〜おきに

About

～おきに, when used in the 「XおきにY」 pattern, literally means that "Y happens with X intervals 'placed' in between each occurrence".

For smaller, more specific intervals (e.g. hours, minutes, seconds, meters),～おきに simply means "every X".

E.g. 2時間じかんおきに = "Every two hours"

E.g. 4分ぷんおきに = "Every four minutes"

For larger, less specific intervals, ～おきに does not count the initial year/month/week/day, and therefore does not mean "every X". We can adjust for this in translation by adding one interval from the Japanese sentence (e.g. 3年ねんおきに = every four years, rather than every three years).

E.g. 5年ねんおきに = "Every six years" (five years in between each occurrence)

E.g. 4か月げつおきに = "Every five months" (four months in between each occurrence)

～ごとに (a similar grammar point) and ～おきに are only interchangeable for small, specific time intervals (e.g. hours, minutes, seconds) and distances (e.g. meters).

Summary

• 

  「XおきにY」 means that "Y happens with X intervals 'placed' in between each occurrence"

• 

  ～おきに means "every X" for specific intervals (e.g. hours, minutes, seconds, meters)

• 

  For larger, less specific intervals, ～おきに does not count the initial year/month/week/day, and therefore does not mean "every X" (e.g. 3年ねんおきに = every four years, rather than every three years)