52 weeks ≠ 12 months
A calendar year has about 52.18 weeks, so “12 months” and “52 weeks” don’t line up perfectly—handy to remember for milestone charts.
Counting quirks
Age in Weeks & Months (Baby-Friendly)
Dates
Result: Enter dates and click Calculate.
How we count weeks & months
Weeks: Whole weeks + remaining days (e.g., 12 weeks, 3 days) and a decimal view (e.g., 12.43 weeks).
Months:
- Calendar months — whole months + days (e.g., 3 months, 5 days).
- Decimal months — total days ÷ average month length ≈ 30.44.
Leap years and real month lengths are respected. Feb 29 birthdays follow calendar arithmetic naturally.
FAQs
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.
5 Fun Facts about Weeks & Months of Age
1,000-week milestone
Hitting 1,000 weeks old lands around 19 years and 3 months—a fun, under-celebrated birthday.
Hidden milestone
Leap days add up
Each leap day adds ~0.033 months. Over a decade that’s roughly 10 extra days—enough to nudge “months old” totals.
Leap lag
Baby growth by the week
Many babies double birth weight near 4–5 months (≈20 weeks) and triple it by ~12 months—why weekly check-ins matter early on.
Milestone math
“40 weeks” isn’t exact
Full-term pregnancies range from 37 to 42 weeks; those extra weeks shift newborn ages by nearly a whole month on early charts.
Early variance