Baby Age Calculator converts your baby's age to weeks, months, and days. Includes corrected age for premature babies, milestone chart, and vaccination schedule.
Baby Age in Weeks: Why the Exact Number Matters
A baby's age in weeks is the primary unit pediatricians, neonatologists, and public health agencies use for the first year of life. When your child's doctor asks "how many weeks old is your baby?" they are not asking for an approximation—vaccination timing, medication dosing, developmental screening, and feeding volume recommendations all depend on precise week-count arithmetic, not loose month estimates. This baby age calculator converts your baby's birth date to exact weeks, total days, and total months simultaneously, and also generates a complete developmental milestone chart for months 1–24.
How to Calculate Baby Age in Weeks: The Formula
The calculation is straightforward: Baby Age in Weeks = Total Days Since Birth ÷ 7. Total days is the count of calendar days from the date of birth to today, inclusive. For example, a baby born on March 1 who is being assessed on May 29 has lived through 89 days, which equals exactly 12 weeks and 5 days (89 ÷ 7 = 12 remainder 5). The remainder days are always displayed separately—"12 weeks + 5 days"—because developmental medicine does not round to the nearest week.
The chronological age calculator on this platform uses the same precision date-interval arithmetic reviewed against the ISO 8601 standard. The baby age in weeks calculator extends this with pediatric-specific outputs: milestone dates, vaccination notes, and corrected age for premature infants.
Baby Age Weeks to Months: Conversion Table
Because months have unequal lengths (28 to 31 days), the conversion between weeks and months is not an integer. One average month contains 30.44 days = 4.345 weeks. This is why the same number of months can correspond to slightly different week counts depending on which specific months are involved. Use the table below as a reference.
| Age in Months | Approximate Weeks | Exact Days | Pediatric Stage |
|---|---|---|---|
| 1 month | 4.3 weeks | ~30 days | Newborn |
| 2 months | 8.7 weeks | ~61 days | Newborn — First vaccines due |
| 3 months | 13.0 weeks | ~91 days | Early infant |
| 4 months | 17.4 weeks | ~122 days | Early infant — Vaccines due |
| 6 months | 26.1 weeks | ~183 days | Mid-infant — Solid foods |
| 9 months | 39.1 weeks | ~274 days | Mobile infant — Standing |
| 12 months | 52.1 weeks | ~365 days | 1 year — Major vaccine appointment |
| 15 months | 65.2 weeks | ~457 days | Toddler — Vaccines due |
| 18 months | 78.2 weeks | ~548 days | Toddler — Running, 20+ words |
| 24 months | 104.3 weeks | ~730 days | 2 years — Corrected age threshold |
Corrected Age for Premature Babies: The Clinical Standard
This is the feature that most generic baby age calculators do not include, yet it is clinically essential. A baby born at 36 weeks gestation (4 weeks early) who is today 20 weeks old chronologically has a corrected age of 16 weeks. The formula is: Corrected Age = Chronological Age − Weeks Born Early.
Corrected age is also called adjusted age or postmenstrual age minus 40 weeks in neonatal medicine. Pediatricians use it because the premature baby's brain, muscles, and organ systems are developing from the original due date—not from the early birth date. Comparing a 20-week-old premature baby's motor skills against a 20-week developmental standard is clinically misleading; comparing against the 16-week standard is accurate.
| Born Weeks Early | Example: Chronological Age | Corrected Age | Use Corrected Until |
|---|---|---|---|
| 2 weeks early | 12 weeks (84 days) | 10 weeks | 24 months corrected |
| 4 weeks early (1 month) | 20 weeks | 16 weeks | 24 months corrected |
| 6 weeks early | 24 weeks | 18 weeks | 24 months corrected |
| 8 weeks early (2 months) | 30 weeks | 22 weeks | 24 months corrected |
| 12 weeks early (3 months) | 40 weeks | 28 weeks | 24 months corrected |
| 16 weeks early (4 months) | 50 weeks | 34 weeks | 24 months corrected |
The threshold for stopping corrected age is universally set at 24 months of corrected age—not 24 months of chronological age. After that point, the developmental gap between premature and full-term children of similar corrected age typically closes, and chronological age becomes the appropriate reference. This aligns with guidelines from the American Academy of Pediatrics (AAP). For parents tracking their premature baby's development, the adjusted age calculator on this platform provides a dedicated tool with this calculation.
