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Baby age calculator
How old is my baby in weeks and months?
A baby's age in weeks is the number of whole days since birth divided by seven. Age in months counts calendar months, so a baby born on 15 January turns one month old on 15 February — not after 30 days. Babies born before 37 weeks are also given a corrected age, which subtracts the weeks they arrived early, and health professionals use that corrected age to read milestones and sleep needs until around two years.
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Weeks, months and days all mean something different
New parents get asked their baby's age constantly, and the expected unit keeps changing. The maternal health nurse wants weeks. The milestone chart wants months. The six-week check wants the actual date. They do not convert cleanly into one another, because a month is not four weeks — it is closer to 4.35.
That gap compounds. By six months, a baby is about 26 weeks old, not 24. Reading a 26-week-old against a 24-week row is one of the quieter reasons a chart can feel “off” for a perfectly typical baby.
Corrected age for babies born early
If your baby arrived before 37 weeks, their corrected age is the one that matters for reading development. Subtract the number of weeks early from their chronological age and use that figure against any milestone, sleep or feeding guidance.
This is not a technicality. A baby born ten weeks early who is measured against their chronological age will look behind on every chart for the better part of two years, when in fact they are exactly where they should be. Health services use corrected age precisely to avoid that false alarm.
Age questions parents ask
- How many weeks old is my baby?
- Count the whole days since your baby's date of birth and divide by seven. A baby born on 1 January is 4 weeks old on 29 January, not on 1 February — weeks and calendar months drift apart quickly, which is why sleep and milestone guidance for young babies is given in weeks.
- When does a baby turn one month old?
- On the same date of the following month. A baby born on 15 January turns one month old on 15 February, at roughly 31 days, not after 30 days. For birth dates near the end of a month, the anniversary falls on the last day of any shorter month.
- What is corrected age and when do I stop using it?
- Corrected age is a preterm baby’s age minus the weeks they were born early, and it is the age health professionals use to read milestones, sleep needs and growth charts. A baby born at 32 weeks is 8 weeks early, so at 20 weeks old their corrected age is 12 weeks. Most services use corrected age until around two years, by which point the difference has usually evened out.
- Why do sleep charts use weeks but milestone charts use months?
- Sleep changes fastest in the first few months, so weeks give the resolution needed — the difference between a 4-week-old and an 8-week-old is enormous. Developmental milestones unfold over longer spans, so months are the more practical unit. Both are describing the same baby, just at different zoom levels.
Where these numbers come from
- Raising Children Network — Australian government-funded guidance on nappies, wees and poos in babies.
- HealthyChildren.org (American Academy of Pediatrics) — Amount and schedule of formula feedings by age.
Stop doing this maths in your head
Add your baby's date of birth once and BubSync keeps their age current everywhere — on the timeline, on the milestones it suggests, and on every memory you save to their baby book.