How is age calculated from a date of birth?

Count whole calendar-month anniversaries from the date of birth to the chosen date, then count the days since the last anniversary. Divide the whole months into years and remaining months. For example, 31 January 2025 to 1 March 2025 is 1 month and 1 day: the February anniversary falls on 28 February. The birth date itself is age zero; the end date is not added as an extra day.

When does someone born on 29 February have their birthday?

This tool uses 29 February in leap years and 1 March in common years. Under that convention, someone born on 29 February 2004 is 20 years, 11 months and 30 days old on 28 February 2025, and 21 years old on 1 March. This is a stated arithmetic convention; a form, test or eligibility rule may require a different method.

Can I calculate age on a past or future date?

Choose A set date and enter the As of date. All figures, including the next birthday, are measured from that date. If As of is empty, the answer uses today in your device’s local timezone. Today refreshes at local midnight and when you return to the tab. The birth date must not be after the measured date.

What do age in months, weeks, days and hours mean?

Months are completed calendar-month anniversaries. Days are the Gregorian calendar-date difference; whole weeks are days divided by 7 and rounded down. Hours are that whole-day count multiplied by 24. They are nominal calendar hours, not elapsed clock hours: no birth time, time-of-day or daylight-saving adjustment is included. For actual timed spans, use Duration Calculator.

Can this calculate a child’s age or an age in weeks?

Yes, from a known date of birth to today or a supplied date. For a child born 1 July 2026, the age on 1 September 2026 is 2 months, 62 calendar days or 8 completed weeks. This calculates calendar age and does not adjust for prematurity, estimate development or apply test norms.

Can I find my date of birth from an age, or compare two people?

A whole-year age does not identify one birth date: it describes a range. This tool needs the actual birth date and does not reverse an age into a birthday. For the difference between two known birth dates, use Age Gap Calculator; for the mean of a supplied list of numeric ages, use Average Calculator.

Is my date of birth saved or uploaded?

The calculation runs in your browser. Changing the birth date deliberately saves a valid date in local browser storage; opening the page or restoring a saved date does not write it again. The As of date stays only in this visit. Forget saved birth date removes the stored value while keeping the current calculation on screen. If storage is blocked, you can still calculate.

How do I copy the result?

Copy age copies a summary with both dates, the years-months-days breakdown, totals, next birthday and conventions. If clipboard access is refused, open Selectable age summary and copy its text manually. The summary does not include a birth time, biological age or an eligibility decision.

Is this a dog-years or biological-age calculator?

It counts calendar age from a known birth date, including a puppy’s age in completed weeks. Human-equivalent dog years use a separate size-based model in Dog Age Calculator. Biological, mental and metabolic age, animal feeding, lifespan and medical adjustments are different calculations and are not inferred from this date span.

Age Calculator

Enter a date of birth to calculate age today, or choose A set date for age on a past or future date. See whole years, months and days, total calendar days and weeks, the birth weekday, and the next birthday from the chosen date. Dates use the Gregorian calendar, with the anniversary rules stated below.

Age calculatorPick a date of birth to see an exact age.

Date of birth

Age at

Changing a valid birth date saves it in this browser. The As of date is not saved.

Common questions

How is age calculated from a date of birth?: Count whole calendar-month anniversaries from the date of birth to the chosen date, then count the days since the last anniversary. Divide the whole months into years and remaining months. For example, 31 January 2025 to 1 March 2025 is 1 month and 1 day: the February anniversary falls on 28 February. The birth date itself is age zero; the end date is not added as an extra day.
When does someone born on 29 February have their birthday?: This tool uses 29 February in leap years and 1 March in common years. Under that convention, someone born on 29 February 2004 is 20 years, 11 months and 30 days old on 28 February 2025, and 21 years old on 1 March. This is a stated arithmetic convention; a form, test or eligibility rule may require a different method.
Can I calculate age on a past or future date?: Choose A set date and enter the As of date. All figures, including the next birthday, are measured from that date. If As of is empty, the answer uses today in your device’s local timezone. Today refreshes at local midnight and when you return to the tab. The birth date must not be after the measured date.
What do age in months, weeks, days and hours mean?: Months are completed calendar-month anniversaries. Days are the Gregorian calendar-date difference; whole weeks are days divided by 7 and rounded down. Hours are that whole-day count multiplied by 24. They are nominal calendar hours, not elapsed clock hours: no birth time, time-of-day or daylight-saving adjustment is included. For actual timed spans, use Duration Calculator.
Can this calculate a child’s age or an age in weeks?: Yes, from a known date of birth to today or a supplied date. For a child born 1 July 2026, the age on 1 September 2026 is 2 months, 62 calendar days or 8 completed weeks. This calculates calendar age and does not adjust for prematurity, estimate development or apply test norms.
Can I find my date of birth from an age, or compare two people?: A whole-year age does not identify one birth date: it describes a range. This tool needs the actual birth date and does not reverse an age into a birthday. For the difference between two known birth dates, use Age Gap Calculator; for the mean of a supplied list of numeric ages, use Average Calculator.
Is my date of birth saved or uploaded?: The calculation runs in your browser. Changing the birth date deliberately saves a valid date in local browser storage; opening the page or restoring a saved date does not write it again. The As of date stays only in this visit. Forget saved birth date removes the stored value while keeping the current calculation on screen. If storage is blocked, you can still calculate.
How do I copy the result?: Copy age copies a summary with both dates, the years-months-days breakdown, totals, next birthday and conventions. If clipboard access is refused, open Selectable age summary and copy its text manually. The summary does not include a birth time, biological age or an eligibility decision.
Is this a dog-years or biological-age calculator?: It counts calendar age from a known birth date, including a puppy’s age in completed weeks. Human-equivalent dog years use a separate size-based model in Dog Age Calculator. Biological, mental and metabolic age, animal feeding, lifespan and medical adjustments are different calculations and are not inferred from this date span.

Exact Gregorian calendar arithmetic for dates from 0001 through 9999. February 29 uses March 1 in common years; short-month anniversaries clamp to the last day. Hours are calendar days × 24, without a birth time.