Baby Age Calculator

Q: How many weeks old is a 3-month-old baby?

A 3-month-old baby is about 13 weeks old. A month is not exactly four weeks, so the numbers drift apart as your baby grows. 1 month ≈ 4.3 weeks 3 months ≈ 13 weeks 6 months ≈ 26 weeks 12 months ≈ 52 weeks This is why a 12-week-old baby is not quite 3 months old yet.

Q: When does a baby stop being called a newborn?

A baby is a newborn (neonate) for the first 28 days of life. After that first month, doctors call the baby an infant until about 12 months. Many baby products use "newborn" to mean up to 3 months, but the medical meaning is the first 4 weeks.

Q: How many weeks are in a month when counting a baby's age?

About 4.3 weeks , not 4. Months have 28, 29, 30, or 31 days, while a week is always 7 days. If you count 4 weeks per month, you lose a few days every month. By a baby's first birthday, that mistake adds up to almost a whole month.

Q: At what age do you stop using corrected age?

Most doctors use corrected age until about 2 years old . By then, most preemies have caught up with full-term children. For babies born just a few weeks early, some doctors stop correcting at 12 to 18 months. Your pediatrician will tell you when to stop.

Q: Are vaccines given by corrected age or actual age?

Vaccines follow the actual birth date , not the corrected age. A baby born 8 weeks early still gets their 2-month shots 2 months after they were born. Corrected age is used for growth charts and milestones like rolling, sitting, and talking, not for the shot schedule.

Q: How many weeks early counts as premature?

A baby born before 37 weeks of pregnancy is premature. Doctors group it like this: Late preterm: 34 to 36 weeks Moderate preterm: 32 to 34 weeks Very preterm: 28 to 32 weeks Extremely preterm: under 28 weeks Babies born at 37 to 42 weeks are full term.

Q: What is the difference between gestational age and chronological age?

Gestational age is how many weeks a baby had grown in the womb before birth, like "born at 34 weeks." Chronological age starts on the day of birth and counts forward in days, weeks, and months. Corrected age links the two: it is the chronological age minus the weeks the baby was born early.

Q: If my baby was born on the 31st, when is their monthly birthday in a short month?

Use the last day of that month . A baby born January 31 turns 1 month old on February 28 (or February 29 in a leap year). The same rule works for babies born on the 29th or 30th. Months are counted by the calendar, not by 30-day blocks.

Q: How many days old is a 1-year-old baby?

365 days , or 366 if a February 29 falls in that year. That is about 52 weeks and 1 day. This is why weeks and months never line up perfectly. Counting the real days gives the exact answer.

Introduction

The Baby Age Calculator tells you exactly how old your baby is in years, months, weeks, and days. Type in your baby's birth date and press calculate. You can also pick a custom date to see how old your baby was (or will be) on that day.

Parents and doctors often count a baby's age in weeks or months, not years. A 10-week-old baby is not the same as a 3-month-old. The calculator counts using real month lengths and leap years, so the numbers come out right every time.

If your baby was born early, check the premature box and enter how many weeks early they arrived. The calculator will show the corrected age, also called adjusted age. Doctors use this number to track growth and milestones for preemies, because a baby born 8 weeks early is still catching up to full-term babies of the same birth date.

You also get a stage name (like newborn, infant, or toddler), a countdown to the next birthday, and a step-by-step breakdown that shows how each number was found. Everything updates right away, so you can check your baby's age for doctor visits, shot schedules, or feeding and sleep guides.

How to use our Baby Age Calculator

Enter your baby's birth date, and the calculator shows their exact age in years, months, weeks, and days, plus their stage, days to the next birthday, and a corrected age if they were born early.

Day (birth date): Type the day your baby was born, from 1 to 31. After two digits, the cursor jumps to the next box.

Month (birth date): Type the month number your baby was born, from 1 to 12. Use 01 for January and 12 for December.

Year (birth date): Type the four-digit birth year, like 2025. It cannot be in the future.

