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Baby age calculator — years, months, weeks, days
In the first years of life, a child's age is counted differently from an adult's. Paediatricians, health visitors and child-development specialists always talk in months rather than years — because the difference between a 6-month-old and a 9-month-old is enormous. Our baby age calculator instantly converts any birth date into years, months, weeks and days, and also generates a written description such as "2 years 3 months 5 days". That way you always know exactly what to tell the doctor, how many full months your baby has completed, or when the next developmental check-up falls. Simply enter the child's date of birth and the calculator does the rest. You can also change the reference date if you want to calculate the age on a past or future day. Leap years are accounted for automatically, so the result is always accurate to a single day. The tool is completely free and works instantly in the browser without any registration.
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How we calculate the child's age
The calculator works out the exact number of days between the birth date and today (or any chosen reference date) using UTC midnight — so daylight-saving time changes never affect the result. Years = floor(days / 365.25), which properly accounts for leap years. Total months = floor(days / 30.44). Total weeks = floor(days / 7). The written description first extracts complete years, then from the remaining days calculates complete months and remaining days. All values use floor rounding, following the standard medical convention: a child who is 364 days old has not yet turned 1 year old.
Example: child born 10 March 2024, today 10 June 2026
Birth date: 2024-03-10. Calculation date: 2026-06-10. Difference = 822 days. Years = floor(822 / 365.25) = 2. Months = floor(822 / 30.44) = 27. Weeks = floor(822 / 7) = 117. Written description: "2 years 3 months 12 days". The calculator automatically breaks down the age into its components using standard floor rounding — a child who is 364 days old has not yet completed their first year.
Frequently asked questions about baby age
How do I calculate my baby's age in months?
To calculate a baby's age in months, divide the total number of days since birth by 30.44 (the average number of days in a month) and round down. For example, a baby born 180 days ago is floor(180 / 30.44) = 5 complete months old. Our calculator does this automatically — just enter the birth date and you instantly get the total months.
Why is age in months important for babies and toddlers?
In the first two years of life, doctors, health visitors and developmental specialists all refer to age in months because babies develop extremely rapidly. Growth charts, weight and length norms, vaccination schedules and developmental milestones (first smile, sitting, walking) are all expressed in months rather than years. Knowing the exact month age helps ensure your child is meeting the right milestones at the right time.
When should a baby start walking?
Most children take their first independent steps between 9 and 12 months and walk confidently by around 14–15 months. The normal range is wide — some children walk as early as 9 months, others not until 18 months. If a child is not walking by 18 months, it is worth consulting a paediatrician to rule out any developmental concerns.
How do I calculate the age of a premature baby?
Premature babies have two ages: the chronological age (from the actual birth date) and the corrected age (from the original due date). The corrected age is used to assess development for the first 2–3 years of life. For example, a baby born 2 months early who is 6 months old chronologically has a corrected age of 4 months. Our calculator gives the chronological age — for premature babies subtract the weeks of prematurity when comparing to developmental milestones.
When do babies say their first words?
Most babies say their first recognisable words between 10 and 14 months. By 12 months most children say 1–3 words with meaning. By 18 months they should know around 10–20 words. If a child has no words by 16 months, it is worth discussing this with a speech therapist or paediatric neurologist. Bilingual children may take a little longer but usually catch up quickly.
How do I calculate my baby's age in weeks?
Age in weeks is calculated by dividing the total days since birth by 7 and rounding down. Weekly age is especially important for newborns and babies up to about 3 months — doctors often ask for the exact age in weeks when assessing weight gain and responses to stimuli. Our calculator shows total weeks automatically alongside days, months and years.
When should a baby be able to sit unsupported?
Most babies start sitting with support at around 4–5 months and sit independently between 6 and 8 months. Independent sitting requires sufficiently strong back and core muscles. If a baby cannot sit without support by 9 months, it is a good idea to mention this at the next paediatric check-up so the doctor can assess muscle tone and development.
Does the calculator account for leap years?
Yes. The calculator works out the exact number of days between the two dates (using UTC to avoid daylight-saving issues), then divides by 365.25 for years (which correctly averages in the leap-year day), 30.44 for months and 7 for weeks. This means the result is accurate regardless of whether a leap year falls within the period.
When should solid foods be introduced?
The World Health Organization (WHO) recommends exclusive breastfeeding for the first 6 months, then introducing complementary foods from 6 months onwards. Many paediatricians allow introducing foods between 4 and 6 months when the baby shows signs of readiness: good head control, interest in food, and loss of the tongue-thrust reflex. Always discuss the timing with your doctor or health visitor.
What does the written age description mean (e.g. "2 years 3 months 5 days")?
The written description breaks the total days into complete years, then from the remaining days extracts complete calendar months (using 30.44 days per month), and finally shows the leftover days. It uses standard floor rounding, which matches the medical convention that a child who is 364 days old has not yet reached their first birthday. The description helps you communicate the precise age to doctors, nurseries and family members.
The calculator shows the chronological (actual) age from the birth date. For premature babies, the corrected age (from the due date) should also be considered. Results are for informational purposes only — consult a paediatrician for any concerns about your child's development.