Everyday & Conversions
Baby Age Calculator
Convert a baby's date of birth into age in weeks, months and days — the way paediatric growth charts and milestone guides express it.
Your inputs
Change any value to update the result instantly.
Age
17 weeks and 3 days old
Completed months
4 months
Total days
122 days
Calendar age
0 y 4 m 0 d
Stage label
Infant
Ages here are calendar ages. Corrected age for a premature baby is a clinical judgement — ask your paediatrician.
What this calculator does
Expresses an infant's age the way growth charts and milestone guides do.
What your result means
Age in completed weeks and days, with completed calendar months alongside.
Formula and working
Age in weeks = total days ÷ 7; age in months = completed calendar months
- 1
Collect the inputs
Baby's date of birth = 2025-09-01; Age on this date = 2026-01-01
- 2
Apply the formula
Age in weeks = total days ÷ 7; age in months = completed calendar months
- 3
Result
Age = 17 weeks and 3 days old
Frequently asked questions
Symbols and assumptions
- D₁
- Start or birth date on the Gregorian calendar
- D₂
- Reference or end date
- Δd
- Whole days between the two dates
- • Proleptic Gregorian calendar with real month lengths (28, 29, 30 or 31 days).
- • Leap years follow the rule: divisible by 400, or by 4 but not by 100.
- • Dates are handled in UTC so a device time zone cannot shift a calendar day.
- • Age is counted from the date of birth, not from a due date.
Sources and review
Reviewed 2026-09-22 · Next review 2027-09-22
- • Dates before 15 October 1582 used the Julian calendar in most of Europe and are outside the intended range.
- • Clock-time results ignore daylight-saving transitions unless the tool asks you to set them.
- • Corrected age for a premature infant is a clinical judgement and is not calculated here. This is not medical advice.
Important limitation
This is an educational estimate based only on the values and formula shown. Verify current rates, rules, units, and professional standards before relying on the result. It is not personalized financial or investment advice.
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