Age in Months Calculator
Enter a birth date (and optionally an as-of date - blank means today) to get age in whole calendar months plus leftover days, along with the years-months-days breakdown, total weeks, and total days. Built for baby milestones and pediatric forms.
Pediatric convention reports age in months through 24 months and in weeks through about 3 months - CDC growth charts are indexed the same way.
How months are counted
A month of age is a calendar month, not 30 days: a baby born November 20 turns 1 month old on December 20 and 2 months on January 20, even though those intervals are 30 and 31 days. This calculator counts whole calendar months to the last month-day recurrence, then leftover days against the actual length of the preceding month.
Month-end birth dates need one convention: born January 31, a baby has no February 31 to reach, so the first month completes on February 28 (or 29). The clamp-to-month-end rule here matches how pediatric offices and daycare enrollment forms count.
Why months matter for babies
Pediatric milestones, growth charts, vaccine schedules, and daycare room assignments are all stated in months through at least age 2 - CDC growth charts run 0-24 months before switching to the 2-20 year charts. For premature babies, clinicians often track corrected age (months since the due date rather than the birth date) for milestones during the first couple of years; enter the due date here to get it.
How it’s calculated
Whole months = calendar months elapsed to the most recent month-day recurrence (clamping to month-end when the birth day does not exist in a month, e.g., Jan 31 → Feb 28); leftover days are counted against the true length of the preceding month. Total days = round((as-of − birth) / 86,400,000 ms) with both dates at noon; weeks = floor(days / 7). The average calendar month is 30.44 days (365.2425 / 12), which is why 19 calendar months rarely equals 19 × 30 days.
Birth time is ignored, and for milestone tracking of premature infants pediatricians may use corrected age from the due date - ask your pediatrician which applies.
Common baby ages in months, weeks, and days
| Age | Weeks (approx.) | Days (approx.) |
|---|---|---|
| 6 months | 26 weeks | 183 days |
| 12 months (1 year) | 52 weeks | 365 days |
| 18 months | 78 weeks | 548 days |
| 24 months (2 years) | 104 weeks | 730 days |
| 36 months (3 years) | 157 weeks | 1,096 days |
Computed with average month = 30.44 days (365.2425/12); actual counts vary by 1-3 days with the calendar.
Common mistakes
- Multiplying months by 30 to get days - 19 calendar months is 601 days for a November birth, not 570, because real months average 30.44 days.
- Counting a month-end birthday as the 1st of the next month: born January 31, the first month completes February 28, not March 1 - the count clamps to the short month's last day.
- Mixing weeks and months for infants - 8 weeks is not 2 months (it is about 1 month 26 days), and vaccine schedules specify which unit they mean.
- Using calendar age instead of corrected age for a premature baby's milestones - clinicians subtract the weeks born early during roughly the first 2 years.
Frequently asked questions
How do I calculate my age in months?
Count whole calendar months from your birth date to the most recent monthly recurrence of your birth day, then add leftover days: age in months = years × 12 + months, calendar-aware. A 31-year-old is 372+ months old.
How many months old is my baby?
Count month-day recurrences: born November 20, 2024, a baby is 19 months, 24 days old on July 14, 2026. Enter the birth date above; the weeks row helps for appointments that use weeks.
Is 8 weeks the same as 2 months?
No - 8 weeks is 56 days, while 2 calendar months from most dates is 59-62 days. Immunization schedules and growth charts specify the unit deliberately, so match it exactly.
What if the birth date is the 31st and the month has no 31st?
The month of age completes on the target month's last day - January 31 to February 28 (29 in leap years) counts as one month. This clamping convention matches pediatric and legal date math.
What is corrected age for preemies?
Age counted from the due date instead of the birth date, used for milestones and growth during about the first 24 months. A baby born 8 weeks early has a corrected age 2 months less than their calendar age; your pediatrician can tell you when to stop correcting.