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
Last Reviewed: August 2026 • AAP Guidelines ↗Exact Calendar Weeks & Premature Corrected Age Math
Official Baby Age Calculator Report
Input Parameters & Record Details
26 Months, 22 Days (813 Total Days)
Calculation Methodology & Scientific Standards
Calculates exact infant age in completed weeks, months, days, and total elapsed hours according to American Academy of Pediatrics (AAP) and CDC Bright Futures pediatric milestone surveillance guidelines.
Clinical & Pediatric YMYL Medical Disclaimer
This calculation is provided strictly for informational, educational, and developmental screening reference. It does not constitute pediatric medical advice, formal psychological diagnosis, or a personalized individualized education plan (IEP) determination. Gestational age adjustments, chronological age calculations, and developmental quotients must be evaluated in clinical context by a licensed pediatrician, developmental specialist, or certified educational psychologist. If you have concerns regarding infant growth, premature developmental delays, or milestone attainment, consult your healthcare provider immediately.
Commonly scheduled check-up milestone ages (2, 4, 6, 12, 15, 18, 24 months). Checkups vary by country and healthcare provider; confirm exact dates with your pediatrician.
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.
Baby Age Calculator FAQs
What people are saying
Real user ratings and reviews for Baby Age Calculator.
Explore Calculators by Category
- Age CalculatorFree online age calculator: instantly calculate your exact age in years, months, days, hours, and seconds with ISO 8601 leap year precision.
- Chronological Age CalculatorCalculate exact chronological age from a date of birth and reference date for clinical, educational, or legal verification.
- Birthday CalculatorTrack your next birthday countdown in days and hours while exploring milestone age thresholds.
- Age Difference CalculatorCalculate the exact age difference between two birth dates in years, months, and days.
- BMI CalculatorCalculate your Body Mass Index (BMI) using height and weight. Designed for adults and older teens with CDC/WHO weight classification standards.
- Calorie CalculatorEstimate daily calorie needs, BMR, and maintenance calories using age, height, weight, sex used by the calculation, and activity level.
- Body Fat CalculatorEstimate body fat percentage using body measurements with the U.S. Navy circumference method. Supports metric and imperial units.
- BMR CalculatorEstimate your basal metabolic rate (BMR): the energy your body uses at rest for essential functions.
- Ideal Weight CalculatorUse this ideal weight calculator to estimate a standard healthy weight range for your height based on the adult BMI range of 18.5 to 24.9.
- Pace CalculatorCalculate your pace, speed, distance, or finish time for running, walking, cycling, and race planning.
- Child Age CalculatorCalculate your child's exact age in years, months, and days for school registration and milestone tracking.
- Adjusted Age CalculatorCalculate adjusted age for preemie babies based on birth date and weeks of prematurity.
- Corrected Age CalculatorCalculate corrected chronological age for premature infants using due date alignment.
- Developmental Age CalculatorCalculate developmental age and DQ scores across key developmental domains.
- Brigance Age CalculatorCalculate exact chronological age for Brigance Inventory of Early Development scoring.
- Pearson Age CalculatorCalculate exact Y:M:D chronological age for psychological and educational assessments.
- Date CalculatorAdd or subtract days, weeks, or months from any starting calendar date.
- Days Between Dates CalculatorCalculate the exact number of days, weeks, and months between two dates.
- Working Days CalculatorCalculate business days between dates while excluding weekends and public holidays.