Baby Age Calculator (In Weeks, Months and Days)
Enter your baby's birth date and get their precise age in months, weeks and total days — as of today or any date you choose.
Quick samples
Enter a birth date to see the exact age.
Add your baby’s birth date (or try a quick sample), and their age in months, weeks and days appears here.
Months use real calendar months, so the count matches how parents and clinics describe age. Weeks and days are totals from birth.
The formula
Age = calendar difference between birth date and check date · total days = check date − birth date · total weeks = total days ÷ 7
- Calendar months — Whole months between the two dates, counted month to month rather than as fixed 30-day blocks.
- Total days — Every day from birth to the check date, added up.
- ÷ 7 — Converts total days into completed weeks of life.
- Check date — Usually today, but any date works to look back or forward.
Age in months, weeks and days at common checkpoints
| Age | About | Total weeks | Total days |
|---|---|---|---|
| Newborn | 0–2 months | 0–8 | 0–60 |
| 3 months | 12–13 weeks | 13 | 91 |
| 6 months | about half a year | 26 | 183 |
| 12 months | one year | 52 | 365 |
Week and day totals are approximate because months vary in length; the calculator gives exact figures for a real birth date.
This baby age calculator turns a birth date into an exact age: months and years the way you would say them out loud, plus total weeks and total days. You can check the age as of today, or set any date to find how old your baby was or will be then.
That precision is handy because so much of baby advice is written in weeks and months — feeding amounts, sleep schedules, vaccine timings, clothing and gear sizes. Converting a birth date into the right age bucket saves you from doing the arithmetic in your head at 3am.
The maths is simple but the output is framed the way parents actually use it. A six-month-old and a 26-week-old are the same baby, and this tool shows both so you can match whatever unit your guide or appointment uses.
How the age is calculated
Months are counted calendar-wise. From the day of birth, each time that day-of-month comes around counts as another month, so a baby born on the 20th turns one month old on the 20th of the next month, regardless of whether that month had 28, 30 or 31 days.
Weeks and days are totals. The calculator also adds up every single day since birth, then divides by seven to give total weeks. This is the raw count, useful when a chart or guideline is expressed in weeks rather than months.
Because calendar months and 30-day approximations differ slightly, age in months is the one clinics use. That is why the headline figure here is months and years, with the day and week totals shown underneath for reference.
A worked example
A baby born on 10 January is checked on today's date of 15 June. Counting month to month, 10 June is five months, so on 15 June the baby is five months and a few days old — the calculator reports five months with the extra days.
In total days, that same span is about 156 days, which is roughly twenty-two weeks. So you can read the baby as five months, or 22 weeks, or 156 days, and every one is the same age written three ways.
Set the check date forward to the next 10 January and the tool rolls it to twelve months, or one year, making it easy to preview a first birthday or a clinic appointment weeks ahead.
Adjusted age for a premature baby
For babies born early, doctors often use a corrected or adjusted age for the first couple of years: the age the baby would be if born on their due date. You get it by subtracting the weeks of prematurity from their actual age.
A baby born five weeks early and now six months old has an adjusted age of about five months. Feeding, sleep and developmental milestones are usually judged against the adjusted figure until roughly the second birthday.
To find it here, enter the due date instead of the birth date and read the age it returns, or ask your paediatrician which age — actual or adjusted — they want you to track for a given milestone.
Where you will use this number
Feeding guidance shifts on a month-by-month basis, from milk volumes to starting solids around six months and moving up textures after that. Knowing the exact month keeps you on the right page of any feeding chart.
Growth, vaccine and developmental checks are all scheduled by age in months, so an accurate count helps you ask the right questions at appointments and spot which milestone window you are in.
Gear and clothing sizes — nappies, car seats, sleep sacks — are keyed to age as well as weight and height. An exact age helps you buy the next size up just in time rather than too early.
