Age in Months Calculator
An age in months is the completed years times 12, plus the whole months since the last birthday. Enter a date of birth below to get that total, with the leftover days and the same span in weeks, days and hours shown alongside it.
Age in months
—years
Fill in the dates above.
- In months
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- In weeks
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- In days
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- In hours
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Working the conversion by hand
Someone who is 25 years and 7 months old has lived 25 × 12 + 7 = 307 months. The days sitting past that 307th month are not folded in, which is deliberate: a partial month is not a month, and rounding it up would make the figure disagree with any chart plotted on whole months.
The subtlety is what “a whole month” means when the calendar hands you months of unequal length. The rule the calculator follows is the same one courts and clinics use: a month is complete when the same day number comes around again. From the 15th to the 15th is one month, whether 28, 30 or 31 days went by. When that day number does not exist in the shorter month — the 31st landing in February — the month completes on the last day available.
Why pediatric care counts in months
Through the first years, a month is a meaningful amount of development, and American pediatric practice is built on that unit rather than on years.
The Bright Futures periodicity schedule published by the American Academy of Pediatrics places well-child visits at 1, 2, 4, 6, 9, 12, 15, 18, 24 and 30 months, and only then moves to a yearly rhythm at age 3. Growth charts follow the same grid, which is why a chart entry needs “18 months” rather than “a year and a half” — the second phrasing does not identify a plotting point.
Saying a child is 18 months old also carries information that “one and a half” hides: it signals which visit, which milestone checklist and which growth curve segment applies.
Years to months at a glance
| Age | In months |
|---|---|
| 1 year | 12 months |
| 2 years | 24 months |
| 3 years | 36 months |
| 5 years | 60 months |
| 10 years | 120 months |
| 18 years | 216 months |
| 30 years | 360 months |
| 50 years | 600 months |
Past childhood the month count mostly stops being useful and becomes a curiosity, in the same family as counting a life in days.
For the standard reading in years, months and days, use the age calculator. This page performs date arithmetic and does not replace a clinician’s assessment when the age in months is being used to track a child’s development.
Frequently asked questions
How do I convert an age into months?
Multiply the completed years by 12 and add the whole months since the last birthday. A person who is 25 years and 7 months old is 307 months old.
Why is a child's age given in months?
Because change is fast enough that a year is too coarse a bucket. The American Academy of Pediatrics schedules well-child visits by month through 30 months, and growth charts are plotted the same way.
How many months old is a one-year-old?
Twelve months at the first birthday, 24 at the second, 36 at the third. Between birthdays the calculator adds the whole months that have gone by.
Does the month total include the leftover days?
No. The headline total counts complete months only; the days beyond the last full month appear alongside it in the breakdown.
What about a birth date on the 31st?
A month completes on the same day number in the following month, and when that day does not exist the count uses the last day of the shorter month. A January 31 birth date completes a month on February 28 or 29.
Sources
- AAP Schedule of Well-Child Care Visits — American Academy of Pediatrics (HealthyChildren.org)
- Introduction to Calendars — U.S. Naval Observatory, Astronomical Applications Department
Written by Lucas Silva · Reviewed by Lucas Silva · Updated on August 17, 2026