Article — Baby Age Calculator
Baby Age Calculator: Weeks, Months, and Milestones
For the first 12 weeks, baby age is tracked in weeks. From 3 months to 24 months, it switches to months. After age 2, year-based tracking takes over. For premature babies, adjusted age (chronological age minus weeks premature) is used until age 2.
This calculator is meant for everyday tracking and milestone awareness, not medical diagnosis. The CDC's "Learn the Signs. Act Early." program and the American Academy of Pediatrics both publish milestone windows that the result panel cross-references. Talk to your pediatrician about any specific concerns; they have validated screening tools that go well beyond a calculator.
Baby age in weeks
Pediatricians and parents use weeks for the first three months because so much changes so fast. A 2-week-old and a 6-week-old are dramatically different in feeding patterns, sleep, and motor control. Saying "almost two months" loses the resolution that "6 weeks" captures.
The well-baby visit schedule recommended by the AAP locks into the same week-based rhythm early on: birth, 3 to 5 days, 1 month, 2 months, 4 months, 6 months, 9 months, 12 months. Each appointment lines up with a developmental checkpoint and a vaccination block, and the week count helps parents track exactly where their baby sits in that sequence.
Baby age in months
Once a baby is past 12 weeks, monthly tracking takes over. Most developmental milestone tables are organized into windows: 3 to 6 months, 6 to 9 months, 9 to 12 months, 12 to 18 months, and 18 to 24 months. The CDC milestone checklists follow this same structure, with new social, language, motor, and cognitive markers appearing at each window.
1 month = 30.4375 days3 months = 91.3 days6 months = 182.6 days12 months = 365.25 daysThe 30.4375-day figure is the average calendar month (365.25 / 12). Using exact calendar month boundaries gives slightly different counts on either side of a 30-day vs. 31-day month, but for tracking and milestone comparison the average is precise enough and easier to reason about.
The baby age formula
Three formulas cover most cases. Days are straightforward. Weeks and months are floor divisions on the day count.
- Total days = today − birth_date
- Age in weeks = floor(total_days / 7)
- Age in months = floor(total_days / 30.4375)
- Age in years = floor(total_days / 365.25)
- Adjusted age = chronological age − weeks_premature
The CDC updated its milestone checklists in 2022, the first major revision since the program launched in 2004. The change shifted some milestones to later ages to reflect what the typical child actually does, instead of the optimistic 50th-percentile timing that earlier versions used. The new lists are explicitly designed for the 75th percentile so that most children will have hit each milestone by the listed age, making it easier to spot real delays.
Adjusted age for preemies
Premature birth is defined as delivery before 37 weeks gestation. A baby born at 32 weeks is 8 weeks early. When that baby is 6 months old chronologically, their development is closer to that of a 4-month-old, because they spent 2 of those months catching up to where a full-term baby would have started.
Adjusted (or corrected) age accounts for this. The American Academy of Pediatrics recommends using adjusted age for developmental milestones until the child reaches 24 months chronological age. After 2 years, most premature children have caught up developmentally and chronological age becomes the reference.
Vaccinations are scheduled by chronological age because immune response depends on time since birth. Developmental milestones use adjusted age. Both numbers may matter at the same well-baby visit, so it helps to know which one your pediatrician is asking about.
Baby age milestone windows
The CDC organizes milestones into eight age windows from birth to 5 years. The calculator shows the current window for whatever age you enter. These are guidelines based on what most children do by the listed age, not strict deadlines.
- 0-3 months = lifts head, follows objects, social smile
- 3-6 months = rolls over, reaches for objects, laughs
- 6-9 months = sits unsupported, babbles, peek-a-boo
- 9-12 months = pulls to stand, pincer grasp, waves bye
- 12-18 months = walks, 5-50 words, follows instructions
- 18-24 months = runs, kicks ball, combines two words
- 2-3 years = jumps, 3-4 word sentences, sorts shapes
- 3-5 years = climbs stairs alternating feet, asks "why"
When to switch between weeks, months, and years
The transition from weeks to months happens around 12 weeks (3 months) because development starts to space out. Saying "13 weeks" instead of "3 months and 1 week" stops adding information.
The switch from months to years happens around 24 months. By 2 years, monthly milestones consolidate into yearly checkpoints. Pediatric visits move from every few months to once a year. Parents and pediatricians both shift to "2 years 3 months" or "2.5 years" instead of counting months alone.
Calendar months and 4-week months differ by about 2 days on average. If you count "4 months" as four 4-week blocks (112 days), you will land about 10 days earlier than counting it as four calendar months (122 days). Most pediatricians use calendar months for ages; the calculator's "total months" stat uses the same convention.
Questions to ask your pediatrician
The calculator helps you track and compare, but it is not a substitute for professional advice. If you have concerns, the AAP recommends asking specific questions at each well-baby visit rather than waiting for the pediatrician to volunteer information.
Good questions to write down before the visit include: which milestones should I be watching for between now and the next visit, are there any red flags I should know about for this age, and how does my baby compare to expected development for their adjusted age (if premature). Specific questions yield specific answers, which is more useful than a general "are they doing okay?"
Growth percentiles are a separate measurement track from milestones, and the two should not be confused. Weight and length are plotted on World Health Organization growth charts for the first 24 months, then on CDC charts after that. A baby in the 10th percentile for weight is not behind on milestones — they are just smaller than most babies their age. Persistent percentile drops across multiple visits matter more than any single reading.
One more note about the calculator's adjusted age output: it caps at the 24-month mark. Some specialty clinics extend adjustment to 36 months for babies born very preterm (under 32 weeks gestation), but standard AAP guidance ends at 2 years. If your child's care team is using a different convention, follow their guidance over any general calculator output.