The basic age formula
Calculating age sounds simple — subtract your birth year from the current year — but accurate age calculation requires handling months, days, and leap years correctly. Here is the full method:
- Start with the current year minus your birth year: 2026 − 1995 = 31
- Check if your birthday has occurred yet this calendar year. If not, subtract 1.
- For months: if the current month is before your birth month, subtract 1 more year. If it is the same month, check the day.
Example: Born September 20, 1995. Today is April 13, 2026.
2026 − 1995 = 31. But April (month 4) is before September (month 9), so the birthday has not occurred in 2026 yet. Age = 30 years.
How to calculate your age in days
Counting days since birth sounds straightforward, but leap years add complexity. Every 4 years (except century years not divisible by 400), February has 29 days instead of 28.
The approximate formula: (years × 365) + (number of leap years since birth) + remaining days.
A 30-year-old born in 1995 has lived through approximately 7–8 leap years (1996, 2000, 2004, 2008, 2012, 2016, 2020, 2024). 30 × 365 = 10,950 + 8 = approximately 10,958 days as a baseline, then add or subtract remaining months and days to the birthday.
For precise day counts, use the calculator below — it handles leap years automatically and calculates down to the exact day.
How many weeks old are you?
Divide your age in days by 7. A 25-year-old is approximately 25 × 365.25 (accounting for leap years) ÷ 7 ≈ 1,305 weeks old. This number gets used in paediatrics — doctors track infant development in weeks during the first two years, and pregnancy progress is measured entirely in weeks.
Age in different countries and cultures
International (most countries)
Age increments on your birthday each year. You are 0 years old from birth until your first birthday, then 1, and so on. This is the system used in the US, UK, Europe, most of Asia, and most of the world.
Korean age (East Asian counting)
Traditionally, Korean age counts the time spent in the womb, so you are 1 at birth. Age then increments every January 1st rather than on your individual birthday. A baby born in December 2025 would be "2 Korean years old" by January 2026. South Korea officially transitioned to international age counting in June 2023, but the traditional system is still used colloquially.
Legal age considerations
Legal age varies by country and purpose. In the US, 18 is the age of majority for voting and contracts; 21 for alcohol. In the UK, 18 applies to both voting and alcohol. In Canada, the age of majority is 18 in some provinces and 19 in others. For immigration, age is typically calculated as of the date of application submission, not processing.
Pregnancy age: weeks vs months
Pregnancy duration is measured in weeks from the last menstrual period (LMP), not from conception. A full-term pregnancy is 40 weeks (approximately 9 months and 7 days). Trimesters are approximately:
- First trimester: weeks 1–12
- Second trimester: weeks 13–26
- Third trimester: weeks 27–40
If you know your due date, you can calculate how many weeks pregnant you are by subtracting today's date from your due date, then subtracting from 40 weeks.
Age and retirement planning
In the US, Social Security full retirement age (FRA) is 67 for those born in 1960 or later. Early retirement is available at 62 with reduced benefits. Medicare eligibility begins at 65. Required Minimum Distributions (RMDs) from retirement accounts must begin at age 73 under current law. Knowing your exact age and birth date is essential for planning around these thresholds — a few months can make a meaningful financial difference.
Interesting age milestones in days
- 1,000 days old: approximately 2 years and 9 months
- 10,000 days old: approximately 27 years and 5 months
- 20,000 days old: approximately 54 years and 9 months
- 1,000,000 hours old: approximately 114 years (nearly impossible to reach)