Baby Age Calculator

One birthday, four different answers

Ask how old a baby is and you can answer honestly in four ways. Every one of them is correct.

Years, months and days is the everyday answer - the one you give a relative on the phone. It is easy to picture and it matches how birthdays work.

Total months is what forms ask for. Nurseries, passport offices and growth charts want a single number, so 1 year and 3 months becomes 15 months.

Total weeks rules the first three months. Newborn checkups, feeding guides and sleep advice are all written in weeks, because one week is a big change at that age.

Total days is the one parents fall in love with. It is how you know the 100th day is close, or that today is day 1000. Nothing rounds and nothing hides - it is just the count.

The calculator gives you all four at once, so you never have to choose in advance. Enter the birthday and read whichever line the moment calls for.

Baby age calculator: weeks to months

This is the conversion parents get wrong most often, and it is not their fault. Four weeks looks like a month. It is not.

A month is 30.44 days on average, not 28. So to turn weeks into months you divide by 4.345, not by 4. The gap is small at first and then it grows. At 12 weeks the two methods sit only a few days apart. By 52 weeks, dividing by 4 tells you 13 months - but your baby is not 13 months old, they are just under 12.

That is why a 44-week-old is about 10 months, not 11. And why 52 weeks works out to 11 months and 29 days: one day short of the first birthday.

The table below is the quick version. Every row opens a full page for that week, with what babies do around that age and the milestones nearby. For a number that matches your baby's real birthday - leap day and short months included - use the calculator instead of the table.

Month-end birthdays and leap days

Calendar maths gets strange at the edges, and this is where dividing by 30 quietly breaks.

Say your baby was born on 31 January. One month later is 28 February, or 29 February in a leap year - there is no 31 February to land on. So that first month runs 28 days and the second one runs 31. The monthly birthday drifts, and both answers are defensible depending on how you count.

A 29 February baby has the rarer version of the same problem. In three years out of four that date does not exist, so families pick 28 February or 1 March and stay consistent. The day count never wobbles though: day 365 is day 365, leap year or not.

We read actual dates off the calendar, so short months, long months and leap days are all handled. If an answer looks a day off from the rough maths you did in your head, the calendar is usually the reason.

The age your doctor uses

Clinics do not settle on one unit either, and knowing which one they mean saves confusion at the desk.

For the first two months the schedule runs in weeks. The 6-week check, the 8-week shots - these count from the birth date in weeks, and a few days either way matters.

From roughly three months to the second birthday everything switches to months. Growth charts, developmental checks and vaccine schedules are all plotted month by month. This is where the weeks-to-months conversion above stops being academic and starts being practical.

After age two, years are enough.

One important exception: babies born early are often tracked by corrected age for milestones - their age minus the weeks they arrived early - while shots still follow the real birth date. The FAQ above answers that one in full.

Bring the birth date to the appointment, not the age. A date never goes stale; a remembered age does.