ごとに

The expression ごとに means "each," "every," or "at intervals of." It attaches directly to nouns – including time words, counters, and general nouns – to indicate that something occurs repeatedly at regular intervals or applies individually to each item.

Regular intervals

When attached to time expressions or counters, ごとに indicates that something happens at regular, recurring intervals.

This usage is similar to English "every ~" or "at intervals of ~."

Each individual item

When attached to general nouns, ごとに means "for each" or "per," indicating that something varies or applies separately to each one.

Gradual change (～ごとに)

With time-related nouns, ごとに can also express gradual change – something that progresses each time the interval passes.

ごとに vs. おきに

A related expression is おきに, which also means "at intervals of." However, おきに can imply a gap or skip between occurrences, while ごとに straightforwardly means "every." For example, ふつか２日ごとに means "every two days" (day 2, day 4, day 6…), and ふつか２日おきに means "with a two-day gap between" (day 1, day 4, day 7…). In practice, the two are sometimes used interchangeably, so context matters.

Writing

ごとに is sometimes written with kanji as 毎に, but the hiragana form ごとに is far more common.