") becomes 便利さ ("convenience"); 難しい ("difficult") becomes 難しさ ("difficulty"). What 〜さ names is the measurable degree of the quality — how much of it there is. This is the noun you reach for whenever you mean an amount of something: a height you could put in meters, a difficulty you could rate, a kindness you could say there was "a lot" of.
The reason to learn 〜さ early and thoroughly is that it is productive — it attaches to almost any adjective in the language, with no list to memorize. Once you know the two attachment rules below, you can nominalize an adjective you have never seen before and be right. Its rarer cousin 〜み (痛み, 深み) does something different and only works on a fixed handful of words — the 〜み page draws that line — but 〜さ is the workhorse, and it should be your default.
The two rules
い-adjectives: drop the final 〜い, add さ. Only the okurigana changes; the kanji stem stays put.
| Adjective | Noun | Meaning |
|---|---|---|
| 高い (takai) | 高さ (takasa) | height |
| 大きい (ōkii) | 大きさ (ōkisa) | size |
| 長い (nagai) | 長さ (nagasa) | length |
| 重い (omoi) | 重さ (omosa) | weight |
| 速い (hayai) | 速さ (hayasa) | speed |
| 難しい (muzukashii) | 難しさ (muzukashisa) | difficulty |
| 優しい (yasashii) | 優しさ (yasashisa) | kindness |
な-adjectives: add さ directly to the bare word. There is no な to drop — な only appears when the adjective sits in front of a noun, and here it does not.
| Adjective | Noun | Meaning |
|---|---|---|
| 便利 (benri) | 便利さ (benrisa) | convenience |
| 大切 (taisetsu) | 大切さ (taisetsusa) | importance |
| 正確 (seikaku) | 正確さ (seikakusa) | accuracy |
| 豊か (yutaka) | 豊かさ (yutakasa) | richness / abundance |
この山の高さは、およそ二千メートルです。
kono yama no takasa wa, oyoso nisen mētoru desu
This mountain's height is about 2,000 meters.
この問題の難しさは、やってみて初めて分かった。
kono mondai no muzukashisa wa, yatte mite hajimete wakatta
I only understood how hard this problem was once I actually tried it.
彼の優しさに、これまで何度も救われてきた。
kare no yasashisa ni, kore made nandomo sukuwarete kita
His kindness has saved me many times over the years.
Why "measurable degree" is the right way to think about it
The English suffixes -ness, -ity, and the noun forms height, width, speed all get folded into this one Japanese suffix, so it is tempting to file 〜さ under "makes -ness words and move on." But there is a sharper idea underneath, and holding onto it will keep you from misusing the suffix: 〜さ names the quality as a quantity on a scale.
高さ is not "the abstract concept of being tall"; it is "the extent to which something is tall" — a value that could be two meters or two hundred. 優しさ is likewise conceived as an amount of kindness, which is exactly why 彼の優しさ pairs so naturally with expressions of quantity. This is the deep reason 〜さ is so productive: any gradable adjective, by definition, has a degree, so any gradable adjective can take 〜さ.
この靴、大きさはちょうどいいけど、色がちょっとね。
kono kutsu, ōkisa wa chōdo ii kedo, iro ga chotto ne
These shoes — the size is just right, but the color, hmm...
光の速さでメールの返信が来て、びっくりした。
hikari no hayasa de mēru no henshin ga kite, bikkuri shita
The email reply came at the speed of light — it startled me.
いい → よさ (the one irregular)
The adjective いい ("good") is irregular in every derived form because it reverts to its older stem よ- before any suffix. So its 〜さ noun is よさ ("goodness / good points"), never ×いさ. This is the same swap that gives you よくない (not good) and よさそう (looks good) — bank it once and it recurs everywhere.
この町のよさは、住んでみないと分からないよ。
kono machi no yosa wa, sunde minai to wakaranai yo
What's good about this town, you won't get until you've lived here.
〜さ nouns behave like ordinary nouns
Once formed, a 〜さ word is a full-fledged noun. It takes の to be modified (この問題の難しさ), it can be a subject or object, and it slots in anywhere a noun does. A very common frame is 〜の＋[adjective]さ＋が分かる/に驚く/を感じる — "to realize / be surprised by / feel the X-ness of something."
一度この便利さを知ると、もう前の生活には戻れない。
ichido kono benrisa o shiru to, mō mae no seikatsu ni wa modorenai
Once you know this convenience, you can't go back to how you lived before.
荷物の重さを量ってから、送料を計算します。
nimotsu no omosa o hakatte kara, sōryō o keisan shimasu
I'll weigh the package first, then calculate the shipping cost.
改めて、家族の大切さに気づかされた。
aratamete, kazoku no taisetsusa ni kizukasareta
It made me realize all over again how important family is.
