How to use the baby age calculator
- Enter your baby’s date of birth.
- Pick the date you want the age for — leave it as today, or choose a future date such as a checkup or vaccination appointment.
- Born before 37 weeks? Open “Born early?” and choose how many weeks pregnant you were at delivery to also see corrected age.
You’ll get your baby’s age in completed months and days, in weeks and days, and as a total day count — the three formats pediatricians, daycare forms and baby apps ask for.
Baby age in weeks vs months
In the first three months, baby age is usually counted in weeks; after that, in months; and after the second birthday, in years.
Weeks matter early on because so much changes from one week to the next — feeding amounts, sleep stretches, growth spurts and the first social smile all shift in days, not months. A month is also not 4 weeks: the average calendar month is 30.4 days, or about 4.35 weeks. So a 12-week-old baby is about 2.8 months old, and a baby turns “3 months” at around 13 weeks.
Our calculator counts months the way a calendar does: your baby turns one month older on the same date each month (or on the last day of shorter months — a baby born on January 31 turns one month on February 28).
Baby weeks to months chart
Use this quick reference to convert your baby’s age from weeks to months.
| Age in weeks | Age in months (approx.) | Age in days |
|---|---|---|
| 4 weeks | 0.9 months | 28 days |
| 6 weeks | 1.4 months | 42 days |
| 8 weeks | 1.8 months | 56 days |
| 10 weeks | 2.3 months | 70 days |
| 12 weeks | 2.8 months | 84 days |
| 13 weeks | 3.0 months | 91 days |
| 16 weeks | 3.7 months | 112 days |
| 18 weeks | 4.1 months | 126 days |
| 20 weeks | 4.6 months | 140 days |
| 24 weeks | 5.5 months | 168 days |
| 26 weeks | 6.0 months | 182 days |
| 30 weeks | 6.9 months | 210 days |
| 32 weeks | 7.4 months | 224 days |
| 36 weeks | 8.3 months | 252 days |
| 39 weeks | 9.0 months | 273 days |
| 40 weeks | 9.2 months | 280 days |
| 44 weeks | 10.1 months | 308 days |
| 48 weeks | 11.0 months | 336 days |
| 52 weeks | 12.0 months | 364 days |
Corrected age for premature babies
If your baby was born before 37 weeks of pregnancy, your pediatrician will look at two ages:
- Chronological age — time since birth (the age on the birth certificate).
- Corrected age — chronological age minus the number of weeks your baby arrived early (40 weeks minus gestational age at birth).
For example, a baby born at 32 weeks who is now 6 months old was born 8 weeks early, so their corrected age is about 4 months. The American Academy of Pediatrics recommends using corrected age to track growth and development until around age 2, when most preemies have caught up. Vaccines, however, follow chronological age.
Baby milestones by age
Every baby develops at their own pace, but these are the milestones the CDC says most children (75% or more) reach by each age. Use your baby’s age — or corrected age for preemies — when you compare.
| Age | Most babies can… |
|---|---|
| 2 months | Smile when you talk to them, look at your face, hold head up during tummy time |
| 4 months | Smile on their own to get attention, chuckle, hold head steady when held, bring hands to mouth |
| 6 months | Laugh, reach for a toy, roll from tummy to back, lean on hands when sitting |
| 9 months | Sit without support, look when you call their name, bang two things together |
| 12 months | Wave bye-bye, call a parent “mama” or “dada”, pull up to stand, pick things up between thumb and finger |
| 15 months | Take a few steps alone, try to say one or two words besides “mama”/“dada”, point to ask for something |
| 18 months | Walk without holding on, say three or more words, follow one-step directions, try to use a spoon |
| 24 months | Kick a ball, say two words together (“more milk”), point to at least two body parts |
If your baby is not meeting milestones for their age, or loses skills they once had, talk to your pediatrician. Acting early makes a real difference.
Want to see how your baby is growing, not just how old they are? Try our baby percentile calculator for weight, length and head size, or check how much sleep your baby needs with the baby sleep schedule by age. For school-age children and developmental test scoring, use the chronological age calculator.
Frequently asked questions
How do I calculate my baby’s age in weeks?
Count the days from your baby’s date of birth to today and divide by 7. The whole number is completed weeks and the remainder is extra days. A baby born 45 days ago is 6 weeks and 3 days old. The calculator above does this for you and also shows the age in months.
Is 4 weeks the same as 1 month?
No. A calendar month averages about 4.35 weeks (30.4 days), so a baby is 1 month old at roughly 4 weeks and 2–3 days, not exactly at 4 weeks. That is why “12 weeks” and “3 months” are close but not identical. Doctors usually use weeks up to about 3 months and months after that.
When should I stop counting my baby’s age in weeks?
Most parents switch from weeks to months around 3 months (about 13 weeks), and from months to years after 24 months. Pediatricians often keep using months until age 2 because development changes quickly in the first two years.
What is corrected age and should I use it?
Corrected (adjusted) age is a premature baby’s age counted from their due date instead of their birth date. If your baby was born before 37 weeks, doctors use corrected age to judge growth and milestones until about age 2. Tick “Born early?” in the calculator to see it.
How old is my baby if born on the same date last year?
Your baby is exactly 12 months (1 year) old on their first birthday. Between birthdays, the calculator shows the completed months plus extra days, for example “11 months, 18 days”.
Does this baby age calculator store my baby’s birthday?
No. All calculations run in your browser. Nothing you type is sent to us or saved.