How do I work out my exact age from a date of birth?

Take the completed years first, then the whole months left over, then the leftover days — 1 January 1990 to 15 January 2026 is 36 years, 0 months and 14 days. The months figure counts only months that have fully finished, which is why it reads 0 in the middle of a month. Add the three parts back together and you get the whole span.

Why is the months figure zero when I am clearly partway through a month?

Because a month has to complete before it counts, the same rule a registry office uses. On 15 January 2026 someone born on 1 January has 0 months and 14 days rather than 0.5 months, and the 14 appears in the days row instead. Rounding months to a fraction would make the three parts stop adding up to the whole.

How many days have I been alive?

Read the total days row — 13163 for someone born on 1 January 1990 and measured on 15 January 2026. That figure includes every leap day in between, so it is larger than the years multiplied by 365. Multiply it by 86400 for the seconds row, which is exact rather than rounded.

Should I use age in months or age in years for a child?

Use months under about two years old, because the difference matters clinically: a 14-month-old and a 23-month-old are both "1" in whole years. Read both rows at once — 310 months is the same child as 25 years 10 months in the example above. Above two years, years and months together is the usual form.

What happens if I enter a birth date in the future?

The page reports an error rather than a negative age, because age is time already lived. If you are counting down to a due date or a birthday that has not happened yet, the birthday calculator is the right tool — it accepts a future date and counts towards it. A future date here usually means a typo in the year.

Age Calculator

Result

Years

Months: 9
Days: 2 days
Total months: 441
Total weeks: 1,917.7 weeks
Total days: 13,424 days
Total seconds: 1,159,833,600 seconds

How old are you, exactly? This age calculator takes a date of birth and reports your exact age as whole years, months and days, then reads the same span four more ways: total months, total weeks, total days and total seconds. Age is measured from today, and today is a whole calendar day the page reads itself — you never type it in. Because every figure comes from one span, the parts always add back up to the whole. The months figure is the number of whole months inside that span rather than a separate count, so it never drifts away from the years and days beside it.

Formula

age = today minus date of birth, decomposed into whole years, whole months and leftover days; total months, weeks, days and seconds are all derived from the same span rather than counted separately

birthDate: The date of birth. It may be any real calendar date from year 1 onwards, and it may not be later than today
years: Completed years — the headline figure, and the one a form usually asks for
months: Whole months left over after the completed years; 0 to 11
days: Days left over after the whole months; 0 to 30, and it is not a simple subtraction of the two day numbers
totalMonths: The whole span counted in months; 36 years and 0 months is 432
totalWeeks: The span in weeks and a fraction of a week, to one decimal place
totalDays: Every day you have been alive, counting the leap days in between
totalSeconds: Total days multiplied by 86400, for the birthday-milestone kind of question

Use it whenever the answer has to be your age rather than a number of days: a form that asks for years and months, a paediatric or geriatric milestone, a visa or pension rule written as an age, an insurance quote. The month-and-day breakdown is what those forms want, and the total-days figure is the one to reach for when you need to know how many days you have been alive or how many remain before a threshold that is written in days. What it will not do is tell you anything about a date in the future — a birth date later than today has no age yet, and the page says so instead of returning a negative number.

Worked examples

Someone born on 1 January 1990, on 15 January 2026
1. 1 January 1990 to 1 January 2026 is 36 completed years, so years is 36
2. The next month boundary would be 1 February 2026, which has not arrived, so months is 0
3. 15 January 2026 is 14 days past 1 January 2026, so days is 14
4. 36 completed years is 36 x 12 = 432 months, with no extra whole month
5. 13163 days divided by seven is 1880.43, which rounds to 1880.4 weeks
6. 13163 x 86400 = 1137283200 seconds
A birthday that falls early in January makes this reading look odd — 36 years and 14 days with 0 months — but the month figure is whole months only, and 1 February has not arrived yet. Nothing is lost: the 14 days are in the days row.
A leap-day birthday: 29 February 2000
1. 29 February 2000 to 15 January 2026 is 9452 days in total
2. In whole months that is 310: 310 months on from 29 February 2000 lands on 29 December 2025, and 311 months would overshoot past 15 January 2026
3. 310 months is 25 years (300 months) plus 10 months left over
4. 29 December 2025 to 15 January 2026 is 17 days, which is the days figure
5. 9452 days divided by seven is 1350.29, which rounds to 1350.3 weeks
6. 9452 x 86400 = 816652800 seconds
A 29 February birthday is the case that separates a correct implementation from a plausible one. Take the years first and the months second and the anchor lands on 28 December, giving 18 days; the span is 310 whole months from 29 February, and that lands on 29 December. One day apart, and only a leap-day birthday can tell the two apart.

Limitations

Today is taken as the UTC calendar day, not your local one, so in time zones well ahead of UTC the age can be a day behind the date on your wall during the first hours after midnight. The birth date must be a real calendar date: 30 February is rejected rather than silently rolled forward into March. A birth date later than today is an error rather than a negative age, because age is time already lived. The months figure counts whole months only, so it reads 0 for most of any month even when the years figure has just ticked over. There is no way to ask for the age on a different date — the page always measures against today, and the figure is recalculated from your browser clock when the page loads rather than being frozen at the moment the site was built.

Frequently asked questions

How do I work out my exact age from a date of birth?: Take the completed years first, then the whole months left over, then the leftover days — 1 January 1990 to 15 January 2026 is 36 years, 0 months and 14 days. The months figure counts only months that have fully finished, which is why it reads 0 in the middle of a month. Add the three parts back together and you get the whole span.
Why is the months figure zero when I am clearly partway through a month?: Because a month has to complete before it counts, the same rule a registry office uses. On 15 January 2026 someone born on 1 January has 0 months and 14 days rather than 0.5 months, and the 14 appears in the days row instead. Rounding months to a fraction would make the three parts stop adding up to the whole.
How many days have I been alive?: Read the total days row — 13163 for someone born on 1 January 1990 and measured on 15 January 2026. That figure includes every leap day in between, so it is larger than the years multiplied by 365. Multiply it by 86400 for the seconds row, which is exact rather than rounded.
Should I use age in months or age in years for a child?: Use months under about two years old, because the difference matters clinically: a 14-month-old and a 23-month-old are both "1" in whole years. Read both rows at once — 310 months is the same child as 25 years 10 months in the example above. Above two years, years and months together is the usual form.
What happens if I enter a birth date in the future?: The page reports an error rather than a negative age, because age is time already lived. If you are counting down to a due date or a birthday that has not happened yet, the birthday calculator is the right tool — it accepts a future date and counts towards it. A future date here usually means a typo in the year.

References

RFC 3339: Date and Time on the Internet: Timestamps — IETF
Time and Frequency Division — NIST

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