Baby Age Calculator
Find your baby's exact age in weeks, months, and days — the way paediatricians measure it. Includes corrected age for premature babies and upcoming developmental milestones.
Enter your baby's date of birth to see their exact age in weeks, months, and days — plus upcoming developmental milestones and, for premature babies, their corrected age.
More Than Just an Age — What This Baby Age Calculator Actually Shows You
Most age calculators give you a number and stop. This one keeps going. Enter your baby's date of birth and you get their exact age in weeks, months, and days — but you also get a live developmental milestone strip showing which milestones your baby has likely passed, which ones they're in the window for right now, and what's coming next. For premature babies, enable the corrected age toggle and the entire milestone assessment automatically adjusts to use adjusted age instead of chronological age.
This matters because parents and caregivers constantly need three things at once: the exact age their paediatrician is asking for, context about where their baby sits developmentally, and — for preemie parents — a corrected age figure that reflects where their baby actually is, not just how many weeks since birth. This calculator gives you all three in one place.
How Paediatricians Measure Baby Age — Weeks, Months, and Why It Changes
When your baby is born, their age is counted in weeks — not months, not years. In the first three months of life, so much neurological and physical development happens in such a short window that even a week can make a meaningful difference. A 4-week-old and an 8-week-old are at very different developmental stages, even though they're both "about one to two months old." Weeks give precision that months can't.
After around 13 weeks (3 months), most paediatricians switch to counting in months. This continues until age 2, at which point years become the standard measure. This calculator mirrors that clinical convention exactly: for babies under 13 weeks, the primary display shows weeks and days. For babies between 3 months and 2 years, it shows months as the headline figure. The total weeks and total days are always shown in the breakdown regardless of age, so you always have the precise number if a doctor or nurse asks.
The Milestone Strip — Developmental Context Built Into Every Calculation
After you enter your baby's birth date and hit calculate, the results section shows a developmental milestone strip pulled from CDC milestone guidelines. This strip is personalised to your baby's current age — it shows:
- Milestones already passed (shown in grey) — what your baby has likely achieved based on their age.
- Current milestone window (highlighted in green) — what your baby is expected to be working on or achieving right now.
- Upcoming milestones (shown in blue) — what's coming in the next few weeks, so you know what to watch for.
- Next milestone callout — a dedicated amber box showing the very next developmental stage and how many weeks away it is.
This is the same milestone framework used by paediatricians at routine check-ups, drawn from the CDC's "Learn the Signs. Act Early." developmental milestone guidelines. It covers 13 milestone stages from 1 month through to 2 years. Every baby develops at their own pace — the milestone strip shows averages and ranges, not hard deadlines. Always discuss any concerns with your paediatrician.
What Is Corrected Age for Premature Babies?
If your baby was born early, their chronological age (weeks since birth) isn't the same as their corrected age (also called adjusted age). Corrected age accounts for how early they were born by subtracting the number of weeks premature from their chronological age.
For example: a baby born at 32 weeks gestation was born 8 weeks early (full term is 40 weeks). If that baby is now 12 weeks old by the calendar, their corrected age is 12 − 8 = 4 weeks. Their brain and body have had the equivalent development time of a 4-week-old full-term baby, not a 12-week-old. Assessing them against 12-week milestones would be unfair and potentially alarming. Assessing them against 4-week milestones gives an accurate picture of their progress.
Enable the premature toggle in the calculator above, enter how many weeks early your baby was born, and you'll see both figures side by side: chronological age in weeks and months, and corrected age in weeks and months. The milestone strip adjusts automatically to corrected age. Most paediatricians recommend using corrected age for developmental assessment until age 2, and sometimes until age 3 for very premature babies born before 28 weeks, according to guidance from the American Academy of Pediatrics.
Developmental Milestones by Baby Age — Full Reference
The milestones shown in the calculator after you calculate are drawn from this reference table. They cover the key developmental markers from 1 month to 2 years across physical, social, language, and cognitive development. Remember: these are averages, not hard deadlines. A baby who reaches a milestone a week or two later than the guide is almost always within the normal range of development.
| Age | What to look for |
|---|---|
| 1 month | Startles at sounds, fixes gaze on faces, brings hands near face, calms with familiar voice |
| 2 months | Smiles at people, coos, follows moving objects with eyes, holds head up briefly during tummy time |
| 4 months | Laughs, holds head steady, reaches for objects, pushes down with legs when feet on a surface |
| 6 months | Responds to name, rolls both ways, sits with support, passes objects hand to hand, takes turns making sounds |
| 9 months | Stranger anxiety starts, says "mama/dada", crawls, uses pincer grip, waves bye-bye, plays peekaboo |
| 12 months | First steps, says 1–3 words clearly, points to things, drinks from open cup with help, pulls to stand |
| 15 months | Walks independently, uses spoon, says 3+ words, copies simple actions, points to ask for things |
| 18 months | Uses 10+ words, runs (wobbly), climbs furniture, shows affection, simple pretend play |
| 24 months | 2-word phrases, runs well, kicks a ball, follows 2-step instructions, plays alongside other children |
How Many Weeks Old Is My Baby?
Because calendar months vary in length (28 to 31 days), converting between months and weeks isn't exact — which is exactly why this calculator uses your baby's actual birth date rather than an estimate. The table below gives approximate conversions as a quick reference, but the calculator gives you the precise number.
| Baby age | Approx. weeks | Approx. days |
|---|---|---|
| 1 month | ~4–5 weeks | ~30–31 days |
| 2 months | ~8–9 weeks | ~59–62 days |
| 3 months | ~13 weeks | ~90–92 days |
| 6 months | ~26 weeks | ~181–184 days |
| 9 months | ~39 weeks | ~273–276 days |
| 12 months (1 year) | ~52 weeks | ~365 days |
| 18 months | ~78 weeks | ~547–549 days |
| 24 months (2 years) | ~104 weeks | ~730 days |
How to Use This Baby Age Calculator
Enter your baby's date of birth in the first field. The "calculate age as of" field defaults to today but you can change it to any date — useful for calculating how old your baby was at a past appointment, or how old they'll be at an upcoming one. If your baby was premature, tap the toggle marked "My baby was born prematurely" and enter how many weeks early they were born. Then hit Calculate.
The results show your baby's age as weeks and days (under 13 weeks), or months (over 3 months), plus the total weeks and days breakdown. If you enabled premature mode, a purple corrected age panel appears showing adjusted weeks and months. Below that, the milestone strip appears — green for current milestones, blue for upcoming, and an amber "next milestone" callout showing exactly what to watch for next and how many weeks away it is.
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