How to calculate your baby's age
A baby's age is simply the time elapsed between their date of birth and today, but how you express it changes a lot in the first two years. In the newborn period, pediatricians and parents typically think in weeks and days ("she's 6 weeks old"); after about a year, most people switch to months, and after age two, to years. This calculator shows all three so you always have the right unit for the moment — a vaccine schedule, a milestone checklist, or simply answering "how old is your baby?"
Converted into complete years, months, and days, plus total weeks and total days for easy reference.
What is corrected (adjusted) age, and why it matters
A baby born before 37 weeks of pregnancy is considered premature, and premature babies have had less time in the womb to grow than a full-term baby of the same birth date. Corrected age (also called adjusted age) accounts for this by subtracting the number of weeks born early from the baby's chronological age — effectively measuring age from the original due date instead of the actual birth date. A baby who is 6 months old chronologically but was born 2 months early has a corrected age of about 4 months, and it's the 4-month milestones — not the 6-month ones — that are the fair comparison for their development.
Weeks born early = 40 − gestational age at birth (in weeks), or the gap between the birth date and the original due date if that's known — the due date method is more precise.
When do you stop using corrected age?
Most pediatricians use corrected age for growth and developmental milestones until around 24 months of chronological age, since that's roughly when the gap between a premature baby and their full-term peers tends to close. For babies born very early (before about 28 weeks), some pediatricians continue correcting longer — sometimes to 2.5 or 3 years. Always follow your own pediatrician's or neonatologist's guidance, since it depends on how early your baby was born and how they're progressing individually.
Why age in weeks matters early on
In the first few months, a lot can change week to week — feeding patterns, sleep, and growth spurts are often tracked and discussed in weeks rather than months, which is why this calculator always shows total weeks alongside months and years. It's also how most hospital discharge instructions, feeding guides, and early milestone checklists are written.
Newborn, infant, toddler: what's the difference?
These stage names are informal but widely used: a newborn is typically under 1 month old, an infant covers roughly 1–11 months, a toddler is about 1–2 years old, and a preschooler is roughly 3–5 years old. This calculator labels your baby's current stage automatically, using corrected age instead of chronological age if you've entered premature birth details.
