Baby Age Calculator
Enter your baby's birth date and today's date to get their exact age in years, months, weeks, and days. You also get upcoming milestone dates, a 24-month birthday calendar with day-of-week labels, and a plain-language summary of where your baby is in their first two years. Results update instantly as you type.
How to calculate your baby's age accurately
For newborns and young infants, pediatricians count age in days for the first few weeks, then in weeks up to about 12 weeks, and then in months through the first two years. A "month" here means a complete calendar month, not a fixed 30-day period, so a baby born on the 15th reaches their 1-month mark on the 15th of the following month. This calculator uses that same logic: it counts complete calendar months first, then resolves the remainder into weeks and days, which is what your doctor means when they say your baby is "4 months and 2 weeks."
Why age in months matters more than years for babies
During the first two years, babies change so rapidly that age in years is almost meaningless - a 6-month-old and a 12-month-old are in completely different developmental worlds. That is why the age-in-months convention exists: it aligns with pediatric growth charts, vaccination schedules, and developmental screening questionnaires (like the ASQ-3), all of which are keyed to month-age. At your 6-month well-child visit, the nurse will ask your baby's exact age in months; having that number ready helps your provider place your child's measurements on the right percentile curve.
Monthly milestones and what to expect
The 24-month calendar this tool generates gives you the exact date for each monthly "birthday," along with the day of the week - handy for scheduling checkups or small celebrations. In terms of development, the World Health Organization and American Academy of Pediatrics identify broad windows rather than exact ages: most babies roll from tummy to back around 3-5 months, sit without support around 6-8 months, crawl around 7-10 months, pull to standing around 8-11 months, and walk independently around 9-12 months. Language follows a similar broad arc from coos to babbles to recognizable words and eventually two-word phrases by 2 years. If your baby seems significantly behind any of these broad windows, a conversation with your pediatrician is a sensible next step.
Corrected age for premature babies
If your baby was born prematurely, their "corrected age" (also called adjusted age) is their age minus the number of weeks they were early. A baby born 8 weeks premature who is now 6 months old has a corrected age of 4 months, and that is the age you should use when comparing against developmental milestones and growth charts - typically until age 2 for premature infants, or age 3 for those born very early. This calculator computes chronological age from the actual birth date. For corrected age, simply subtract the weeks of prematurity from the total weeks this tool shows.
Baby development stages by age
| Age range | Stage | Key developmental focus |
|---|---|---|
| 0-4 weeks | Newborn | Reflexes, sleep cycles, bonding |
| 1-3 months | Young infant | Social smiling, tracking, head control |
| 3-6 months | Infant | Rolling, reaching, babbling |
| 6-9 months | Older infant | Sitting, object permanence, solid foods |
| 9-12 months | Older infant | Crawling, pulling to stand, first words |
| 12-18 months | Toddler | Walking, 10-20 words, finger foods |
| 18-24 months | Toddler | Running, two-word phrases, imaginative play |
General pediatric developmental stages. Every baby develops at their own pace - these are broad ranges, not strict checkboxes.
Frequently asked questions
How do I calculate my baby's age in months?
Count the number of complete calendar months from the birth date to today. A baby born on March 10 reaches 1 month on April 10, 2 months on May 10, and so on. If the current date has not yet reached that day-of-month, you subtract one month. For example, on April 5 the baby born on March 10 is still 0 months old (25 days). This calculator handles all of that automatically.
When should I stop counting my baby's age in weeks?
Most parents and pediatricians switch from weeks to months around 12 weeks (3 months). Before that point, week-precision is useful because development changes so quickly. After 3 months, months are the standard unit used on growth charts and in developmental screening tools, with the remainder expressed as additional weeks.
What is corrected or adjusted age for a premature baby?
Corrected age accounts for early birth by subtracting the weeks of prematurity from the chronological age. A baby born 6 weeks early who is now 10 weeks old has a corrected age of 4 weeks. Pediatricians use corrected age when plotting a premature baby on growth and developmental charts, usually until age 2 (or age 3 for very premature births).
Why does the number of days in a month affect the calculation?
Calendar months have 28, 29, 30, or 31 days, so a fixed 30-day count can drift. The correct approach is to move the birth-day forward month by month on the same day of the month. This calculator uses that calendar-based method, which matches how your pediatrician counts.
Can I use this to find out how old my baby will be on a future date?
Yes. Simply set the "calculate age as of" date to any future date - such as a holiday, a vacation, or a grandparent's visit - and the calculator will show you the exact age your baby will be on that day, along with which monthly milestone falls next.
What is the difference between 6 months old and 26 weeks old?
6 calendar months and 26 weeks are not the same. Six months is approximately 183 days, while 26 weeks is exactly 182 days - a difference of one day most of the time. More importantly, 6 calendar months from January 31 lands on July 31, while 26 weeks from January 31 lands on July 30. Pediatric charts always use calendar months, not week multiples.