Parents are constantly converting their baby’s age from weeks to months. One moment the midwife is talking about a 6-week-old, the next a relative asks how many months old your baby is — and suddenly you are counting on your fingers. This guide makes it instant. Enter your baby’s date of birth below and you will see their exact age in years, months and days, plus the total weeks and days, and then use the handy chart further down to convert any week count to months at a glance.
Baby Age Calculator
Weeks to months conversion chart
| Weeks old | Months (approx) |
|---|---|
| 4 weeks | 1 month |
| 8 weeks | 2 months |
| 12 weeks | 3 months |
| 17 weeks | 4 months |
| 22 weeks | 5 months |
| 26 weeks | 6 months |
| 30 weeks | 7 months |
| 35 weeks | 8 months |
| 39 weeks | 9 months |
| 43 weeks | 10 months |
| 48 weeks | 11 months |
| 52 weeks | 12 months (1 year) |
A note on accuracy: a calendar month averages 4.35 weeks, not exactly 4 weeks, so week counts and month counts drift slightly as the months go by. That is why 12 weeks is close to 3 months but not exactly 3 months — a baby reaches exactly 3 calendar months at around 13 weeks. The calculator above gives the exact calendar answer for your baby's own dates, so use it whenever you need a precise age.
Why baby age is counted in weeks at first
In the first weeks of life, babies change incredibly fast. Feeding, sleep, weight gain and early milestones can look very different at 2 weeks, 6 weeks and 10 weeks, even though all of those ages could loosely be described as about 1 or 2 months. Counting in weeks gives parents, midwives and doctors a much more precise picture during the newborn stage, which is why appointments, vaccinations and developmental checks are often scheduled by week early on.
When to switch from weeks to months
Most parents switch from weeks to months at around 3 to 4 months, once the newborn stage is over and the weekly changes become less dramatic. From then on, months are easier to say, easier to remember and match how clothing sizes, weaning guidance and developmental milestones are usually described. There is no strict rule — answer in whatever unit feels clearest, and use the calculator above whenever you need the exact figure.
More baby growth tools
For your baby's full age in years, months, weeks and days, try our main Baby Age Calculator. To see how your baby's weight and length compare with other babies of the same age, use the Baby Percentile Calculator, and follow their growth over time on our Baby Growth Chart page.
Frequently asked questions
How many weeks is a 2-month-old baby?
A 2-month-old baby is about 8 to 9 weeks old. Two calendar months are usually 59 to 62 days, which works out to roughly 8 and a half weeks, so parents often round it to 8 or 9 weeks depending on the exact dates.
Is 12 weeks the same as 3 months?
Almost, but not quite. 12 weeks is 84 days, while 3 calendar months are usually 89 to 92 days because calendar months vary in length. A baby is closer to exactly 3 months at about 13 weeks.
How old is my baby in months if they are 20 weeks?
A baby who is 20 weeks old is about 4.5 months old. Dividing 20 weeks by the average month length of 4.35 weeks gives roughly 4.6 months, and the exact calendar age depends on their date of birth.
Why does my pediatrician count age differently?
Pediatricians often count age in weeks for young babies because development changes so quickly in the early months and weeks are more precise. For babies born early, they may also use corrected age, which counts from the due date instead of the birth date, usually until around age 2.
Disclaimer: This page is for general information only and is not medical advice. If you have concerns about your baby's growth or development, talk to your pediatrician.