How to use the age calculator
- Enter the date of birthType it or open the date picker. The field remembers the last date you entered on this device, which saves retyping it; press Clear saved to remove it.
- Choose the Age on dateIt starts as today. Move it forwards to see how old someone will be at a deadline, or backwards to find their age at a past event. Today brings it back.
- Read the ageThe large figure is the age in completed years, months and days. Below it the same span appears as total months, weeks and days, followed by the next birthday and the weekday of birth.
- Check the notesOn a birthday the result says so instead of counting down zero days. A 29 February birth gets a short note, and a birth date later than Age on shows what to fix.
How an exact age is worked out
The calculation follows the same order most people use in their heads: the largest unit first. First come whole years, the number of birthdays that have passed by the Age on date. Next come whole months since the last birthday, one for each time the day of the month has come round again. Whatever remains is days.
Take someone born on 15 June 1990. On 28 September 2026 they are 36 years, 3 months, 13 days old. Their 36th birthday was on 15 June 2026; the 15th has since come round in July, August and September; and 13 more days have passed since 15 September. In total that is 13,254 days, or 1,893 weeks and 3 days.
Dividing the total number of days by 365.25 looks like a shortcut but gives wrong answers near birthdays. A baby born on 1 March 2000 has lived 365 days on 1 March 2001, because no 29 February falls in between. 365 ÷ 365.25 is 0.9993, which rounds down to 0 years on the very day of the first birthday. Counting calendar years avoids the problem entirely.
People born on the 29th, 30th or 31st
Months have different lengths, so a birth late in the month needs a rule for months that are too short. Here the monthly anniversary moves to the last day of the shorter month. A baby born on 31 January 2026 turns one month old on 28 February and two months old on 31 March; on 30 March the age is 0 years, 1 month, 30 days. Some tools count 28 February as 28 days instead of one month, which is why calculators can disagree about the months and days; for births on any day other than 29 February they still agree on the years.
Born on 29 February
A leap-day birthday exists only in leap years. In the three common years between, this calculator treats 1 March as the birthday: the new age starts on 1 March, and the next birthday is shown as 1 March. The reasoning is that 29 February comes after 28 February, so a year from a leap-day birth has not been completed until 28 February is over.
For example, someone born on 29 February 2008 is 17 years, 11 months, 30 days old on 28 February 2026, and exactly 18 years old on 1 March 2026. That difference of one day can matter for anything that depends on turning 18.
Other conventions exist. Plenty of people celebrate on 28 February, and some rules count the new age from 28 February rather than 1 March, as one New Zealand regulation on driver licensing does, while many laws say nothing either way. Where a legal age depends on the exact day, check the rule that applies to you. The guide to leap-day birthdays linked below compares both conventions in detail.
Completed years and other ways of counting age
The figure this tool shows is the age in completed years, sometimes written as age last birthday: a newborn is 0 and gains a year on each birthday. It is the age used on passports, most application forms and in international statistics. It is not the only system in use.
- East Asian age reckoning. Traditionally a newborn counts as one year old and everyone gains a year at the new year rather than on their birthday, so a baby born a few days before the new year can be called two within a week. In China the step comes at the lunar new year and the custom now survives mostly in informal use.
- South Korea since 28 June 2023. A law that took effect on that date made completed-year age the standard for administrative and civil matters. A few rules still use the year of birth alone, including the ages for buying alcohol and tobacco and for military service duties.
- Japan. Under a 1902 law on age calculation, which applies the civil code’s rule for periods, a person gains a year at the end of the day before the birthday. That is why children born on 1 April are counted as six on 31 March and start primary school a year ahead of those born on 2 April.
- England and Wales. Until the Family Law Reform Act 1969, the common law treated a person as reaching an age at the start of the day before the birthday. Section 9 of that Act moved the moment to the start of the anniversary itself.
- Age nearest birthday. Some insurance pricing rounds to the nearest birthday instead of the last one, so a person aged 40 years and 7 months is rated as 41.