Use a custom date: Leave this off to get your baby's age today. Turn it on if you want to know how old your baby was, or will be, on another day.

Custom Day, Month, and Year: If you turned on the custom date, type that date here. It must be on or after the birth date.

My baby was premature: Tick this box if your baby was born before their due date. This adds the corrected age result.

Weeks Early: Type how many weeks early your baby arrived, from 1 to 16. The tool takes these weeks off the age to show the corrected (adjusted) age.

Calculate Age: Click this button to see the results, charts, and step-by-step math. Click Reset to clear every box and start over.

Baby Age: What It Means and Why It Matters

A baby's age is counted from the day they were born. But babies change fast, so doctors and parents don't just say "1 year old." In the first months, age is counted in days and weeks. After that, it is counted in months. Only after a child turns two do people usually switch to years.

Days, Weeks, and Months

These three ways of counting are used at different times:

Days: Used in the first month. A newborn is a baby under 28 days old.
Weeks: Used in the early months. Feeding, sleep, and growth checks often use weeks.
Months: Used up to about age two. Milestone charts, vaccine schedules, and growth charts all use months.

Months are tricky because they are not all the same length. Some have 28 days, some 30, and some 31. February gains an extra day in a leap year. That is why counting months by hand can give the wrong answer.

Corrected Age for Premature Babies

A baby born early has two ages. The chronological age counts from the birth day. The corrected age (also called adjusted age) counts from the original due date. To find it, you take the chronological age and subtract the number of weeks the baby came early.

Example: a baby is 6 months old and was born 8 weeks early. Their corrected age is about 4 months. This baby may sit, crawl, or talk closer to a 4-month-old than a 6-month-old, and that is normal. Doctors usually use corrected age for growth and milestone checks until about age two. Vaccines, though, are given based on the real birth date.

Why Baby Age Is Used So Often

Checkups: Well-baby visits are set at ages like 2, 4, 6, 9, and 12 months.
Vaccines: Shots are scheduled by age in weeks and months.
Milestones: Rolling over, sitting, crawling, and first words are all tied to age ranges.
Feeding: Solid foods usually start around 6 months.
Growth charts: Weight, length, and head size are compared to other babies the same age.
Sleep and gear: Car seats, cribs, and sleep habits change with age.

Common Baby and Toddler Stages

Newborn: birth to 28 days
Infant: about 1 to 12 months
Toddler: about 1 to 3 years
Preschooler: about 3 to 5 years

Every baby grows at their own speed. Age ranges are guides, not rules. If you are worried about your child's growth or milestones, talk to your pediatrician.

Formulas used

Total days since birth

D_{\text{total}} = \frac{t_{\text{ref}} - t_{\text{birth}}}{86400000}

Complete calendar months since birth

M_{\text{total}} = (y_{\text{ref}} - y_{\text{birth}}) \times 12 + (m_{\text{ref}} - m_{\text{birth}}) - \delta, \quad \delta = \begin{cases} 1 & \text{if birth} + M \text{ months} > \text{ref} \\ 0 & \text{otherwise} \end{cases}

Years and months breakdown

\text{years} = \left\lfloor \frac{M_{\text{total}}}{12} \right\rfloor, \qquad \text{months} = M_{\text{total}} \bmod 12

Remaining days after the last complete month

d_{\text{rem}} = \frac{t_{\text{ref}} - \left(t_{\text{birth}} + M_{\text{total}}\ \text{months}\right)}{86400000}

Total weeks and leftover days

W = \left\lfloor \frac{D_{\text{total}}}{7} \right\rfloor, \qquad d_{7} = D_{\text{total}} \bmod 7

Corrected (adjusted) age for prematurity

D_{\text{corrected}} = D_{\text{total}} - (w_{\text{early}} \times 7)

Days until next birthday

D_{\text{next}} = \frac{t_{\text{next birthday}} - t_{\text{ref}}}{86400000}

Progress through the first year and toward next birthday

P_{\text{year}} = \frac{D_{\text{total}}}{D_{\text{first year}}} \times 100, \qquad P_{\text{bday}} = \frac{t_{\text{ref}} - t_{\text{last bday}}}{t_{\text{next bday}} - t_{\text{last bday}}} \times 100

Frequently asked questions

How many weeks old is a 3-month-old baby?

