Age Calculator

Calculate your exact age in years, months, and days with our precise age calculator. Find out your age at any specific date, calculate birth date from age, and discover how many days you've lived. Perfect for determining age requirements, planning birthday celebrations, or calculating age-related milestones. A comprehensive tool for age verification and birth date calculations.

Birth Date

Target Date

(Optional) Select a date to know your age at that time

How to Use Age Calculator?

1. Enter your birth date (day, month, and year)
2. Optionally, enter a target date to calculate your age at that time
3. Click the "Calculate" button
4. The results will show:
- Your exact age in years, months, and days
- Your birth date and day of the week
- Days until your next birthday
- Total days lived

Tips:
- Use target date to calculate birth date from age
- Birth dates must be between years 1900-2100
- You can calculate future ages as well
- Perfect for age verification and planning
- Automatically shows which day of the week you were born

Date Calculations
Age Calculator Date Calculator Add/Subtract Days Calculator Date and Time Calculator

How age is calculated, and why answers differ

This calculator reports your age the way civil registries and most Western countries do: complete years first, then the leftover months and days. The arithmetic looks trivial until February, leap years, and end-of-month dates get involved, which is where most manual attempts go wrong.

Formula

years = target.year - birth.year, minus 1 if (target.month, target.day) < (birth.month, birth.day)

The remainder is then expressed as whole months and the days left over after those months are subtracted.

The borrow rule that trips people up

Age is not a simple subtraction of two numbers. You subtract three components independently - day, month, year - and borrow when a component goes negative, exactly like long subtraction in base 10 except the bases are irregular.

When the day count goes negative you borrow one month, but a month is not a fixed quantity. The convention this calculator follows, and the one used by ISO 8601 duration arithmetic, is to borrow the length of the month preceding the target date.

  • Born 31 January, measured on 1 March 2025: days go negative, so borrow February (28 days) - the result is 1 month 1 day, not 1 month 29 days.
  • Born 15 May 1990, measured 14 May 2025: the day is one short, so the year has not completed. Age is 34 years 11 months 29 days, not 35.
  • Born 29 February 2000, measured 28 February 2025: non-leap years have no 29 February, so the birthday is treated as complete on 28 February in most legal systems.

East Asian age reckoning is a different system

If a result here does not match what you expected, the reason is often that you are thinking in a different counting system rather than that the arithmetic is wrong.

Traditional East Asian age counting starts a newborn at 1 and adds a year at Lunar New Year, so a baby born in December can be "2" a few weeks later. South Korea legally moved to international age reckoning in June 2023, but the traditional count still appears in conversation. This calculator always uses international reckoning.

Where exact age actually matters

Precision matters more than people expect once a threshold is attached to a date.

  • School enrolment cut-offs, which are usually "age on 31 December" or "age on the first day of term" rather than age today.
  • Pension and retirement eligibility, where a single day can shift entitlement into a different scheme year.
  • Paediatric dosing and growth percentiles, which are indexed by age in completed months for children under three.
  • Insurance underwriting, where many insurers use "age nearest birthday" and will round you up six months before your birthday.
  • Sports age groups, almost always calculated from a fixed cut-off date rather than from the date of competition.

Worked example

Birth date 12 August 1987, target date 7 March 2026. Start with the year difference: 2026 - 1987 = 39. The target month and day (3, 7) come before the birth month and day (8, 12), so subtract one: 38 complete years.

From 12 August 2025 to 7 March 2026 is 6 months to 12 February 2026, then 23 more days to 7 March 2026. Final answer: 38 years, 6 months, 23 days - which is 14,087 total days.

Frequently Asked Questions

Because a calendar year is 365 or 366 days depending on leap years. Over 38 years you accumulate roughly 9 or 10 leap days, so total days divided by 365 will overstate your age by about a tenth of a year.

In non-leap years there is no 29 February, so a convention is needed. Most jurisdictions, including the UK and most US states, treat the birthday as occurring on 28 February. A minority use 1 March. This calculator uses 28 February, which matches the more common legal treatment.

No. It compares two calendar dates, not two instants. If you were born in Tokyo and are measuring in Los Angeles, the calendar date of your birth in the local records is the one to enter.

Yes. The target date is free, so you can ask how old someone was on a historical date or how old they will be at a future event. Dates before the Gregorian calendar adoption in your region will not match historical records, since countries switched at different times between 1582 and 1923.

It rounds your age to the closest birthday rather than the last one, so you become one year older six months before your actual birthday. Insurers use it because mortality risk changes continuously, and rounding to the nearest year is a closer approximation than rounding down.

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References

Last reviewed: 2026-08-07. This page is informational. For legal, medical, tax, or financial decisions, confirm the result with a qualified professional.