What this calculator does
A baby age calculator answers a question that adult ages do not usually need to bother with: is this baby better described in weeks or in months? For the first couple of months, "9 weeks old" is more informative than "2 months old", because feeding, sleep and developmental guidance for infants is almost always given in weeks at that stage. This calculator gives both, worked out from the date of birth, so the answer to how old is my baby is never stuck in the wrong unit.
Enter a date of birth and, if needed, a different reference date, and it returns the age as a plain sentence (in weeks while the baby is under about a month old, then in months and days), alongside the exact totals in weeks, months, and days lived so far.
The formula
Total days lived is the plain difference between the date of birth and the reference date. Age in weeks is that day total split into whole weeks and a remainder of days. Age in months is counted forward calendar month by calendar month from the date of birth, the same way a birthday is counted, so "3 months old" always means three full calendar months have passed, not simply 90 days.
| Term | Meaning |
|---|---|
| Age in weeks | Whole weeks since birth, with the leftover days shown separately, the standard way a newborn or young infant’s age is described. |
| Age in months | Whole calendar months since birth, counted the same way a birthday is counted, with any leftover days shown separately. |
| Decimal months | Total days lived divided by the average month length (30.4375 days), the format used on some growth-chart tools that expect a single decimal number rather than a months-and-days split. |
The inputs explained
| Field | What to enter |
|---|---|
| Date of birth | The baby’s date of birth. |
| As of this date | The date to calculate the age as of, normally left on today. |
When to use it
Filling in a form that asks for age in months
Many baby forms, growth charts and vaccination schedules ask for age in completed months rather than a birthdate. The months-and-days figure here gives that answer directly, and the decimal-months figure covers tools that want a single number instead.
Working out how old my baby is in weeks
Feeding schedules, sleep guidance and some medical advice for the first months are commonly given in weeks rather than months, since a lot changes between week 4 and week 12. The weeks-and-days figure answers that directly.
Checking age as of a past or future date
Changing the "as of" date works out how old the baby was at an earlier appointment, or how old they will be by an upcoming date, using the same breakdown.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
How the age description changes as a baby grows from newborn to one year old
A baby born on 1 January, checked at a series of ages through the first year, to show how the weeks-versus-months framing shifts as the baby grows.
| Age checked on | Age | Age in weeks |
|---|---|---|
| 2026-01-15 | 2 weeks, 0 days old | 2 weeks, 0 days |
| 2026-02-01 | 1 month, 0 days old (4 weeks) | 4 weeks, 3 days |
| 2026-03-01 | 2 months, 0 days old (8 weeks) | 8 weeks, 3 days |
| 2026-04-15 | 3 months, 14 days old (14 weeks) | 14 weeks, 6 days |
| 2026-07-01 | 6 months, 0 days old (25 weeks) | 25 weeks, 6 days |
| 2027-01-01 | 12 months, 0 days old (52 weeks) | 52 weeks, 1 day |
Questions
How old is my baby in weeks?
Enter the date of birth above and the "Age in weeks" figure gives the exact answer: a whole number of weeks plus any leftover days, updated to today by default.
Why does the headline age sometimes show weeks and sometimes months?
For roughly the first month, the headline is given in weeks, since that is how very young infants are usually described. From about one month onward it switches to a months-and-days figure, with the equivalent week count shown alongside it.
What is the difference between this and the how old am I calculator?
The maths is related, but the framing is different: this page is built for infants, where weeks matter as much as months, and leads with that breakdown. The how old am I calculator is built for the years-months-days framing that suits older children and adults.
What are decimal months used for?
Some growth-chart tools and paediatric references plot age as a single decimal number of months rather than a months-and-days split. The decimal-months figure here converts total days lived into that same format.
For the years-months-days framing used for older children and adults, see the how old am I calculator. To count down to an upcoming date, such as a due date or a milestone appointment, use the days until calculator.