早い vs 速い — same reading, different kanji?

Question

Both are はやい but I keep mixing up which one to use for "the train is fast" vs "I woke up early." Is there a clean rule or do I just have to memorize per word?

Accepted Answer

There's a clean rule actually: 早い = early / soon (time), 速い = fast / quick (speed). 朝早く起きる = wake up early. 電車が速い = the train is fast. If you can swap in "early" in English, use 早い; if you can swap in "fast/quick," use 速い.

Answer

This tripped me up in anime subs too lol. 足が速い (fast runner) vs 朝が早い (early morning) — same word in furigana but totally different kanji depending on which meaning you want.

Answer

Good rule of thumb. One edge case: 頭がいい isn't related but 頭の回転が速い ("quick-witted") uses 速い for mental speed, not just physical — so think "rate/speed" broadly, not only literal running.