Baby Age Calculator
A baby born on July 1 is exactly 2 weeks, 1 day old on July 16. This baby age calculator turns a birth date and an as-of date (it defaults to today) into the exact age of an infant or toddler, shown as completed weeks plus extra days, as a calendar-accurate years, months, and days breakdown, and as a total day count. Parents and caregivers use the weeks-and-days format because that is how pediatricians talk about newborns at early checkups, while the months figure matches how most people describe a baby's age once the first year is underway. Enter the date of birth and, if you are not checking today's age, the date you want the age calculated as of. The tool then answers all of the common versions of the same question at once: how old is my baby, how many weeks is my baby, how many months old is my baby, and how many days old is my baby.
Quick answer
The primary result is completed weeks plus extra days, such as 14 weeks, 3 days, which is the format most pediatric visit schedules use for infants.
What this tells you
- •The primary result is completed weeks plus extra days, such as 14 weeks, 3 days, which is the format most pediatric visit schedules use for infants.
- •Total days is the exact whole-day count between the birth date and the as-of date, including every day in between.
- •The months-old figure adds calendar-accurate years and months together, so an 18-month-old toddler shows 18 rather than a fraction of a year.
- •The years, months, and days breakdown uses real calendar arithmetic, borrowing from the month and year the same way a person counting age by hand would.
- •The as-of date defaults to today but can be set to any date on or after the birth date, useful for checking age on a past or upcoming appointment date.
- •A birth date entered after the as-of date returns no result, since a baby cannot have a negative age.
How to Use
- 1Enter the baby's date of birth in the first field.
- 2Leave the as-of date on today for the current age, or change it to a specific appointment, milestone, or document date.
- 3Click Calculate to see the age in weeks and days alongside the months-old and years-months-days breakdowns.
- 4Use the weeks-and-days figure for early pediatric visits, since many newborn and infant schedules are set by week of age.
- 5Use the months-old figure once the baby is closer to or past their first birthday, since ages are commonly described in months through about age two.
How It Works
Formula
Total days = as-of date minus birth date, then weeks = floor(days / 7), remainder days = days mod 7The calculator first finds the exact whole-day gap between the birth date and the as-of date using the two calendar dates directly, so leap days are counted automatically. That day count is divided by 7 using floor division to get completed weeks, and the leftover using a modulo operation gives the extra days, producing the weeks-and-days headline such as 14 weeks, 3 days. Separately, the calculator builds a calendar-accurate years, months, and days breakdown by subtracting year from year, month from month, and day from day, then borrowing a month when the day subtraction goes negative and borrowing a year when the month subtraction goes negative, using the actual number of days in the relevant month rather than a flat 30-day assumption. The months-old figure is calculated as years times 12 plus the leftover months from that same breakdown, giving an exact total month count rather than an approximation based on average month length.
Calculation note: values are processed in the order shown above, using the current input units.
Worked Examples
Two-week-old newborn checkup
A baby born July 1, 2026 is 15 total days old on July 16, 2026, which breaks down to 2 completed weeks plus 1 extra day. This is the exact format used to schedule a two-week newborn checkup, and it also shows as 0 years, 0 months, 15 days on the calendar breakdown.
Two-and-a-half-month-old at a pediatric visit
From May 1 to July 16, 2026 is 76 total days, which is 10 completed weeks and 6 extra days. The calendar breakdown reports the same span as 0 years, 2 months, 15 days, and the months-old figure rounds that down to 2 completed months since the 15 extra days have not reached a third full month.
Exactly 6 months old with a clean weeks figure
This span comes out to exactly 182 total days, which divides evenly into 26 completed weeks with 0 leftover days. The calendar breakdown still shows 0 years, 6 months, 1 day because a week-based count and a month-based count do not always land on the same day, even when the weeks figure is a clean round number.
Toddler past the first birthday
A child born January 1, 2025 is 561 total days old on July 16, 2026, which is 80 completed weeks plus 1 extra day. The calendar breakdown reports 1 year, 6 months, 15 days, and the months-old figure combines the year and month parts into 18, matching how most people would describe this child's age in conversation.
Weeks to Months Milestone Reference
Common infant age milestones shown in both weeks and months, based on calendar-accurate month lengths rather than a flat 30-day estimate.
| Weeks old | Approx. months old | Common milestone |
|---|---|---|
| 4 weeks | About 1 month | First well-baby checkup window |
| 6 weeks | About 1.5 months | Postpartum and newborn follow-up window |
| 8 weeks | About 2 months | First round of infant vaccinations in many schedules |
| 13 weeks | About 3 months | Common start of more predictable sleep patterns |
| 26 weeks | About 6 months | Typical age for starting solid foods |
| 52 weeks | 12 months | First birthday |
These milestones are general reference points only. The calculator itself always computes exact weeks, days, months, and years from the two dates entered, not from this rounded table.
Why weeks, months, and days all matter for baby age
Newborn and infant care commonly tracks age in weeks for the first couple of months because feeding schedules, vaccination timing, and early checkups are often set by week of age rather than by month. That is why the weeks-and-days figure, such as 14 weeks, 3 days, is the headline result on this page instead of a rounded month count.
Once a baby is a few months old, age is usually described in whole months instead, such as a 4-month-old or a 9-month-old, up through roughly the second birthday. This calculator's months-old figure is built from the same calendar-accurate years-and-months breakdown used for older children and adults, so it lines up with how a pediatrician or growth chart would describe the same age.
The total-days figure and the full years, months, and days breakdown are included for anyone who needs an exact record, such as filling in an exact age on a medical form, checking eligibility for an age-based program, or simply satisfying curiosity about how many days old a child is.
Common mistakes
- Reading the weeks figure as a month count. 14 weeks is not 14 months, and it is also not exactly 3.5 months on the calendar breakdown, since weeks and calendar months do not divide evenly into each other.
- Entering a birth date that falls after the as-of date. The calculator requires the birth date to be on or before the as-of date and will not return a result otherwise.
- Leaving the as-of date on today when checking age for a specific past or future appointment. Set the as-of date to the exact date you need, such as a vaccination visit or a program cutoff date.
- Assuming every month is 30 days for the months-old figure. The calculator uses the actual number of days in each calendar month, so the leftover days in the years-months-days breakdown will vary depending on which months fall in the range.
- Mixing up total days lived with total weeks times 7. Total days is the exact figure the weeks-and-days result is derived from, so multiplying the rounded weeks number back out by 7 will not always match the original day count once there is a remainder.
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