A baby age calculator exists because the first two years are measured in units nobody uses for anyone else. Nobody asks how many weeks old an adult is. For a baby the answer changes what applies: appointments are booked in weeks, growth charts are plotted in weeks and months, and a paediatric team asking "how old" during the first two years of a baby born early usually means something quite specific — the corrected age, not the age since birth.
Arb Digital keeps a free tools library for the everyday date and measurement arithmetic that is easy to get wrong by hand. This page gives the exact age in weeks, months and days on any date you choose, works out corrected age for a baby born before the due date, and shows the calendar dates the next round-number milestones fall on.
What This Baby Age Calculator Does
Enter a date of birth and, if you want the age on a date other than today, an "as at" date. The headline is the age in months and days as most people say it out loud. The grid gives the age in completed weeks, the age in plain days, the corrected age where one applies, and the gestational age at birth. Underneath, the bars show how far through the first and second years the baby is.
For a baby born early there are two ways to give the tool what it needs. If you know the original estimated due date, enter that and the calculator works out how many weeks early the birth was. If you were told a gestational age at birth instead — "32 weeks exactly", say — select that option and enter the weeks and days. Either route produces the same corrected age.
The boundary with the adult tool on this site is simple. The age calculator reports age in whole years, months and days, which is the right resolution for a person of any age but far too coarse for the first year of life. It has no concept of gestational age or corrected age. Use that page for everyone over two and this one below it. The Korean age calculator handles the different East Asian reckoning system, which is a separate question again.
How to Use It
- Enter the date of birth. The day of birth is day zero, so a baby born last Tuesday is seven days old this Tuesday, not eight.
- Leave the second date as today unless you are checking an age on a specific future or past date — an appointment, a vaccination schedule, a passport application.
- Say whether the baby was born early. If the birth was at or after the due date, choose no and the corrected age field simply reports that none applies.
- Give a due date or a gestational age. Either works. If you have both and they disagree slightly, the due date from the dating scan is normally the one clinicians use.
- Read both ages where both exist. Chronological age is how long the baby has been alive. Corrected age is how long since the original due date.
The Formula / How It's Calculated
Age in days is a plain date subtraction. Weeks are days divided by seven, reported as completed weeks plus leftover days, which is the "23 weeks and 4 days" form used on paperwork. Months are counted by calendar rather than by dividing days, because months vary in length: from 10 March to 22 August is five whole calendar months plus twelve days, not 165 ÷ 30.44.
Corrected age subtracts the prematurity from the chronological age. Weeks early is 40 weeks − gestational age at birth, or equivalently due date − date of birth in days. Corrected age is then chronological age − weeks early. Put another way, the corrected age is the age counted from the original due date instead of from the birthday. The American Academy of Pediatrics sets out the same method on its corrected age for preemies page on HealthyChildren.org: start with the age in weeks since birth, then subtract the number of weeks the baby was preterm.
Worked example, using the values the page loads with. A baby born on 10 March 2026 with an original due date of 5 May 2026 is, on 22 August 2026, 165 days old — 23 weeks and 4 days, or 5 months and 12 days. The gap between birth date and due date is 56 days, so the baby was 8 weeks early and was born at a gestational age of 32 weeks. Corrected age is 165 − 56 = 109 days, which is 15 weeks and 4 days, or 3 months and 17 days. Both numbers are correct; they answer different questions.
Corrected Age: What It Is and Who Uses It
Corrected age, also written as adjusted age, is the age a baby would be if they had been born on their due date. It exists because a baby born two months early has spent two fewer months developing than a baby born at term on the same calendar day, and comparing the two against the same expectations would compare things that are not alike.
Paediatricians and neonatal teams use corrected age rather than chronological age when they discuss development in babies born preterm, and typically continue doing so through roughly the first two years. That is the single most useful thing to know about the number, and it is the reason this page reports it prominently: if a health visitor or a growth chart asks for an age and the baby was born early, corrected age is usually the one being asked for. Ask the clinician which they want if it is not stated — the two can differ by months.
Chronological age does not stop mattering. It is the legal age, the age on documents, the age used for anything dated from the birth itself, and the one everyone will use at the birthday party. Both are real; they are simply counted from different starting points.
