Calculate age from date of birth
Enter a date of birth and the date on which you want to calculate age. Choose “Use today” for your current local calendar date, or enter an appointment date to calculate age at that visit. The result includes completed years, remaining months and remaining days.
For a form asking for age in years, use the completed-years value. For a form asking for age in months, use completed calendar months. Total weeks and total days are separate ways of expressing the same elapsed interval; do not add them to the years-and-months result.
How calendar age is calculated
We count completed calendar months from the birth date, divide that count into years and remaining months, then count elapsed days from the last monthly anniversary. A year contains 12 calendar months. A week contains seven days. Months have different lengths, so we do not divide elapsed days by 30 or 365 to estimate age.
Date arithmetic uses calendar dates without a time of day. Daylight-saving changes therefore do not add or remove a day. Dates from 1900 through 2100 are supported, and the reference date must not precede the birth date.
Worked example
Someone born June 15, 1990 is 36 years, 3 months and 24 days old on October 9, 2026 under this method. They have completed 435 calendar months. If the form asks for whole years, the answer is 36, not 37.
For a newborn, a birth date of October 1 and reference date of October 9 gives eight elapsed days, or one week and one day. The birth date itself is zero elapsed days. A hospital may label “day of life” differently, so follow the record system’s convention.
Leap birthdays and month-end dates
If a monthly anniversary would fall on a day that does not exist, we use the last day of that month. January 31 to February 28, 2026 therefore counts as one completed month. We anchor every anniversary to the original birth day, so the following anniversary is March 31.
For February 29 birthdays, February 28 is used in non-leap years. This is a disclosed arithmetic convention, not a rule for legal age, benefit eligibility or every clinical record system. Use an organization’s required convention if it differs.
Chronological age and health calculations
This tool measures elapsed calendar time. It does not calculate corrected age for premature birth, developmental age, biological age or gestational age. Those concepts require different information and methods. A calendar-age result alone does not determine which medical reference range or treatment is appropriate.
Use the eligibility instructions on each separate tool. For example, the adult BMI calculator has a different purpose from this age calculation. Our due date calculator handles pregnancy dating rather than time since birth.
Frequently asked questions
Can I calculate age on a past date?
Yes. Set the reference date to the date of the visit or record, provided it is on or after birth.
Why are months not the same as four weeks?
Four weeks is 28 days, while a calendar month may have 28, 29, 30 or 31 days. Use the unit requested on your form.
Are dates saved in the link?
No. The inputs remain in this page’s browser state and are not placed in URL parameters. Reloading the page clears the inputs.
Method reference
This tool uses Gregorian calendar arithmetic, with the month-end convention documented above. It is not based on a clinical prediction equation. See the U.S. Naval Observatory explanation of leap years for the calendar rule. Method documented October 9, 2026.