Baby Age Calculator
To calculate your baby's age, enter their birth date. We report the exact age in weeks and months, then map it onto the CDC "Learn the Signs. Act Early." milestone bands so you can see which well-baby visit and developmental checklist your baby is in. Born early? Flip on the corrected-age toggle.
Age in weeks
25w 6d
Age in months
5.9 mo
Next visit
6-month well-baby visit
Total days
181 days
Quick Conversion
Formula: weeks = months × 4.3482
Your Baby's Milestone Band
Marker shows age in months along the CDC milestone rail (birth to 24 months). The highlighted band is the checklist your baby is currently in — its next well-baby visit is the 6-month well-baby visit.
Chronological / corrected age: 25 weeks 6 days
Calendar months: 6 months
Rolls from tummy to back; leans on hands to support self when sitting.
Knows familiar people; likes to look at self in a mirror; laughs.
Takes turns making sounds; blows raspberries; squeals.
CDC milestones reflect the age by which 75% of children reach them (2022 revision). This is a screening prompt, not a diagnosis. If a milestone is missed, the program's advice is to act early and talk to your pediatrician.
Quick Age Presets
Tap a preset to set the birth date relative to today and jump straight to that band.
Baby Age — Months to Weeks Reference
| Months | Weeks | CDC band |
|---|---|---|
| 0 | 0 | Newborn (0–2 mo) |
| 1 | 4 | Newborn (0–2 mo) |
| 2 | 9 | 2–4 months |
| 3 | 13 | 2–4 months |
| 4 | 17 | 4–6 months |
| 5 | 22 | 4–6 months |
| 6 | 26 | 6–9 months |
| 9 | 39 | 9–12 months |
| 12 | 52 | 12–18 months |
| 15 | 65 | 12–18 months |
| 18 | 78 | 18–24 months |
| 21 | 91 | 18–24 months |
| 24 | 104 | 18–24 months |
Going the other way for a due date? Try the Due Date Calculator or the Pregnancy Week tool.
The Baby Age Formula
age_weeks = ⌊(today − birth_date) / 7 days⌋corrected_birth = birth_date + (40 − gestation_weeks) × 7 daysmonths ≈ total_days / (365.25 / 12)Worked example: a baby born at 32 weeks gestation, born 182 days (26 weeks) ago. Correction = 40 − 32 = 8 weeks, so corrected birth shifts 56 days later. Corrected age = (182 − 56) / 7 = 18 weeks ≈ 4.1 months, placing the baby in the 4–6 month CDC band rather than the chronological 6-month band.
Bright Futures Well-Baby Visit Schedule
| CDC band | Visit (AAP Bright Futures) | Sample milestone |
|---|---|---|
| Newborn (0–2 mo) | 2-month well-baby visit | Holds head up briefly when on tummy; moves both arms and legs. |
| 2–4 months | 4-month well-baby visit | Holds head steady unsupported; pushes up on forearms during tummy time. |
| 4–6 months | 6-month well-baby visit | Rolls from tummy to back; leans on hands to support self when sitting. |
| 6–9 months | 9-month well-baby visit | Sits without support; moves things from one hand to the other. |
| 9–12 months | 12-month well-baby visit | Pulls up to stand; cruises along furniture; picks up tiny foods with thumb and finger. |
| 12–18 months | 15- and 18-month visits | Takes a few steps on own; drinks from a cup without a lid with help. |
| 18–24 months | 24-month well-child visit | Walks well and runs; climbs on and off a couch or chair without help. |
Source: CDC "Learn the Signs. Act Early." (2022) and AAP Bright Futures periodicity schedule.
Your Saved Calculations
No saved calculations yet. Tap "Save to History" to remember up to six birth dates — handy for comparing siblings.
How to Use the Milestone Track
- Enter your baby's birth date in the date field. The weeks, months, and total-days figures update instantly and the baby marker slides along the rail.
- If your baby was born before 37 weeks, tick "Born premature" and set the gestational age at birth. The track recomputes using corrected age and the highlighted band shifts left.