Gestational Age vs. Chronological Age vs. Corrected Age
These three age measures are frequently confused in parenting and clinical contexts. Gestational age (measured in weeks) describes how far along a pregnancy is—from the last menstrual period (LMP) to birth. A full-term pregnancy is 40 weeks gestational age. Chronological age begins at birth and counts forward in days, weeks, and months. Corrected age adjusts chronological age by subtracting the weeks a premature baby was born before 40 weeks. For due-date calculation from conception, see the conception calculator and pregnancy due date calculator.
Korean Age for Babies: How It Works
In the traditional Korean age system, a baby is considered 1 year old at birth—the time in the womb counts as the first year. On January 1st of each new calendar year, every person gains an additional year regardless of birth date. So a baby born in November is 1 year old at birth, becomes 2 on January 1st of the following year, and is chronologically only 2 months old at that point. South Korea officially moved toward the international chronological age system in 2023, but Korean age remains widely used socially and in many pediatric contexts. Our calculator displays Korean age alongside all other formats.
Vaccination Schedule: Age-by-Month Reference
The US CDC pediatric immunization schedule is organized by the baby's age in months, not weeks. Knowing your baby's exact month age ensures you schedule vaccines at the correct time. The milestone chart generated by this calculator marks the key vaccine appointment months (2, 4, 6, 12, 15, 18, and 24 months) directly alongside developmental stage descriptions. Always confirm current vaccine schedules with your pediatrician, as recommendations are updated annually.
Frequently Asked Questions
How do I calculate my baby's age in weeks?
Count the total days from the birth date to today, then divide by 7. A baby born 70 days ago is exactly 10 weeks old. Our baby age calculator performs this automatically and also shows remaining days beyond complete weeks.
How many weeks is a 3-month-old baby?
A 3-month-old baby is approximately 13 weeks old. The precise conversion is 3 months × 4.345 weeks/month = 13.04 weeks. Because months have different lengths, the exact figure ranges from 12 to 14 weeks depending on specific birth and assessment dates.
What is corrected age for a premature baby?
Corrected age equals chronological age minus the weeks born early. A baby born 6 weeks early who is now 20 weeks old chronologically has a corrected age of 14 weeks. Pediatricians use corrected age for developmental assessments until 24 months corrected age.
How many weeks old is a 6-month-old?
A 6-month-old baby is approximately 26 weeks old (6 × 4.345 = 26.07 weeks). The exact count depends on birth date and whether any of the intervening months were 28, 30, or 31 days.
When do I stop using corrected age for my premature baby?
Use corrected age until your baby reaches 24 months of corrected age. After that point, developmental gaps between premature and full-term peers typically close, and chronological age is used for all assessments.
What is the difference between gestational age and baby age in weeks?
Gestational age counts weeks of pregnancy before birth (0–40 weeks). Baby age in weeks starts at birth and counts forward. A baby born at 36 weeks gestation who is now 10 weeks old chronologically has a gestational age at birth of 36 weeks and a current chronological age of 10 weeks.
How do I calculate baby age weeks to months?
Divide the total weeks by 4.345. For example, 35 weeks ÷ 4.345 = 8.05 months (approximately 8 months and 2 days). To go the other direction, multiply months by 4.345 to get weeks.
What is Korean age for a newborn baby?
In the Korean age system, a newborn baby is 1 year old at birth. On the following January 1st, the baby becomes 2 years old—even if they are only a few months old chronologically. South Korea officially adopted the international (Western) age system in 2023, but traditional Korean age remains commonly referenced.