Sino-Japanese · Native Japanese · Common Counters
Japanese has two number systems. 和語 (wago, native Japanese) — ひとつ, ふたつ, みっつ… — is the original counting system and only reaches とお (10); after that everything switches to the borrowed system. 漢語 (kango, Sino-Japanese) — いち, に, さん… — has no upper limit and is what pairs with every counter word below (人, 個, 回, and the rest).
Most counters trigger a sound change at 1, 6, 8, and 10: the number's ending drops into a small っ (gemination), and a counter starting with か, さ, た, or は行 hardens to match — 一 + 本 (ほん) becomes いっぽん, not いちほん. は行 counters go a step further, shifting to ぱ行 after that gemination (ほん→ぽん) and to ば行 after ん at 3 (さん + ほん → さんぼん).
四, 七, and 九 are usually よん, なな, and きゅう before a counter — but not always. Clock hours are the well-known exception: 四時 is よじ, 七時 is しちじ, and 九時 is くじ, never よんじ, ななじ, or きゅうじ. Watch each item's note for the counters that break the general rule.
Calendar dates and the days of the week live on the Calendar page. This page counts durations instead — how many days, weeks, months, or years something lasts (二日間, 三週間, 一か月) — which read differently from the date itself.