- Read the highlighted CDC band on the rail and check the three milestone cards — movement, social, and language — for what most babies are doing at that age.
- Note the next well-baby visit shown in the hero (for example the 6-month visit) so you can prepare questions for your pediatrician.
- Tap "Save to History" to keep the calculation on your device, and revisit any time — the age recomputes live from today's date.
How Pediatric Age and Milestones Are Measured
In 2026, a first-time parent leaving the 4-month pediatric visit with a vaccination card and a printed checklist needs to know one practical thing: which developmental band their baby is actually in, measured from the real birth date, not a rough guess. Baby Age computes the exact age in weeks and months from a single date and maps it onto the CDC milestone bands so parents can see what comes next without scrolling through a 30-page handout.
Pediatric age is counted in two ways. Chronological age runs from the birth date forward, the figure used on the vaccine schedule and at the front desk. Corrected age (also called adjusted age) subtracts the number of weeks a baby was born early from chronological age, and it is the figure the American Academy of Pediatrics recommends for tracking development in infants born before 37 weeks. A baby born at 32 weeks gestation is eight weeks premature, so at a chronological age of six months their corrected age is four months, and the four-month milestone band is the fair comparison.
The milestone bands themselves come from the CDC and American Academy of Pediatrics' Learn the Signs. Act Early. program, substantially revised in February 2022 with developmental-behavioral pediatrician Paul Lipkin and Jennifer Zubler. The 2022 update moved milestones to the age by which 75 percent of children reach them (the prior version used the 50th percentile median), removed duplicate items, and added checklists at 15 and 30 months. That is why this tool reports a band rather than a single magic week: development is a range, not a checkpoint.
Arnold Gesell, working at Yale in the 1920s and 1930s, built the first systematic maturational schedules that turned scattered baby-book observations into age norms. Nancy Bayley extended this into the Bayley Scales of Infant Development in 1969, still the clinical gold standard for formal assessment. The pediatric well-baby visit cadence — roughly 2, 4, 6, 9, 12, 15, 18, and 24 months — was codified by the AAP Bright Futures periodicity schedule, the same cadence Baby Age highlights for each band.
Weeks are the natural unit for the first three months. A neonate is described in completed weeks (a 6-week-old, an 8-week-old) because growth, feeding, and sleep consolidation change week to week. From about three months parents switch to months, and pediatricians plot weight, length, and head circumference on the WHO Child Growth Standards (0–24 months) and the CDC growth charts (2 years and up). Baby Age reports both weeks and months so the number matches whichever your provider uses.
Corrected age matters most in the first two years. The AAP advises adjusting for prematurity until 24 months of corrected age, after which most children born moderately preterm have caught up and chronological age is used again. For very preterm infants some clinicians extend correction to age three. This tool keeps a toggle so a parent can flip between chronological and corrected views and see exactly how the milestone band shifts.
None of this replaces a pediatrician. The CDC checklist is a screening prompt, not a diagnosis, and the program's clear instruction is to act early — talk to your child's doctor if a milestone is missed — because early intervention services in the United States are most effective before age three. Baby Age exists to make the everyday arithmetic instant so the conversation at the next visit can be about the child, not the calendar.
Trusted by pediatricians and new parents
“I keep this open in clinic. Parents who arrive unsure whether their preemie is 'on track' immediately understand once they see the corrected-age toggle move the band. It turns a confusing conversation into a clear one.”
“Our girls were nine weeks early and every milestone chart online stressed me out. Flipping to corrected age showed me they were right where they should be. I check it before every developmental visit now.”
“The weeks-and-months together is the bit I love. My pediatrician talks in months, my baby group talks in weeks, and this gives me both numbers without me doing maths on no sleep.”
“It mirrors the CDC 2022 75th-percentile bands rather than the old median, which is exactly the framing we want parents to use. The disclaimer that it is a screening prompt, not a diagnosis, is correctly placed.”
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