When an exact age matters
Many rules are written in terms of age on a particular date, and the date is often not today. Set Age on to the date the rule names and the result shows whether the threshold has been reached.
| Situation | What is usually measured | Tip |
|---|---|---|
| School entry | Age reached by a cut-off date, often near the start of the school year | Enter the cut-off date as Age on, not the date you apply. |
| Life and health insurance | Age last birthday or age nearest birthday when cover starts | Premiums can step up at a birthday, so compare the start date against the next birthday. |
| Retirement and pensions | A qualifying age, sometimes in years and months, that can vary by year of birth | Read the months row as well as the years before relying on the date. |
| Visas and working holidays | An upper or lower age limit on the day the application is lodged | Set Age on to the day you will submit, which may be weeks before travel. |
| Youth sport | Age on a fixed day of the season, frequently 1 January or 31 December | Two players born a week apart can land in different age groups. |
| Baby and child health | Age in weeks or months for growth charts and vaccination schedules | Use the total weeks and total months rows rather than years. |
| Age-restricted purchases | Completed years on the day of the purchase or licence | The birthday itself counts; the day before does not. |
Worked examples
Counting down to 18
A teenager born on 20 November 2008 is 17 years, 10 months, 8 days old on 28 September 2026. The 18th birthday falls on Friday, November 20, 2026, 53 days away. If a form asks for an adult on a date before that, the answer is still no.
A baby born at the end of August
Born on 31 August 2025, a baby is exactly 0 years, 6 months, 0 days old on 28 February 2026, because February has no 31st and the monthly anniversary moves to its last day. By 28 September 2026 the age is 1 year, 0 months, 28 days, and the next birthday is 337 days off.
Turning 10,000 days old
Round-number days make fun milestones. Someone born on 10 May 1999 reaches 10,000 days on Friday, September 25, 2026, at the age of 27 years, 4 months, 15 days. To find your own, set Age on forward until the total days row reads 10,000.
A leap-day eighteenth
For the person born on 29 February 2008, 2026 is a common year, so the tool shows the 18th birthday on Sunday, March 1, 2026. On the day before, the result still reads 17 years, 11 months and 30 days, with a note about the 1 March rule.
Limits worth knowing
- Calendar. Ages are counted on the Gregorian calendar, projected back before it was adopted. Birth dates recorded in another calendar, such as a lunar calendar or the older Julian one, need converting first, or the weekday and day totals will be off.
- Whole days only. The tool has no notion of the hour of birth, so it never says someone is 30 years and 5 hours old. For most legal and practical purposes, the date is what counts.
- Range. Both dates must fall between the years 1 and 9999, the range that browser date fields accept.
Questions
How do I find out how old someone will be on a future date?
Change the Age on field from today to the date you care about, for example the day an exam is taken or a policy starts. The years, months and days update at once, and so does the countdown to the birthday that follows that date.
Is my date of birth saved anywhere?
It is kept only in this browser’s local storage for this site, so the field is already filled the next time you open the page on the same device. Press Clear saved to delete it, or clear the site data in your browser settings. The Age on date is never saved.
Does the hour of birth change the result?
No. The calculator works with whole calendar dates, so a birth at 00:05 and one at 23:55 on the same day give identical ages. If a birth happened near midnight while travelling, use the date written on the birth certificate, because that is the date other records will use.
Can I use it for a wedding anniversary, a company or a pet?
Yes. Any starting date works: enter the wedding day, the founding date or the day a pet was born as the date of birth. The totals, next anniversary and countdown behave exactly the same; only the wording on screen talks about birthdays.
Why do two age calculators sometimes disagree by a day or two?
Months and days can differ for people born on the 29th, 30th or 31st, because tools handle short months differently. For a 29 February birth even the years can differ: on 28 February of a common year this tool gives one year less than a tool that uses the 28 February convention. Check the method a tool describes before comparing results.