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Baby Age Calculator: Age in Weeks and Months

What is this tool?

The baby age calculator shows a baby's age in weeks and months side by side, which is how parents, doctors and health visitors actually talk about the first two years. Babies are counted in weeks at first, then in months, and the switch point confuses almost everyone, so this page shows both units at once and never makes you convert. Enter the baby's date of birth below and you get weeks, months, days and the next month milestone in one result.

How it works

It subtracts the birth date from today using completed months and days, and divides total days by 7 for completed weeks. The result is the baby's chronological age, which is the age from birth. Corrected age for a premature baby is a different, clinical calculation and is not shown here — ask your paediatrician for that, because it depends on the due date.

Worked example

Worked example: born 1 August 2026, on 21 October 2026 the baby is 2 months and 20 days old, which is 11 completed weeks and 81 days in total. The next month milestone is 1 November 2026, when the baby turns 3 months.

When to use it

The first two years of a child's life are the one period when age is routinely discussed in weeks and months rather than years, and that switch between units is the reason this page shows both side by side. In the earliest months, a difference of a week is a large share of the child's whole life, so parents, clinics and health visitors talk in weeks. As the months accumulate, months become the more useful unit, because weekly differences matter less and monthly milestones are easier to track. A single result that gives weeks, months and days at once removes the need to convert between those units by hand, which is where most errors creep in.

Forms and records are the most practical use. Many forms for young children ask for age in months, while others ask for age in weeks, and a parent who only knows one figure has to work out the other under pressure, often in a waiting room. Having both figures from one birth date means the form can be filled in either way without a second calculation. The same applies to messages to family: grandparents and relatives often ask how many weeks old a baby is in the early months and how many months old later, and the answer changes every week, so a fresh calculation beats memory.

Tracking the passage of time is the other use, and it is less about forms than about keeping a record. Monthly milestones, such as the day a baby turns three months, six months or one year, are easy to miss if they are worked out roughly, because months are not all the same length. A baby born on 1 August turns three months on 1 November, not on a date found by adding twelve weeks, and the difference between those two approaches grows over the year. The next-milestone figure in the result exists for exactly this purpose: it gives the calendar date of the next monthly milestone so it can be noted down or celebrated on the right day.

• Filling in forms that ask for age in weeks, months or days for a young child.
• Answering questions from clinics, health visitors and family about a baby's current age.
• Finding the date of the next monthly milestone, such as three months, six months or one year.
• Keeping a simple record of a child's age at a particular date, for example in a baby book or a photo caption.
• Comparing the ages of siblings or twins at the same date, using the same units for both.
• Checking a rough age given in weeks against the exact age in months, or the other way round.

One limit of this page should be understood before it is used for a child born early. The result here is chronological age, counted from the actual birth date. For a baby born before the expected time, clinicians sometimes use a different figure, known as corrected age, which adjusts for how early the birth was. That figure is clinical, it depends on information this page does not ask for, and it is not shown here. If corrected age is what a clinic has asked for, ask a paediatrician for it rather than trying to derive it from the result on this page.

Step by step

Using this page takes one input and gives four figures. The steps below explain each figure so the result is read correctly rather than just glanced at.

1. Enter the baby's date of birth in the field at the top of the page. Only the birth date is needed; the reference date is today, and the calculation runs entirely in the browser.
2. Read the headline result first: completed months and remaining days. This is the figure most forms want, and it reads like ordinary age, for example two months and twenty days.
3. Read the completed weeks next. Weeks are total days divided by seven and rounded down, so a baby who is eighty-one days old has completed eleven weeks, with four days of the twelfth week already passed.
4. Read the total days. This is the simplest figure of all: the number of calendar days from the birth date to today, counting leap days, starting from zero on the birth date itself.
5. Note the next month milestone. This is the date on which the baby completes one more month, found by adding one month to the birth date's day-of-month, and it is the date to use for monthly celebrations or records.

The relationship between the figures is worth understanding, because it explains why they do not convert neatly into each other. Completed months are calendar months, counted from the same day-of-month as the birth date, while completed weeks are simply days divided by seven. A baby born on 1 August is two months old on 1 October, but the number of weeks at that point depends on how many days August and September contained, which is sixty-one days, or eight weeks and five days. There is no fixed number of weeks in a month, and any chart that claims otherwise is an approximation.

Month-end birth dates need one extra note. A baby born on 31 January has a monthly milestone on 28 February in a non-leap year, because February has no thirty-first day, and the milestone day is clamped to the end of the month rather than spilling into March. The same clamping applies to any birth date whose day-of-month does not exist in the milestone month. The result handles this automatically, but it is worth knowing when checking the result by hand, because a hand calculation that adds thirty days per month will land on a different date.

Finally, the reference date matters. This page always calculates to today, so a result read on Monday will differ from a result read on Friday of the same week if a weekly or monthly milestone falls in between. For a record of age on a specific past date, note the result on that date or record the date alongside the age, because re-running the calculation later will give the age on the later date, not the original one.

