Baby Age Calculator: Weeks, Months, and Days from Date of Birth
Working out exactly how old your baby is sounds simple until someone asks “how many weeks?” and you’re counting on your fingers. Vaccines, milestones, and growth charts all go by precise age, so a rough guess isn’t always enough. The baby age calculator below turns your baby’s date of birth into an exact age in years, months, days, total weeks, and total days, and it converts weeks to months too.
Baby age calculator: find the exact age
Enter your baby’s date of birth and the age shows instantly. You can also set any “as of” date to find the age on a past or future day, like a doctor’s appointment.
Why counting a baby’s age is confusing
Baby age gets counted three different ways, and that’s where people trip up.
- Calendar age is years, months, and days, like “7 months and 12 days.” This is how most people say it.
- Weeks matter in the early months. A newborn’s vaccines and checkups are scheduled at 6, 10, and 14 weeks, so parents count weeks for the first few months.
- Months take over after that. Milestones and growth charts switch to months, like the 9-month vaccine or the 6-month weight check.
The tricky part is that months aren’t all the same length. One month can be 28 days, another 31. So “6 months old” isn’t exactly 180 days. The baby age calculator handles this by counting real calendar months, not just dividing days, so the number matches what your doctor uses.
Weeks to months: a quick reference
Parents often need to flip between weeks and months, especially around vaccine time. Here’s the rough guide, using about 4.3 weeks per month.
| Weeks | Roughly |
|---|---|
| 4 weeks | about 1 month |
| 8 weeks | about 2 months |
| 13 weeks | about 3 months |
| 26 weeks | about 6 months |
| 39 weeks | about 9 months |
| 52 weeks | about 12 months |
The weeks-to-months box in the baby age calculator does this for any number you type, so you don’t have to memorize it.
Where an exact age actually helps
- Vaccines. The first doses are scheduled by week, so knowing the exact week keeps you on time.
- Growth charts. Weight and height charts read by month. The right month gives you the right healthy range.
- Milestones. Development guides use age in months, so an accurate figure tells you what to expect now.
- Corrected age for premature babies. If your baby was born early, doctors often use a “corrected age” counted from the due date, not the birth date. You can get that by entering the due date as the start.
Frequently asked questions
How do I calculate my baby’s age in months?
Count full calendar months from the date of birth to today, then add the leftover days. The baby age calculator does this automatically and also shows total months, since months vary in length and simple division by 30 gives a slightly wrong number.
How many weeks is my baby?
Divide the total days since birth by 7 for completed weeks, with any remainder as extra days. The calculator shows both total weeks and weeks-plus-days. Weeks matter most in the first few months, when vaccines are scheduled by week.
Why doesn’t 6 months equal 180 days?
Because calendar months have different lengths, from 28 to 31 days. Six real calendar months usually add up to about 181 to 184 days, not exactly 180. The calculator counts actual months, which is why its number matches your doctor’s.
What is corrected age for a premature baby?
Corrected age counts from your baby’s original due date instead of the birth date, so it reflects how developed the baby actually is. To find it, enter the due date as the starting date. Doctors often use corrected age for premature babies up to about age two.
