Baby Age Calculator
Enter your baby's birth date to get their age three ways at once: weeks and days (how pediatricians track the first months), calendar months and days, and total days old — plus a note on the developmental stage that age falls in.
Weeks are exact 7-day counts; months are calendar months (birth date to same date next month). That's why '8 weeks' and '2 months' aren't quite the same age.
Weeks, months, and why parents track both
Pediatric care runs on weeks for the first couple of months (the 1-week and 8-week visits, vaccine schedules) and then on calendar months (4-, 6-, 9-, 12-month checkups). Weeks are unambiguous — exactly 7 days each — while a 'month' stretches from 28 to 31 days depending on where it starts. A baby who is 8 weeks old (56 days) hasn't necessarily reached 2 calendar months.
This tool reports all three counts from the same day tally, so the numbers always agree with each other. For babies born prematurely, development is usually judged against adjusted age (counted from the due date, not the birth date) until about age 2 — use the adjusted age tool for that.
How it’s calculated
Days old = round((today − birth date) / 86,400,000 ms), both dates at noon. Weeks = floor(days ÷ 7) with remainder days. Months use calendar anniversaries with borrow arithmetic: if today's day-of-month is smaller than the birth day, one month is borrowed as the length of the preceding month. Stage notes follow the age bands pediatric milestone checklists use.
An educational tracker, not a growth or development assessment — milestone timing varies widely, so bring schedule and development questions to your pediatrician.
Well-child visit ages in weeks and days
| Checkup age | In days (avg) | In weeks |
|---|---|---|
| 1 month | 30 | 4 weeks, 2 days |
| 2 months | 61 | 8 weeks, 5 days |
| 4 months | 122 | 17 weeks, 3 days |
| 6 months | 183 | 26 weeks, 1 day |
| 9 months | 274 | 39 weeks, 1 day |
| 12 months | 365 | 52 weeks, 1 day |
Computed with 1 month = 30.44 days (Gregorian average); visit ages per the AAP periodicity schedule.
Common mistakes
- Equating 4 weeks with 1 month — a month averages 30.44 days, so month counts fall behind week counts by roughly a day per month.
- Counting the birth date as day 1; age in days is midnights since birth, so the birthday itself is day 0.
- Using calendar age instead of adjusted age for a preemie's milestones — clinicians correct for prematurity until about age 2.
- Month math at month-ends: born Jan 31, a baby turns '1 month' on Feb 28/29 by the borrowing rule, not on a nonexistent Feb 31.
Frequently asked questions
How is a baby's age calculated?
Everything starts from days old = (today − birth date) in whole days. Weeks = days ÷ 7 with a remainder; months count calendar anniversaries of the birth day, borrowing the previous month's length when the day-of-month hasn't arrived yet.
When do you switch from weeks to months?
Convention: weeks until about 8-12 weeks old, months until the second birthday, then years. That mirrors how checkup schedules and milestone checklists are written.
Is 8 weeks the same as 2 months?
Not exactly. Eight weeks is 56 days; two calendar months from a birth date averages about 61 days. The gap widens with age, which is why 52 weeks is a bit more than 11 calendar months, not 12.
My baby was premature — which age do I use?
For growth and milestones, use adjusted age: calendar age minus the weeks born early, typically until age 2. Your pediatrician will tell you when to stop correcting; ask them about anything developmental.