〜さ / 〜み — abstract noun formation from adjectives
Japanese has two productive suffixes that turn adjectives into nouns. 〜さ is the all-purpose 'measurable quality' suffix — attach it to almost any i-adjective or na-adjective stem and you get an objective, measurable noun: 高い → 高さ ('height'), 重い → 重さ ('weight'), 大きい → 大きさ ('size'). 〜み is the limited but evocative 'subjective feel / experiential quality' suffix — only attaches to a small fixed set of adjectives, and produces a noun that suggests the felt or experienced version of the quality: 高い → 高み ('high vantage / heights'), 重い → 重み ('gravity / weight of meaning'), 痛い → 痛み ('pain'). 〜さ is fully productive; 〜み is closed-class and lexicalized.
〜さ — productive measurement
〜さ attaches to the stem of almost any i-adjective (drop the い) or na-adjective (use the bare stem). It always produces a measurable, objective noun — a quantity, dimension, or degree. i-adjective examples: 高い → 高さ (height), 重い → 重さ (weight), 長い → 長さ (length), 大きい → 大きさ (size), 早い → 早さ (speed/earliness), 強い → 強さ (strength), あつい → あつさ (heat — temperature). na-adjective examples: しずか → しずかさ (quietness), べんり → べんりさ (convenience), きれい → きれいさ (cleanness/beauty as a degree). The noun is what you'd put a number on: 山 の 高さ は 三千メートル です。 = 'The mountain's height is 3000 meters.' にもつ の 重さ を はかる。 = 'measure the weight of the luggage.' If you can ask 'how much / how + adjective?', 〜さ will form the noun for the answer.
〜み — fixed, subjective feel
〜み only works on a small lexicalized set, and the resulting noun feels evocative rather than measurable — a quality you experience or sense, not one you measure. The classic ones: 高い → 高み ('high vantage / spiritual height'), 重い → 重み ('gravity / weight of meaning'), 深い → 深み ('depth — of feeling, of taste, of experience'), 強い → 強み ('one's strong point / strength'), 弱い → 弱み ('one's weakness / weak point'), ありがたい → ありがたみ ('felt gratitude / preciousness'), おもしろい → おもしろみ ('inherent fun / interest'), 痛い → 痛み ('pain'), 苦い → 苦み ('bitterness — taste'), 甘い → 甘み ('sweetness — taste'). Note these often involve taste, feeling, or character — the experiential side. 彼 の 言葉 に は 重み が ある。 = 'His words have weight (gravity).' この お茶 に は 深み が ある。 = 'This tea has depth.' You can't freely make new ones — やすみ doesn't mean 'cheapness' (it means 'rest/holiday', historically a different formation).
〜さ vs. 〜み for the same root
Several adjectives form both, with a clear meaning split. 高い: 高さ = height-as-measurement (3000 m), 高み = lofty position, vantage point, spiritual heights — 高み の けんぶつ = 'watching from the sidelines'. 重い: 重さ = weight in kilograms, 重み = the felt weight of words / responsibilities — 責任 の 重み. 深い: 深さ = depth in meters, 深み = experienced depth (of taste, of meaning, of feeling) — 味 に 深み が ある. 強い: 強さ = strength as a measurable degree, 強み = a personal strong point — 彼 の 強み は 集中力 だ. 弱い: 弱さ = how weak something is (degree), 弱み = a personal weak point. The pattern: 〜さ measures, 〜み characterizes. When you're talking numbers, 〜さ. When you're talking felt qualities or someone's traits, 〜み.
Productivity rule of thumb
〜さ is productive — you can take essentially any descriptive adjective and form a 〜さ noun, even one you've just made up. おかしい → おかしさ ('funniness / strangeness'), やさしい → やさしさ ('kindness'), むずかしい → むずかしさ ('difficulty'). If a learner needs an abstract noun and isn't sure, 〜さ is almost always safe. 〜み is closed-class — stick to the lexicalized set and don't invent new 〜み forms. おかしみ exists in archaic/literary Japanese but feels stilted; やさしみ is not standard. Some adjectives only form 〜さ (長さ, 広さ), some form only 〜み (痛み — there's no 痛さ in standard usage, though 痛さ does appear informally), and some form both (the 高さ/高み set above). When in doubt, default to 〜さ.
In sentences
Treat 〜さ / 〜み nouns like any other noun. They take particles (〜が, 〜を, 〜の), modify other nouns with 〜の, and behave normally. この 山 の 高さ は どれくらい です か。 = 'About how tall is this mountain?' 彼 の 強み は 何 です か。 = 'What's his strong point?' 痛み が 取れない。 = 'The pain won't go away.' 甘み が 足りない。 = 'It's not sweet enough (lit. the sweetness is insufficient).' Compound use: 重要さ ('importance' from 重要), 新しさ ('newness'), 正しさ ('correctness') — all built from familiar adjectives. The suffix is one of Japanese's most economical word-building tools.
