Baby's Age Calculator
Calculate your baby's exact age in calendar weeks, total days, months, premature corrected age, and track Wonder Weeks mental leaps.
Baby Age Calculator
Exact Calendar Weeks & Premature Corrected Age Math
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.
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.
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. Use the table below as a clinical reference.
| Age in Months | Approximate Weeks | Exact Days | Pediatric Milestone 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 intro |
| 9 months | 39.1 weeks | ~274 days | Mobile infant — Pulling to stand |
| 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
A baby born at 36 weeks gestation (4 weeks early) who is today 20 weeks old chronologically has a corrected age of 16 weeks. Formula: Corrected Age = Chronological Age − Weeks Born Early. The threshold for stopping corrected age is universally set at 24 months of corrected age per American Academy of Pediatrics (AAP) guidelines.
| 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 mo) | 20 weeks | 16 weeks | 24 months corrected |
| 6 weeks early | 24 weeks | 18 weeks | 24 months corrected |
| 8 weeks early (2 mos) | 30 weeks | 22 weeks | 24 months corrected |
| 12 weeks early (3 mos) | 40 weeks | 28 weeks | 24 months corrected |
| 16 weeks early (4 mos) | 50 weeks | 34 weeks | 24 months corrected |
Gestational vs. Chronological vs. Corrected Age
Gestational Age: Weeks of pregnancy before birth (0–40 weeks).
Chronological Age: Begins at actual birth and counts forward in days/weeks.
Corrected Age: Adjusts chronological age by subtracting weeks born before 40 weeks.
Korean Age System for Babies
In traditional Korean age, a baby is considered 1 year old at birth (gestational womb time), gaining another year on January 1st. While South Korea legally transitioned to international chronological age in 2023, traditional Korean age remains widely referenced socially.
- American Academy of Pediatrics (AAP): Clinical Guidelines on Premature Birth Corrected Age Assessment — Corrected age math (chronological age minus weeks premature) used up to 24 months corrected age.
- World Health Organization (WHO) & CDC: Child Growth Standards & Preventive Pediatric Check-up Schedule (2, 4, 6, 12, 15, 18, 24 months).
- The Wonder Weeks Research: van de Rijt, H., & Plooij, F. Predictable Mental Leap Progression Phases in Infant Brain & Neurological Development.
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Community Reviews
Based on 1485 user ratings and calculations worldwide.
Sub-second exact age calculation down to the second! The live seconds counter is super fun to watch, and I love that it doesn't send my birth date to any server.
I needed an exact age in years, months, and days for a legal trust document and HR cutoff. ISO 8601 accuracy gave me 100% confidence.
The leap year handling for Feb 29 birth dates is spot on. Handled my brother's leap birthday perfectly without any day drift.
The traditional Korean age mode is such a thoughtful feature. Really clean Vercel-like design with instant dark mode!
Used the age difference tab to compare my kids' exact age gap for school enrollment. Simple, fast, and 100% private.