Baby Age Calculator

This page gives general information about how a baby’s age is counted, not medical advice. Milestones are ranges, not deadlines, and every baby develops at their own pace. If you have a concern about your baby’s development, talk to your pediatrician.

In the first year of life, parents and pediatricians measure age in months and weeks rather than years, because early development moves so incredibly fast. Enter your baby’s birth date below for an exact age in months, weeks, and days.

This guide walks you through each calculation step by step, with worked examples you can follow along with. You will also learn how to adjust the math for a premature baby and which mistakes to avoid.

How to Use the Baby Age Calculator

Enter the birth date: the month, day and year. You can also open the native date picker with the “Show date picker” link beside the birth date, and beside the “Calculate as of” date. The as-of date is optional and defaults to today, so leave it blank to find the age right now.

Four quick chips sit below the form: Born 14 May 2026, Born 1 January 2026, Born 29 February 2024, and Born 5 November 2026. Each one calculates at once, and each one clears the as-of date.

Born 14 May 2026 gives 5 months and 24 days as of 7 November 2026, or 177 days, 25 weeks and 2 days. Born 1 January 2026 gives 10 months and 6 days, or 310 days, 44 weeks and 2 days.

Born 29 February 2024 gives 2 years, 8 months and 9 days, or 982 days, 140 weeks and 2 days. Born 5 November 2026 gives 2 days old. You can also add ?dob=YYYY-MM-DD to the page address, and the calculator fills in the birth date and calculates on load.

The result shows the age in completed months and days, with the total months, total weeks and total days underneath. The big line reads “7 months 28 days”, and the line underneath reads “Your baby was born on Tuesday, 10 March 2026.”

Below that, a line reads “That is 34 weeks and 4 days old.” Leave the birth date blank and the tool replies: “Please enter your baby’s date of birth.” An impossible date draws this answer: “That date is not valid.

Please check the date of birth.” When the birth date falls after the as-of date, the tool greets you with: “Your baby’s date of birth is after the calculation date. A baby cannot be born in the future.” Nothing you enter is saved or sent anywhere.

Why Babies’ Ages Are Measured in Months (Not Years)

There are three practical reasons the whole baby world runs on months: Pediatrician visits and vaccinations. Well-baby checkups follow a tight schedule, newborn, 1 month, 2 months, 4 months, 6 months, 9 months, 12 months, 15 months, 18 months, and 24 months, and vaccines are timed to specific months.

The first big round of shots, for example, is due at 2 months (8 weeks). Saying a baby is “0 years old” tells the doctor nothing.

Developmental milestones. Babies hit milestones on a monthly rhythm: first social smiles around 6 to 8 weeks, rolling over around 4 to 6 months, first steps around 12 months.

Milestone charts are organized by month because a single month can bring dramatic change. WHO growth charts. The World Health Organization’s Child Growth Standards plot weight, length, and head circumference against age in months for children under 2.

Doctors compare your baby against these curves, so knowing the exact month (and week) of age genuinely matters. In short: during the first two years, months, and weeks for newborns, are the language everyone speaks.

How to Calculate a Baby’s Age in Months

The month-counting method is simple: Start from the baby’s date of birth. Count full calendar months forward to the same-numbered day in the current month. Count the remaining days after that date. Worked example.

Suppose a baby was born on March 10, 2026, and today is November 7, 2026: March 10 to October 10 = 7 full months October 10 to November 7 = 28 days Answer: the baby is 7 months and 28 days old.

Babies born on the 29th, 30th, or 31st need a small adjustment in shorter months. We follow the same month-end rules described in our methodology.

How to Calculate a Baby’s Age in Weeks

To get the age in weeks, count the total days the baby has lived and divide by 7. The whole number is the week count; the remainder is the extra days. Worked example.

Using the same baby (born March 10, 2026; today November 7, 2026): Total days lived = 242 days 242 divided by 7 = 34 full weeks, with 4 days left over Answer: the baby is 34 weeks and 4 days old.

Weeks are the preferred unit in the first two months, your pediatrician will likely say “6-week-old” rather than “one-and-a-half months,” because early growth and vaccine timing are tracked weekly.

How to Calculate a Baby’s Age in Days

The simplest unit of all: a baby’s age in days is just the number of days between the birth date and today. Worked example. March 10, 2026 to November 7, 2026 = 242 days. That is it. The baby has been alive for 242 days.

Day counts are the foundation of the other two units (divide by 7 for weeks; use the month-counting method for months), and they are the most precise way to describe a newborn’s age in the first few weeks of life.

Corrected Age for Premature Babies

Babies born before 37 weeks of pregnancy are considered premature, and for the first couple of years their development is measured against their corrected age (also called adjusted age), the age they would be if they had been born on their due date.

Formula: Corrected age = Chronological age minus (40 minus gestational age at birth in weeks). In plain terms: subtract the number of weeks the baby arrived early. Worked example.

A baby born on March 10, 2026 at 34 weeks gestation arrived 6 weeks early (due date April 21, 2026):

Chronological age on November 7, 2026: 242 days (7 months, 28 days)

Subtract 6 weeks (42 days): 242 minus 42 = 200 days Corrected age: 6 months and 17 days (or 28 weeks and 4 days) Use corrected age for milestones, growth charts, and feeding guidance until about age 2 (24 months), unless your pediatrician advises otherwise.