A 3-month-old baby is about 13 weeks old. A month is not exactly four weeks, so the numbers drift apart as your baby grows.

1 month ≈ 4.3 weeks
3 months ≈ 13 weeks
6 months ≈ 26 weeks
12 months ≈ 52 weeks

This is why a 12-week-old baby is not quite 3 months old yet.

When does a baby stop being called a newborn?

A baby is a newborn (neonate) for the first 28 days of life. After that first month, doctors call the baby an infant until about 12 months.

Many baby products use "newborn" to mean up to 3 months, but the medical meaning is the first 4 weeks.

How many weeks are in a month when counting a baby's age?

About 4.3 weeks, not 4. Months have 28, 29, 30, or 31 days, while a week is always 7 days.

If you count 4 weeks per month, you lose a few days every month. By a baby's first birthday, that mistake adds up to almost a whole month.

How do you work out a premature baby's corrected age?

Take the baby's real age and subtract the number of weeks they were born early.

Example: a baby is 6 months (26 weeks) old and was born 8 weeks early. 26 − 8 = 18 weeks, which is about 4 months corrected age.

You can also count how many weeks early by subtracting the birth gestation from 40 weeks. A baby born at 32 weeks was 8 weeks early.

At what age do you stop using corrected age?

Most doctors use corrected age until about 2 years old. By then, most preemies have caught up with full-term children.

For babies born just a few weeks early, some doctors stop correcting at 12 to 18 months. Your pediatrician will tell you when to stop.

Are vaccines given by corrected age or actual age?

Vaccines follow the actual birth date, not the corrected age. A baby born 8 weeks early still gets their 2-month shots 2 months after they were born.

Corrected age is used for growth charts and milestones like rolling, sitting, and talking, not for the shot schedule.

How many weeks early counts as premature?

A baby born before 37 weeks of pregnancy is premature. Doctors group it like this:

Late preterm: 34 to 36 weeks
Moderate preterm: 32 to 34 weeks
Very preterm: 28 to 32 weeks
Extremely preterm: under 28 weeks

Babies born at 37 to 42 weeks are full term.

What is the difference between gestational age and chronological age?

Gestational age is how many weeks a baby had grown in the womb before birth, like "born at 34 weeks."

Chronological age starts on the day of birth and counts forward in days, weeks, and months.

Corrected age links the two: it is the chronological age minus the weeks the baby was born early.

If my baby was born on the 31st, when is their monthly birthday in a short month?

Use the last day of that month. A baby born January 31 turns 1 month old on February 28 (or February 29 in a leap year).

The same rule works for babies born on the 29th or 30th. Months are counted by the calendar, not by 30-day blocks.

How many days old is a 1-year-old baby?

365 days, or 366 if a February 29 falls in that year. That is about 52 weeks and 1 day.

This is why weeks and months never line up perfectly. Counting the real days gives the exact answer.

When do you stop counting a child's age in months?

Most parents and doctors switch to years around age 2. Before that, months matter because babies change so fast.

You will still hear "18 months" or "30 months" at checkups, because growth charts and milestone lists use months until age 3.

Should solid foods start at corrected age or actual age for a preemie?

Use corrected age. Most babies are ready for solids around 6 months corrected, when they can sit with support and hold their head steady.

A baby born 10 weeks early may be closer to 8 months old by the calendar before they are ready. Always check with your pediatrician first.

When does a leap day baby have a birthday in normal years?

Babies born on February 29 usually celebrate on February 28 or March 1 in years without a leap day.

For age counting, most systems roll the date back to the last day of February, so the baby turns a year older on February 28.

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