Age Calculator by Date of Birth

This age calculator by date of birth gives an exact age in years, months and days, counted the way birthdays are counted. It also shows the age in completed years, in months, in days and in weeks, the date of the next birthday and how many days away it is, and the weekday the person was born on.

By default the age is worked out for today. The site is built ahead of time as static pages, so today's date has to come from your browser rather than from the page: when the page loads, the "Age on" box is set to the date on your phone or computer. You can change it to any other date, which is how you answer questions like "how old will I be on my wedding day" or "was my child five by the school cutoff date".

An age is not the number of days divided by 365. People count age in whole calendar months and years: you are a year older on your birthday, whatever the length of the year in between. This calculator does the same, so its answer matches the one on a birthday card, a passport form or a school enrolment rule.

1. Enter the Date of birth.
2. Leave Age on as today, or pick another date to see the age then.
3. Read the age at the top in years, months and days.
4. The cards show the age as whole years, total months, total days and weeks, the next birthday with the age it brings, the days left until then, and the weekday of birth.

The presets show five cases with fixed dates: the default, a February 29 birthday, a birthday on the 31st of a month, the age gap between two people, and a baby's age in months and weeks.

1. Read both dates. In the browser, "Age on" starts as today's date on your device. Both dates are plain calendar days on the Gregorian calendar, with no time of day, so time zones and daylight saving cannot change the answer once the dates are set.

1. Count whole calendar months. The calculator finds the largest number of months that can be added to the birth date without passing the age-on date. Adding a month keeps the day of the month; when that day does not exist in the month, the month's last day is used. Twelve months make a year, so the years are the months divided by 12 and the months are what is left.

1. Count the leftover days. From the last monthly anniversary up to the age-on date, the days are counted, with the anniversary itself not counted.

1. Totals. Age in days is the number of days from the birth date to the age-on date, the birth date not counted. Age in weeks splits that into whole weeks and days. Age in months is the whole-month count from step 2.

1. Next birthday. The next birthday is the birth date plus one more year than the current age, using the same month rule. A February 29 birthday therefore lands on February 28 in common years. The days to it are counted from the age-on date.

Born Tuesday, May 15, 1990, age on January 1, 2026:

1. Adding 427 months to May 15, 1990 reaches December 15, 2025. One more month would be January 15, 2026, which is past January 1. So the whole months are 427, which is 35 years and 7 months.
2. From December 15, 2025 to January 1, 2026 is 17 days.
3. The age is 35 years, 7 months, 17 days.
4. In days, that is 13,015, or 1,859 weeks and 2 days.
5. The next birthday is Friday, May 15, 2026 (turning 36), which is 134 days away.

A baby born on August 3, 2026 is 0 years, 2 months, 14 days old on October 17, 2026, which is 75 days, or 10 weeks and 5 days.

Put the older person's date of birth in Date of birth and the younger person's date of birth in Age on. The result is the gap between them, counted the same way as an age. For someone born March 10, 1988 and someone born July 24, 1992, the gap is 4 years, 4 months, 14 days, or 1,597 days.

It depends on whether you have had your birthday yet this year. If you are 18 and your birthday has passed, you were born 18 years before this year; if it is still to come, you were born the year before that.

On October 17, 2026, someone born October 17, 2008 has just turned 18, while someone born a day later, October 18, 2008, is still 17 years, 11 months, 29 days old. The table below gives the age reached by the end of 2026 for a range of birth years.

• Time of birth. Ages are counted in whole days. Someone born at 11 pm is treated as a day old at midnight, the same as someone born at 1 am.
• Where you were born versus where you are. A birth date is a date in the place of birth. If you now live many time zones away, your birthday arrives at a different moment, but the calculator only compares dates.
• Legal rules for February 29. This calculator places a February 29 birthday on February 28 in common years. Some laws and agencies use March 1 instead for deciding when a person born on February 29 reaches a legal age. If that matters, check the rule that applies to you.
• Other ways of counting age. Some cultures count a newborn as one year old, or add a year at the new year rather than on the birthday. This calculator uses the birthday convention.
• Dates before the Gregorian calendar. Historical dates are counted as if the Gregorian calendar had always been in use.

• Dividing days by 365. Turning a count of days into years by dividing by 365 ignores leap days, and turning the remainder into months assumes every month is the same length. Both make the answer drift by days, and near a birthday the year can come out wrong.
• Subtracting birth years only. 2026 minus 1990 is 36, but on January 1, 2026 someone born in May 1990 is still 35. Years alone are only right after the birthday.
• Expecting months to be equal. Born January 31, 2014, a child is 12 years, 1 month, 0 days old on February 28, 2026, because February has no 31st and its last day stands in for it.
• Forgetting the age-on date is today by default. A link someone shares with you keeps their dates. Set "Age on" back to today if you want your own.
• A device with the wrong date. "Today" comes from your device. If its date is wrong, type the date you mean.

• NIST Time and Frequency Division, Time and Frequency from A to Z, Leap Year: the Gregorian leap year rule used for every date. nist.gov/pml/time-and-frequency-division/popular-links/time-frequency-z/time-and-frequency-z-l
• NIST Time and Frequency Division, Time and Frequency from A to Z, Daylight Saving Time: when US clocks change. nist.gov/pml/time-and-frequency-division/popular-links/time-frequency-z/time-and-frequency-z-d