After that, the gap becomes small relative to the normal range of development, and chronological age takes over.

The American Academy of Pediatrics recommends using corrected age for growth and developmental assessment through at least 24 months corrected age for infants born at or before 32 weeks gestation, and through at least 12 months for late-preterm infants born between 34 and 36 completed weeks.

For babies born before 28 weeks, it is sometimes used until age 3. The calculator on this page gives chronological age only, so work the corrected age out by hand: take the total weeks the tool shows and subtract the weeks the baby arrived early.

A baby born 8 weeks early who is now 6 months old has a corrected age of 4 months, and that is the age to use when comparing against developmental milestones and growth charts.

The full method, with the worked examples and the ages to use it until, is in our guide to corrected age for premature babies.

Baby Age Milestones by Month

Every baby develops at their own pace, but these are the widely used reference points pediatricians watch for. (For premature babies, judge these against corrected age.)

Milestones are ranges, not deadlines, but if your baby consistently misses several for their age, mention it at the next checkup. The CDC checklists describe what about 75% of babies can do by each age, so a baby who has not reached a milestone yet is not necessarily behind.

24-Month Milestone Calendar

The calculator gives the age, not the dates ahead. The table below is the 24-month calendar for a baby born on 15 June 2026, with the date each monthly milestone falls on, the days old, and what most babies do by then:

At 15 months most babies take a few steps on their own, try to say one or two words besides “mama” or “dada”, and use their fingers to feed themselves.

At 18 months most walk without holding on, try to say three or more words besides “mama” and “dada”, follow one-step directions without a gesture, and scribble. At 24 months most run, kick a ball, say at least two words together, point to things in a book when asked, and eat with a spoon.

Well-child visits follow the same rhythm: 3 to 5 days, 1 month, 2 months, 4 months, 6 months, 9 months, 12 months, 15 months, 18 months and 24 months.

Milestone Dates for One Baby

All four milestones for one worked birth date, with both the day count and the week count. Born June 15, 2026:

Worked Example: Born June 15, 2026

On November 7, 2026: Months: June to November = 5. Day 7 is less than day 15, so borrow: 4 months. Days: 7 plus 31 (October) minus 15 = 23 days.

Total days: June 15 to 30 = 15, July = 31, August = 31, September = 30, October = 31, November 1 to 7 = 7. Total: 145 days = 20 weeks, 5 days. Result: 4 months, 23 days (20 weeks, 5 days).

Common Mistakes Parents Make When Calculating Baby Age

Counting from the due date instead of the birth date. A baby’s age always starts on the actual day of birth. The due date only enters the picture when computing corrected age for a premature baby.

Treating a month as exactly 4 weeks. Calendar months average about 4.345 weeks, so 16 weeks is roughly 3.7 months, not 4. Convert weeks to months with the month-counting method, not by dividing by 4.

Forgetting month-end birthdays. A baby born on January 31 turns one month old on February 28 (or 29 in a leap year), not March 3. The anniversary falls on the last day of the shorter month.

Rounding up. A baby who is 5 months and 29 days old is 5 months old, completed months are what count, just like completed years for adults.

If you need a child’s exact age on a specific past or future date, for a school enrolment cutoff, a vaccination or a competition, work it out with the Age on a Specific Date calculator.

Baby Age Facts

Weeks-first convention

Doctors count age in weeks for the first few months, then months until about two years, because development changes so quickly. The conversion factor: 4.345 weeks per month.

Corrected age for preemies

Chronological age minus weeks born early. Standard for premature babies through age 2 for developmental assessments.

Day 0 vs day 1

Days-old counts the birth day as day 0. The Korean 100-day custom counts it as day 1, so the 100th day is birth date plus 99 days.

Month rollover rule

A baby born January 15 is 2 months old on March 15. If the next month is shorter than the birth date, the month rolls over on the month’s last day.

How This Calculator Compares

Four baby age calculators sit at the top of the results for the same query, and they count a baby’s age in different units. Here is how the four stack up, feature by feature.

The gaps in this tool, stated plainly: it shows no corrected-age input, no 24-month milestone calendar, no milestone dates, no growth-chart lookup and no vaccine schedule.

OnlyCalculators gives the 24-month calendar with day-of-week labels and the upcoming milestone dates, ShortCalc gives the hours, minutes and seconds plus the next birthday countdown, and AgeCalculatorForBaby has a corrected-age toggle.

What you only get from this guide is the 24-month milestone calendar, the corrected-age worked example with the prematurity categories, and the milestone dates for one baby, all worked by hand in the guide above.

Keep Exploring

Age on a Specific Date Calculator: work out a child’s exact age on a cutoff date, for school enrolment or a competition.

School Age Calculator: check whether a child is old enough to start school, with the cutoff dates.

Days Lived Calculator: count the days lived since any birth date, with the milestones along the way.

Life in Weeks: see a life as a grid of weeks, with the weeks lived and the weeks left.

Sources

World Health Organization: Child Growth Standards (the under-2 growth tracking standard, from 6 countries on 4 continents)

American Academy of Pediatrics (HealthyChildren.org): Corrected Age for Preemies (the corrected-age method, used for the first 2 years)

CDC: Developmental Milestones (the checklists, revised in 2022; use corrected age if born more than 3 weeks prematurely)

Frequently Asked Questions