Reading the result with common sense
Age is a helpful guide, not a rule. Babies develop on a spectrum, so a guideline that says four to six months means a range, and your baby sitting near either edge is normal rather than behind or ahead.
Do not treat the day total as though precision means importance. A baby is not meaningfully different at 156 days than at 154; the number simply helps you line up with charts and sizes that happen to use days or weeks.
When a decision genuinely depends on age — starting solids, an immunisation, a car-seat transition — confirm the timing with your health visitor or paediatrician, who can weigh age against your individual child's readiness.
Age in months for milestones and schedules
Most first-year guidance is written in months, so an exact month count is what lets you use it with confidence. Feeding amounts, sleep and wake windows, vaccine timings and developmental checkpoints are each tied to a month or a short range of months rather than to an exact birthday.
That is why the same baby can appear to have several ages at once — a month for the feeding chart, a week count for a sleep guide, and a day total for a growth record. None is more correct than the others; they are one age expressed in the unit a given source happens to use.
Reading your baby's precise month also helps you anticipate what is coming. Knowing they are four months and two weeks, rather than vaguely 'around four', lets you see that a sleep regression or the move towards solids is on the horizon, so you can prepare rather than be surprised by it.
Key terms
- Chronological age
- Time since actual birth, the usual way we describe how old a baby is.
- Adjusted (corrected) age
- Age recalculated from the due date for a premature baby, used for early milestones.
- Calendar month
- A month counted date-to-date, which is why months are not all exactly 30 days.
- Total weeks
- Every day since birth divided by seven, useful for week-based charts.
Pro tips
- For a premature baby, enter the due date to read an adjusted age for early milestones.
- Use the 'age on this date' field to preview a milestone or birthday before it happens.
- Match the unit your guide uses — months for feeding, weeks for some growth charts.
Common mistakes
- Treating age as a hard deadline for milestones rather than a range.
- Using chronological age for a very premature baby when adjusted age is wanted.
- Assuming a month means 30 days when comparing to week-based charts.
Baby age calculator — your questions
How do I calculate my baby's age in months?
Count from the birth date to today one calendar month at a time, from the same day-of-month. A baby born on the 12th turns another month old on the 12th of each following month. This calculator does that count and also shows the leftover days, plus the total in weeks and days, so you can match whichever unit your book or appointment uses without doing the arithmetic yourself.
Why does it give weeks and days as well as months?
Because different sources use different units. Feeding and milestone guides usually talk in months, while some growth charts, sleep resources and gear sizing talk in weeks or even exact days. Showing the same age in months, weeks and total days means you never have to convert between them, and you can copy the figure straight into whatever chart or size guide you are reading.
Should I use actual age or adjusted age for a preemie?
For babies born early, doctors commonly use adjusted age — the age calculated from the due date rather than the birth date — for at least the first year or two. A baby born five weeks early and six months old has about a five-month adjusted age. Feeding, sleep and developmental milestones are usually judged on that corrected figure, so enter the due date to read it, and follow your paediatrician's guidance on which age applies.
Can I find my baby's age on a future date?
Yes. Change the 'age on this date' field to any future day and the calculator shows how old your baby will be then. It is handy for checking whether a baby will be six months at an upcoming trip before you start solids planning, or how old they will be at a first birthday, a sibling's milestone or a booked clinic appointment weeks away.
Is age in months the same as age in weeks divided by four?
Not exactly, which is why the tool shows both. A calendar month averages about 4.3 weeks, not four, so 26 weeks is closer to six months than the simple divide-by-four rule suggests. Relying on weeks-divided-by-four can make a baby seem a few weeks off. The month count here follows real calendar dates, matching how clinics and parents describe age.
What is this useful for beyond curiosity?
It lines your child up with age-based guidance: the right milk volume or solid, a sleep and wake-window schedule, an immunisation or developmental check, and the next size up in nappies, clothing or a car seat. Getting the month or week exactly right means you follow the correct row of a chart rather than guessing, and can plan purchases and appointments at the right time.