Age Calculation: Full Age (周岁) vs Nominal Age (虚岁)

Key Takeaways

周岁 (full age): 0 at birth, +1 on each birthday — the legal and medical standard

虚岁 (nominal age): 1 at birth, +1 at each Lunar New Year — traditional East Asian reckoning

The gap: 虚岁 − 周岁 = 1 or 2, biggest just before your birthday after New Year

Legal rule: contracts, voting, drinking all use 周岁 — the day AFTER the birthday in some jurisdictions' interpretation

Core answer: full age (周岁) = completed years since birth (+1 on your birthday); nominal age (虚岁) = 1 at birth, +1 every Lunar New Year. Born May 1995, in March 2025: full age 29 (birthday not yet passed), nominal age 31. Law, contracts and school enrollment all use full age.

How full age works

Rule: 0 at birth, +1 each time the birthday passes.

Birth date	Reference date	Full age
2000-06-15	2025-06-14	24 (day before birthday)
2000-06-15	2025-06-15	25 (on the birthday)
1990-01-01	2025-12-31	35

Exact test: if today's month-day ≥ birth month-day → current year − birth year; otherwise subtract 1 more.

How nominal age works

Rule: 1 at birth, +1 at every Lunar New Year (Spring Festival).

Nominal age = current lunar year − birth lunar year + 1

Born on the 28th of the twelfth lunar month 1995: nominal age 1 at birth, and 2 two days later at New Year — "two years old" at three days of life, the classic counterintuitive case.

Side-by-side comparison

Born May 1995, reference March 2025 (after New Year):

System	Math	Result
Full age	2025−1995, May not reached → −1	29
Nominal age	2025−1995+1	31
Exact age	29 years 10 months	29.83 years

When the gap is 1 vs 2

Period	Nominal − Full
After birthday, after New Year	1
Before birthday, after New Year	2
After birthday, before New Year	0 (both just incremented)
Before birthday, before New Year	1

Which one do legal contexts use?

Criminal/civil law: full age (criminal responsibility at 12/14/16 full years)
School enrollment: 6 full years by Aug 31
Retirement/social insurance: full age, precise to the month
Driving license: 18/70 full-year thresholds
Traditional settings: birthday celebrations for elders and zodiac talk often use nominal age

Common mistakes

Reporting nominal age as full age: resumes and legal documents need full age — errors can have real consequences.
Miscounting on the birthday: you ARE the new age on the birthday itself (25 on your 25th birthday).
Adding nominal age at Jan 1: it increments at Spring Festival, not the Gregorian new year.
Month-age conversion: 18 months = 1.5 full years; infant developmental assessments are precise to the month.

Use the [age calculator](/c/date/age) — enter a birthday for full age, nominal age, months and the next-birthday countdown instantly.

Frequently Asked Questions

How is 周岁 (full age) calculated?

Zero at birth, +1 each time your birthday passes. Born 1995-06-15, today 2025-03-10: not yet this year's birthday → 2025 − 1995 − 1 = 29. After June 15: 30. This is the age for contracts, ID, medical records, and school enrollment — the international standard (international age = 周岁).

How is 虚岁 (nominal age) calculated?

One year old AT BIRTH (counting womb time symbolically), then +1 at every Lunar New Year — regardless of birthday. A baby born on Lunar New Year's Eve turns 2 虚岁 the next day. Everyone in the culture ages together on the same holiday, which is its original social function. Traditional ceremonies, some zodiac readings and elder speech still use it.

Why is the gap sometimes 1 and sometimes 2?

虚岁 jumps at Lunar New Year; 周岁 jumps at your birthday. Before your birthday in a year where New Year passed: 虚岁 = 周岁 + 2. After your birthday: +1. Born 1995-06-15, on 2025-03-10: 周岁 29, 虚岁 31 (gap 2). On 2025-08-01: 周岁 30, 虚岁 31 (gap 1). The gap never exceeds 2.

How do I compute exact age in years, months and days?

Subtract year-month-day from year-month-day, borrowing carefully: today 2025-03-10 minus birth 1995-06-15. Days: 10 − 15 → borrow a month (February has 28 days in 2025): 10 + 28 − 15 = 23 days; months: 3 − 1(borrowed) − 6 → borrow a year: 2 + 12 − 6 = 8 months; years: 2024 − 1995 = 29. Age: 29 years 8 months 23 days.

When does legal age actually kick in?

Statutory ages use 周岁: 18 for contracts/voting, with the precise moment being the START of the birthday in most readings — some jurisdictions rule it begins the day AFTER (a historical quirk in civil codes). School enrollment cutoffs (e.g., age 6 by Aug 31) create the "September baby advantage" — children born days apart start school a year apart. Check the exact cutoff date, not just the year.

What mistakes happen with age?

(1) Quoting 虚岁 on official forms — forms want 周岁; (2) zodiac-year confusion: 本命年 follows the lunar calendar, and some schools count zodiac from 立春 (Feb 3-5), not Lunar New Year; (3) "year subtraction" without checking whether the birthday passed; (4) Korean age — a third system (was +1 at birth, +1 every Jan 1) — Korea officially switched to international age in 2023; (5) assuming age at death = death year − birth year without the birthday check.