Age in Weeks and Months Calculator

Baby Age Calculator in Weeks and Months

For newborns and infants, age is often counted in weeks first, then months. This baby age calculator helps parents convert a date of birth into exact weeks, remaining days, calendar months, decimal months, and total days old.

For example, a baby may be 12 weeks and 3 days old, but not exactly 3 calendar months old because calendar months are longer than 4 weeks. This is why the calculator shows both week-based and month-based results.

These formats are useful for baby milestones, pediatric forms, vaccination planning, newborn appointment notes, and questions like "how many weeks old is my baby?" or "how many months old is my child today?".

How to Calculate Age in Weeks

To calculate age in weeks, count the total number of days from the date of birth to the target date, then divide by 7. Whole weeks are the completed 7-day blocks, and the remainder is the extra days. For example, 87 days old is 12 weeks and 3 days old because 84 days makes 12 complete weeks.

Need a full years-months-days breakdown? Try our Age Calculator. Need the number of days between two dates? Use the Date Calculator.

How to Calculate Age in Months

Age in months can be counted two ways. Calendar months count full real months from the birth date, then any remaining days. Decimal months divide total days by the average Gregorian month length of about 30.44 days. Calendar months are usually better for baby milestones because they follow real dates on the calendar.

Weeks to Months Baby Conversion Chart

Weeks and months do not line up perfectly. One average month is about 30.44 days, or about 4.345 weeks, so this chart gives approximate month labels for common baby ages.

Calendar Months vs Decimal Months

Calendar months answer questions like "is my baby 6 months old today?" by counting real month anniversaries. Decimal months are useful for charts and comparisons because they turn the total day count into one number. A baby can be 12 weeks old but still not be exactly 3 calendar months old, because calendar months are longer than 28 days.

Corrected Age for Premature Babies

Corrected age adjusts a premature baby’s chronological age by subtracting the number of weeks they were born early. For example, a baby who is 16 weeks old but was born 6 weeks early may have a corrected age of about 10 weeks. This can be useful for development tracking, but medical questions should be discussed with a healthcare professional.

Baby Age Milestones by Week and Month

Weeks are often used for newborn appointments and early development notes. Months become more common as babies get older, especially around 2 months, 4 months, 6 months, 9 months, 12 months, and toddler milestones. This calculator shows both so you can match the format used by milestone charts, childcare forms, pediatric paperwork, or family updates.

FAQs

How many weeks old is my baby?

Enter your baby’s date of birth and today’s date. The calculator counts the total days since birth, divides by 7, and shows whole weeks plus remaining days.

Is 12 weeks the same as 3 months?

Not exactly. Twelve weeks is 84 days, while three calendar months can be 89, 90, 91, or 92 days depending on the months involved.

How do I convert baby weeks to months?

For an approximate conversion, divide weeks by 4.345. For baby milestones, calendar months are usually more accurate because real months have different lengths.

Should I use weeks or months for baby age?

Weeks are often useful for newborns and young infants. Months are commonly used for older babies and toddlers. This calculator shows both.

What is corrected age for a premature baby?

Corrected age adjusts a premature baby’s chronological age by subtracting the number of weeks they were born early. This can be useful for development tracking, but medical questions should be discussed with a healthcare professional.

Which month should I use?

For baby milestones, many parents prefer calendar months. Decimal months are handy for charts or spreadsheets.

Can I choose a future date?

Yes—set any target date to see age at that time (e.g., first day of school).

Is my data private?

Yes. Everything runs in your browser; no dates are uploaded.