A note on 静かさ vs 静けさ
Some な-adjectives have a slightly poetic, lexicalized alternative alongside the regular 〜さ form. 静か ("quiet") gives the perfectly correct 静かさ, but the everyday noun for stillness is often 静けさ (shizukesa) — an older formation inherited from classical grammar. Both are understood; 静けさ (literary) simply sounds more evocative and is what you will see in descriptions of a hushed forest or a quiet night. This is one of the very few places where the regular 〜さ form has a competing partner, so it is worth flagging rather than pretending the pattern is seamless.
夜の森の静けさが、かえって心を落ち着かせた。
yoru no mori no shizukesa ga, kaette kokoro o ochitsukaseta
The stillness of the forest at night was, if anything, calming.
How this differs from English
English has no single suffix that does this job. You would use -ness (kindness), -ity (difficulty, convenience), -th (length, depth, width), or a suppletive noun (high → height, fast → speed, heavy → weight), and there is no way to predict which. A learner of English simply has to memorize each pair. Japanese collapses all of that into one rule: stem + さ. There is no irregular noun form to look up — 高さ, 長さ, 速さ, 重さ are all built the same way, where English gives you height, length, speed, weight, four unrelated words.
The flip side is that English lets you attach -ness rather freely to coined or nonce adjectives ("the just-rightness of it"), and Japanese 〜さ is, if anything, even freer — it genuinely attaches to almost any adjective, including ones invented on the spot. That productivity is the single most useful fact on this page: when in doubt about how to noun-ify an adjective, add さ.
Common Mistakes
1. Forgetting to drop the final い. You replace い with さ; you do not stack them.
❌ この山の高いさを調べた。
kono yama no takai-sa o shirabeta
Incorrect — the final い must drop: 高い → 高さ.
✅ この山の高さを調べた。
kono yama no takasa o shirabeta
I looked up this mountain's height.
2. Deleting a な that was never there on a な-adjective. な-adjectives add さ to the bare word; there is no な in the noun form to begin with, and there is certainly no ×なさ to insert.
❌ このアプリの便利なさに驚いた。
kono apuri no benri-na-sa ni odoroita
Incorrect — no な: 便利 + さ = 便利さ.
✅ このアプリの便利さに驚いた。
kono apuri no benrisa ni odoroita
I was amazed by how convenient this app is.
3. Writing いさ for いい. いい reverts to the よ- stem, so its noun is よさ.
❌ この店のいさは値段だ。
kono mise no isa wa nedan da
Incorrect — いい → よさ, never ×いさ.
✅ この店のよさは値段だ。
kono mise no yosa wa nedan da
The good thing about this shop is the price.
4. Reaching for 〜み when you mean a measurement. For a quantifiable amount — a height, a weight, a size — use 〜さ. 〜み names a felt quality, not a number, and many み-forms simply do not carry a measurement sense (the word 高み exists, but it means "a lofty vantage point," not "height as a measure").
❌ このビルの高みは百メートルだ。
kono biru no takami wa hyaku mētoru da
Incorrect — for a measured height use 高さ; 高み means 'lofty heights,' not a measurement.
✅ このビルの高さは百メートルだ。
kono biru no takasa wa hyaku mētoru da
This building's height is 100 meters.
Key Takeaways
- い-adjective: drop 〜い, add さ (高い → 高さ). な-adjective: add さ to the bare word (便利 → 便利さ).
- 〜さ names the measurable degree of a quality — the amount of it, on a scale — which is why it is objective and why it fits measurements.
- It is fully productive: it attaches to almost any adjective, with no list to learn. Make it your default for turning an adjective into a noun.
- Irregular: いい → よさ (via the よ- stem), never ×いさ.
- Use 〜さ, not 〜み, whenever you mean an amount or a measurement; 〜み is the limited, felt-quality suffix covered on its own page.
Now practice Japanese
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Start learning Japanese→Related Topics
- 〜み: Concrete Nouns from AdjectivesN3 — The limited, semi-lexicalized suffix 〜み that turns certain adjectives into nouns naming a felt quality or a concrete instance — 痛み (pain), 深み (richness), 重み (gravity) — and how it differs from the measuring 〜さ.
- Two Adjective ClassesN5 — Japanese has two structurally different kinds of adjective — い-adjectives that conjugate themselves like verbs, and な-adjectives that are really nouns borrowing the copula — and this single split explains every adjective form you will ever meet.
- Japanese Nouns: No Gender, No Articles, No PluralN5 — Japanese nouns don't inflect for gender, definiteness, number, or case — the grammatical work English does with articles, plural -s, and word order is handled instead by particles and context.
- Adverbial Form: 〜く / 〜にN4 — Turning adjectives into adverbs — i-adjectives change 〜い to 〜く (早く走る), na-adjectives add 〜に (静かに歩く) — the same stem that also feeds なる 'become' and する 'make', plus the よく polysemy.