Common mistakes

The most common mistake is treating four weeks as one month. Four weeks are twenty-eight days, but a calendar month is twenty-eight to thirty-one days, so four weeks is always shorter than a month except in February of a non-leap year. A baby described as twelve weeks old is not three months old; twelve weeks are eighty-four days, while three calendar months from most birth dates are eighty-nine to ninety-two days. The gap is small in the first months and grows with time, which is why weekly and monthly ages drift apart if they are converted by multiplying or dividing by four.

The second mistake is counting the current, incomplete month as if it were complete. A baby born on 15 March is not seven months old on 15 October in the sense of having completed seven months and then some; the completed-months figure on 21 October is seven months and six days, because the seventh month was completed on 15 October and six days of the eighth month have passed since. Saying seven months without the days is fine as a rough figure, but writing it on a form that asks for exact age understates the precision the form is asking for.

• Converting weeks to months by dividing by four, which understates the monthly age by several days per month.
• Converting months to weeks by multiplying by four, which gives a week count several weeks too low by the end of the first year.
• Counting the month a baby is currently in as a completed month, adding one month too many.
• Adding thirty days per month to find a milestone, which lands on the wrong date whenever a thirty-one-day month or February is involved.
• Using the same weekday for monthly milestones. Monthly milestones fall on the same day-of-month, not the same weekday, so they move through the week over the year.
• Comparing a weekly age with a monthly age from different dates, for example a week count from last week with a month count from today.
• Forgetting that the result is calculated to today, and quoting a result from an earlier date as if it were current.

A further mistake is assuming that a baby's age in weeks stops being useful at a fixed point. There is no universal switch date: some clinics ask for weeks for longer, some switch to months earlier, and family questions follow no rule at all. The practical approach is to give the unit that was asked for, and to keep both figures available, which is what this page does. Trying to settle on a single unit for every purpose creates more confusion than it removes, because the person asking usually has a reason for the unit they chose.

One more error is worth naming because it affects records: writing an age without its date. An entry that says a baby was two months old is only useful if the date of the entry is also recorded, because two months old on one date is a different child, in calendar terms, from two months old on another. Baby books, photo captions and clinic notes all become more useful when the date sits next to the age, and the next-milestone date in the result is designed to be written down alongside the current age for exactly that reason.

More worked examples and notes

The examples below are all calculated to 21 October 2026, the reference date used in the worked example on the main part of this page. Each one can be checked by entering the same birth date into the tool on that date, and the arithmetic behind each figure is shown so it can be followed by hand.

• Born 1 August 2026: on 21 October 2026 the baby is 2 months and 20 days old, which is 81 days in total and 11 completed weeks plus 4 days. The next month milestone is 1 November 2026, when the baby turns 3 months.
• Born 1 January 2026: on 21 October 2026 the baby is 9 months and 20 days old, which is 293 days in total and 41 completed weeks plus 6 days. The baby is close to ten months, but the completed-months figure is still nine until 1 November 2026.
• Born 15 March 2026: on 21 October 2026 the baby is 7 months and 6 days old, which is 220 days in total and 31 completed weeks plus 3 days. This example shows the week count and the month count side by side: thirty-one weeks sounds much larger than seven months, but they describe the same age.
• Born 10 June 2025: on 21 October 2026 the child is 1 year, 4 months and 11 days old, which is 16 completed months, 498 days in total and 71 completed weeks plus 1 day. Past the first birthday, months remain the more useful unit, but weeks are still shown for anyone who asks for them.
• Milestone dates for a baby born 1 August 2026: the 100th day after birth is Monday 9 November 2026, the six-month milestone is Monday 1 February 2027, and the first birthday is Sunday 1 August 2027. Note that the 100-day date is found by counting days, while the monthly milestones are found by counting calendar months, and the two methods give different kinds of dates.
• Fifty-two weeks is not a year: a baby born 1 January 2026 reaches 52 completed weeks on Thursday 31 December 2026, which is 364 days after birth, one day short of the first birthday on 1 January 2027. This is the clearest proof that weeks and months cannot be converted by a fixed rule.
• Month-end clamping: a baby born 31 January 2026 has a one-month milestone on 28 February 2026, because February 2026 has no thirty-first day. The milestone is clamped to the last day of February rather than moving into March.

A useful habit follows from these examples: when recording a young child's age, write all three figures together, for example nine months and twenty days, forty-one weeks, two hundred and ninety-three days, with the date. Any one figure on its own can be misread, but the three together pin the age down exactly, and they let anyone who needs a different unit read it off without a fresh calculation. The figures also check each other: if the weeks and the months disagree wildly, one of them has probably been converted by the four-weeks-per-month shortcut rather than counted from the birth date.

Over the first two years the balance between the units shifts gradually. In the first weeks, days and weeks carry the most information, because a single week is a large fraction of the child's life. By six months, months carry the most information, and weeks become a secondary figure. By the second birthday, years and months take over, and weeks are rarely asked for. None of these shifts happens on a single day, and the page therefore keeps all units visible at every age, so the reader can use whichever unit fits the question being asked rather than whichever unit fits a rule about switching.

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Frequently asked questions

Written and checked against the calculator above. Last updated: 21 October 2026. Calculation runs entirely in your browser — no date is sent to a server. This tool gives calendar results only, not medical, legal or financial advice.