Enter the date of birth
How to use the age calculator, and how ages are counted in Japan
Working out your age today is easy enough in your head, but it gets fiddly as soon as you need a different reference date, a school year, or the traditional Japanese count. Enter a birth date and this tool shows the full age, the traditional "kazoedoshi" count, the total days lived and the days until the next birthday together.
How to use it
Enter the date of birth and the age for today appears immediately. The reference date can be changed freely, so you can check "how old on the joining date" or "how old next April" without doing the arithmetic yourself — useful when filling in forms or checking eligibility.
In Japanese law, age increases the day before the birthday
In everyday life people turn a year older on their birthday, but Japanese law adds the year at the end of the day before the birthday (the Act on Calculation of Age, and Article 143 of the Civil Code). It rarely matters day to day, but it is the reason a child born on 1 April belongs to the earlier school year: they turn six at the close of 31 March, so they fall inside that year's intake.
Full age vs. kazoedoshi
Full age starts at 0 at birth and increases on each birthday — the same convention used in most countries. Kazoedoshi starts at 1 at birth and increases for everyone on 1 January. Traditional milestones such as Shichi-Go-San, unlucky years (yakudoshi) and the 60th-year kanreki are usually quoted in kazoedoshi, so the same person can be one or two years older by that count. This tool shows both side by side.
School years
A Japanese school year groups children born from 2 April to 1 April the following year. Because of the rule above, 1 April is the last day of the earlier year rather than the first day of the later one.
"Contract age" on insurance forms
Some life insurance products use an insurance age rather than the full age, rounding up when six or more months have passed since the last birthday. The definition varies by product, so check the policy documents for the exact wording. If you simply need to count days, try the date calculator; for pregnancy dates, see the due date calculator.
Frequently asked questions
How is age calculated?+
It is the common full-year age.
How are the zodiac and Chinese zodiac determined?+
The Chinese zodiac comes from the birth year's 12-year animal cycle.
How do you count days lived?+
Can I use a reference date other than today?+
Is my date of birth stored?+
・Age = number of birthdays reached by the reference date (ref year − birth year, minus 1 if the birthday has not yet occurred)
・Kazoedoshi = full age + 1 (+ 2 before the birthday), incremented on 1 January
・Chinese zodiac = birth year's animal in the 12-year cycle ((year − 4) mod 12)
・Western zodiac = month and day mapped to the usual sign cut-offs
・Act on Calculation of Age (Act No. 50 of 1902) and Civil Code Art. 143: age increases at the close of the day before the birthday (this tool displays it on the birthday itself)
・School Education Act, Art. 17: school years group births from 2 April to 1 April, which is why a birth on 1 April falls in the earlier year
This site does not represent any organization and does not guarantee the accuracy of the results. Age handling for schooling or various systems varies by law and rules. For official procedures, confirm the prescribed basis.