Baby age calculator, exact in weeks and months.
Enter your baby’s date of birth to instantly calculate their exact age in weeks and days, months, and total days — the precise units pediatricians, vaccination schedules, and milestone trackers use. Calculate today’s age or age on any milestone date. 100% private and runs in your browser.
Your exact age
Enter a date of birth
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- Seconds alive
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Every unit of your baby’s age
Age in weeks, months, and days
Read your baby’s age in whichever unit your pediatrician, daycare, or milestone book asks for. Everything updates as you type.
Weeks and days
The primary unit pediatricians use for the first 6 months.
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Months and days
The standard milestone unit for baby photos and checkups.
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Total weeks
Used for vaccination schedules and developmental leaps.
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Age in whole months
For monthly milestone markers.
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Total days alive
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1st Birthday / Next birthday
Countdown to the big celebration!
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Half birthday (6-month milestone)
Six months from birth.
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Decimal age
Used on pediatric growth percentiles.
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How baby age is calculated
A baby’s age is the exact calendar time elapsed since birth. During the first few weeks and months of life, calculating age is more nuanced than simple year subtraction because development happens at breakneck speed. Pediatricians and parents measure time in weeks rather than months for the first half-year.
This calculator handles the exact calendar length of every month (28 to 31 days) and leap years, giving you both the weeks-and-days figure and the calendar months-and-days figure without rounding errors.
Developmental stages: Newborn to Toddler
Early childhood development is categorized into distinct stages, each with unique sleep, feeding, and physical milestones:
| Stage | Age range | Key developmental focuses |
|---|---|---|
| Newborn | 0 to 2 months (0–8 weeks) | Focusing on faces, feeding every 2–3 hours, startle reflex, rapid growth tracked weekly. |
| Infant | 2 to 12 months (9–52 weeks) | First smiles, rolling over, sitting up, babbling, starting solid foods at 6 months. |
| Toddler | 1 to 3 years (12–36 months) | First steps, vocabulary explosion, running, climbing, early independence. |
| Preschooler | 3 to 5 years | Complex sentences, interactive play, motor skills, preschool readiness. |
Why doctors count in weeks instead of months
Parents often wonder why a pediatrician says their baby is “10 weeks old” instead of “2 and a half months”. Because calendar months have between 28 and 31 days (averaging 4.33 weeks per month), four weeks is only 28 days — which is less than almost every calendar month. Calling a 4-week-old baby “1 month old” creates a cumulative error.
For vaccines, developmental screenings, and weight gain tracking, weeks are the medical gold standard until roughly 6 months of age, after which months become the standard measurement.
Corrected age for premature babies
If your baby was born preterm (before 37 weeks of pregnancy), their developmental milestones should be measured against their corrected age (also called adjusted age), not their chronological birth date.
To find corrected age: take your baby’s chronological age and subtract the number of weeks they arrived early (40 weeks minus gestational age at birth). For example, if a baby is 16 weeks old chronologically but was born 6 weeks premature, their corrected age is 10 weeks. Pediatricians typically use corrected age until a child turns two years old.
Key milestones in your baby’s first year
While every child develops at their own pace, milestones provide a general guide to major developmental steps:
1 Month
Tracks faces with eyes, responds to familiar voices, lifts head briefly during tummy time.
2 Months
First real smiles, cooing sounds, smoother arm and leg movements.
4 Months
Reaches for toys, laughs out loud, pushes up on elbows, rolls from tummy to back.
6 Months
Sits with support, passes objects between hands, responds to own name, ready for solids.
9 Months
Crawls, sits unsupported, develops pincer grasp, babbles syllables like “mama” or “dada”.
12 Months
Pulls to stand, takes first steps, waves goodbye, says first intentional words, celebrates 1st birthday!
Practical tips for tracking baby age
- Use weeks for the first 12 weeks. Growth charts, feeding amounts, and doctor appointments in the fourth trimester are nearly all indexed by week.
- Use calendar dates for monthly birthdays. If your baby was born on January 15th, celebrate their month-iversary on the 15th of each month, not every 4 weeks.
- Always tell your pediatrician if born premature. Plotting growth on chronological charts can cause unnecessary worry; preemies should always be plotted against adjusted age.
- Save the 6-month half birthday. Six months is a major milestone when solid foods start, sitting begins, and sleep patterns mature.
Why track it
Everyday milestones and medical checks
From weekly weigh-ins to 1st birthday milestones, here is where exact baby age matters.
Pediatrician visits and growth charts
Doctors track height, weight, and head circumference percentiles against precise weeks and months. Knowing exact weeks ensures accurate growth curve plotting.
Vaccination schedules
Childhood immunizations are scheduled around strict age thresholds (e.g., 2 months / 8 weeks, 4 months, 6 months). Knowing the exact week keeps vaccines on track.
Developmental leaps and sleep regressions
Parents track “wonder weeks” and classic sleep regressions (like the 4-month and 8-month regressions) calculated from the due date or birth date.
Monthly milestone photos
Documenting your baby turning 1 month, 2 months, or 6 months old requires knowing the exact calendar date of each monthly anniversary.
Daycare and nursery enrollment
Infant care rooms and baby classes have strict age minimums (often 6 weeks or 12 weeks). Exact day-to-week conversion confirms eligibility.
Corrected age for premature babies
Premature babies reach milestones according to gestational age (corrected age) rather than chronological age during their first two years.
FAQ
Frequently asked questions about baby age
How do I calculate my baby’s exact age in weeks and months?
Enter your baby’s date of birth and leave today’s date selected. The calculator instantly breaks down their age into exact weeks and days, total weeks, and months and days. You can also change the target date to calculate how old your baby was on a past doctor visit or will be on a future date.
Why do pediatricians measure infant age in weeks rather than months?
A calendar month averages 4.33 weeks (30.4 days), not 4 weeks. Measuring in weeks provides the high precision needed for rapid infant growth, feeding adjustments, and time-sensitive vaccination schedules during the first 6 months.
What is corrected age, and when should I use it?
Corrected age (or adjusted age) is calculated by subtracting the weeks of prematurity from your baby’s chronological age. If your baby was born at 32 weeks (8 weeks early) and is 20 weeks old today, their corrected age is 12 weeks. Use corrected age for developmental milestones and growth percentiles until age two.
When does a baby officially become an infant or a toddler?
Medical organizations classify children as newborns from birth to 2 months (0–8 weeks), infants from 2 months to 12 months (1 year), and toddlers from 1 to 3 years old (12–36 months).
How do I know when my baby turns another month old?
Your baby turns a month older on their birth date each month. For example, a baby born on March 18 turns one month on April 18, two months on May 18, and so on. For months with fewer days (such as February), the last day of the month is used.
Is my baby’s date of birth sent to any server?
No. All calculations run strictly inside your browser using JavaScript. No birth date is uploaded, stored, or tracked anywhere.