Why "Weeks Old" and "Months Old" Disagree
Parents constantly meet two answers that will not reconcile: an app says 23 weeks and a form says 5 months. Neither is wrong. There are about 4.35 weeks in an average month, not 4, so twelve months is roughly 52 weeks while twelve lots of four weeks is only 48. By the first birthday the two counting methods have drifted a full month apart.
The practical rule is to use whichever unit the thing in front of you uses, and never to convert between them by multiplying by four. Health services generally use weeks for the early schedule and months later on; growth charts are usually plotted against completed weeks or months from birth. This tool reports weeks and months independently, each counted properly from the birth date, so you can read off whichever one a form is asking for. The date difference calculator handles the general case of any two dates, and the days until date calculator counts down to a fixed date.
Gestational Age, Term and Preterm
Gestational age at birth is how far along the pregnancy was when the baby arrived, counted in weeks and days from the first day of the last menstrual period, which is why a "40 week" pregnancy is dated from about two weeks before conception. The World Health Organization preterm birth fact sheet defines preterm as a live birth before 37 completed weeks of pregnancy, and subdivides it into extremely preterm at under 28 weeks, very preterm at 28 to under 32 weeks, and moderate to late preterm at 32 to under 37 weeks.
Those bands are why the corrected-age adjustment can be so large. A baby born at 34 weeks is about six weeks early, an adjustment that fades in significance over the first year. A baby born at 26 weeks is fourteen weeks early — more than three months — and for the whole of the first year the corrected age will be dramatically smaller than the age on the birth certificate. If your baby was born after the due date, there is no correction to make and the calculator says so. The due date calculator works forwards from a last-period date, and the conception date calculator works backwards from a due date.
Arb Digital's free tools library covers dates, scheduling and budgeting maths for family and business planning, and our team is happy to talk through anything the tools cannot answer.
Browse Free Tools Talk to Arb DigitalCommon Mistakes to Avoid
- Counting the birthday as day one — the day of birth is day zero, the same convention used for every other date interval.
- Converting months to weeks by multiplying by four — a month averages 4.35 weeks, and by the first birthday that error is a whole month.
- Giving chronological age when corrected age was asked for — for a baby born early the two can differ by months. Ask which one is wanted.
- Using the due date the app guessed rather than the dating scan — where the two disagree, the scan date is normally the one clinicians work from.
- Applying a correction to a baby born at or after term — there is nothing to correct, and the tool will report that no adjustment applies.
Related Free Tools From Arb Digital
Use the age calculator for anyone past the first couple of years, the due date calculator during pregnancy, the baby percentile calculator to place a weight or height on simplified growth bands, the date difference calculator for any two dates, and the cost of a baby calculator for the budgeting side of the first year. Everything else is in the free online tools hub.
Frequently Asked Questions
Take the age in weeks since birth and subtract the number of weeks the baby was born early. Weeks early is 40 weeks minus the gestational age at birth, which is the same as the number of days between the birth date and the original due date.
Corrected age is the one clinical teams generally use when discussing development in babies born preterm, typically through about the first two years. Chronological age remains the legal age and the one used on documents. If a form does not say which it wants, ask.
No. The day of birth is day zero, so a baby born on a Tuesday is seven days old the following Tuesday. This is the same convention used for every other kind of date interval.
Because a month averages about 4.35 weeks rather than 4. Twelve months is roughly 52 weeks, while twelve four-week blocks are only 48. Use whichever unit the form in front of you asks for and do not convert by multiplying by four.
The World Health Organization defines preterm birth as a live birth before 37 completed weeks of pregnancy, subdivided into extremely preterm under 28 weeks, very preterm from 28 to under 32 weeks, and moderate to late preterm from 32 to under 37 weeks.
No. Corrected age only applies to babies born before their due date. If the birth was at or after term, select that option and the calculator reports that no adjustment applies, leaving the chronological age as the only figure.
Yes. Change the "work out the age as at" field to any date and every figure recalculates for that day, which is useful when an appointment letter asks for the age the baby will be on the day of the visit.
This tool performs date arithmetic only. It provides no medical or developmental advice, makes no claims about what a baby of any age should be doing, and is not a substitute for a paediatrician, midwife or health visitor.
