Age Calculator by Date of Birth
Enter a date of birth and the calculator reports the age today in years, months and days, the total number of days lived, and how long it is until the next birthday.
Each bar shows one unit of the result against its own scale: years against 100 years, months against 12, days against 31. The right-hand labels report the total day count and the two dates the result comes from.
How the age by date of birth is worked out
The calculator counts completed months from the date of birth to today, then reports the days left over, so the years figure is the age most forms ask for and the months and days add the detail. It compares civil dates rather than clock times, which keeps a daylight-saving change from adding or removing a day, and it does not use the year-zero remapping of the JavaScript Date object. The birthday countdown uses the next occurrence of the birth month and day, with a 29 February birthday observed on 28 February in a non-leap year.
What the age by date of birth answers
This page answers one question with one field: how old is someone today, given the date they were born. The headline is the age in years, months and days, and the lines underneath add the total days lived, the age in whole months, the next birthday with its weekday, and how many days away that birthday is. Nothing is stored and no date is sent anywhere; the reference date is today on your own device.
How the birthday countdown is worked out
The countdown finds the next occurrence of the birth month and day on or after today. If that date has already passed this year, the next one falls in the following year. The result shows the date, the weekday it lands on and the number of days between today and that day, so a party plan, a form deadline or a milestone reminder can be written down without counting on a calendar by hand.
February 29 birthdays
A birth on 29 February has no anniversary in a common year. The calculator observes that birthday on 28 February in the years that are not leap years and on 29 February when the day exists, which keeps the countdown stable and matches how most organisations record the date. The age in years is unaffected, because it counts completed months rather than anniversaries.
Age in months, weeks and days
Some checks use months rather than years: an infant health form may ask for age in months, and a project timeline may be measured in weeks. The result line reports the age in whole months, and the total day count can be divided by seven for weeks. These figures come from the same civil-day count as the years, months and days, so they never disagree with each other.
Using the result in a form
Forms usually ask for age in completed years on a stated date. Read the years figure for that, and check the date the form names: if it asks for the age on a cut-off rather than today, use the age-on-a-specific-date calculator on this site and enter the cut-off as the target date. When a form asks for years and months, the two figures appear together in the same result.
Frequently asked questions
How do I calculate my age from my date of birth?
Enter your date of birth and press the calculate button. The result reports your age today in years, months and days, followed by the total number of days lived and the age in whole months. The reference date is today on your own device, so no date has to be typed twice.
How many days until my next birthday?
The result lists the next birthday with its date, its weekday and the number of days between today and that day. The countdown uses the next occurrence of your birth month and day, so once this year's birthday has passed the figure counts to the following year.
What happens if I was born on 29 February?
In a common year the calculator treats 28 February as the birthday and in a leap year it uses 29 February. Your age in years, months and days is not affected, because it counts completed months from the birth date rather than anniversaries.
How do I convert my age to months or weeks?
The result line reports the age in whole months directly. For weeks, take the total day count and divide it by seven; the day count covers every day from the birth date to today, so it includes leap days in the years in between.
Does the calculator use today's date automatically?
Yes. The reference date is the current day on your device, so the answer reflects the calendar day you are reading the page on and no server clock is involved. Reload the page on a later day and the age and the countdown move on with it.
How accurate is the age in years, months and days?
The split is exact for the calendar it uses: months are anchored to the birth day, leftover days are counted one by one, and a short month clamps to its last day. It is not a legal age determination, and organisations may apply their own cut-off date, so check the rule you are being measured against.