Baby Age Calculator

Weeks, then months, then years

A baby's age is reported in weeks for the first couple of months, in months until about two years, and only then in years — and the shifting unit is not arbitrary. It reflects how quickly development proceeds, and how much difference a few weeks makes early on.

The difference between a six-week-old and a ten-week-old is enormous. The difference between a six-year-old and a six-year-and-one-month-old is negligible. The unit follows the rate of change.

Why the precise age matters

Immunisation schedules are defined in weeks and months. India's Universal Immunisation Programme schedules BCG, OPV and hepatitis B at birth; pentavalent, OPV and rotavirus doses at 6, 10 and 14 weeks; measles-rubella at 9 to 12 months; and boosters at 16 to 24 months. Missing or mistiming a dose matters, and the intervals are minimums rather than suggestions.

Developmental milestones are assessed against age bands. Social smiling around 6–8 weeks, head control by 3–4 months, sitting unsupported around 6–8 months, first words around 12 months, walking between 9 and 18 months. The ranges are wide, and a child at the late end of a range is usually developing normally.

Feeding guidance — the WHO recommends exclusive breastfeeding for the first 6 months, with complementary feeding introduced from 6 months alongside continued breastfeeding.

Sleep and routine guidance also shifts by month, and paediatric medication dosing is frequently weight-based but age-gated.

Corrected age for premature babies

This is the most important adjustment on this page, and it is routinely missed.

For a baby born preterm, development should be assessed against corrected age — chronological age minus the number of weeks born early.

A baby born at 32 weeks, eight weeks early, who is now six months chronologically, has a corrected age of about four months. Expecting six-month milestones from them is inappropriate, and measuring them against a six-month chart will produce unnecessary alarm.

Corrected age is generally used for developmental assessment until about two years, after which most preterm children have caught up. Note that immunisations, by contrast, are given according to chronological age, not corrected — this distinction catches many parents out.

Reading the result

The months figure counts completed calendar months, which is what paediatric charts and schedules use. The weeks and days figures are useful in the early period and for precise scheduling.

Note that months are not uniform in length, so a figure in months is not simply days divided by 30. The calculator counts calendar months properly.

On milestone ranges

Developmental milestones describe ranges, not deadlines, and the normal range is much wider than most parents expect. Walking anywhere between 9 and 18 months is within normal limits. First words anywhere between 10 and 18 months likewise.

What warrants attention is not being at the later end of a range but loss of previously acquired skills, no response to sound, no eye contact or social smiling by around three months, or a consistent pattern of falling further behind across multiple domains rather than one.

This is general information, not medical advice. Every child should have routine well-baby checks, and any concern about development, feeding or growth is worth raising with a paediatrician rather than researching alone — early intervention is consistently more effective than waiting.

Common mistakes

Not using corrected age for a preterm baby. It is the most consequential error here.

Using corrected age for immunisations. Those follow chronological age.

Treating milestones as deadlines. The normal ranges are wide.

Comparing against another child. Variation within normal is substantial.

Converting months by dividing days by 30. Calendar months vary in length.