Age in Weeks Calculator
Work out how many weeks old you are. Enter a date of birth and a reference date to see total weeks lived, the day remainder, total days, and the day of the week you were born.
Age in weeks
1,878 weeks, 2 days
- Total days lived
- 13,148
- Exact age
- 35 years, 11 months, 30 days
- Day of the week born
- Friday
- Weeks until next birthday
- 0
A week is defined as seven calendar days (ISO 8601 §3.4.4 / NIST). Total weeks lived equals the floor of total days divided by seven, with the remainder shown as extra days.
How to use this calculator
Enter the date of birth (year, month, day) and the reference date you want the age in weeks calculated for. Leave the reference date as today for current age in weeks, or set it to a future date to find a milestone. The calculator returns total completed weeks lived plus the day remainder of the current week, alongside total days, the exact age in years/months/days, the day of the week you were born, and the weeks until the next birthday.
How the calculation works
A week is defined by ISO 8601 (§3.4.4) and NIST as exactly seven calendar days. Total weeks lived is therefore the floor of total days lived divided by seven, and the day remainder is the modulo. Total days lived is the exact count of calendar days between the two dates using the proleptic Gregorian calendar in UTC — so daylight-saving offsets, leap years, and the variable length of months all wash out correctly. The underlying day-count algorithm matches PHP DateInterval, date-fns, and Wolfram Alpha.
Worked example
Date of birth: 15 June 1990. As of: 14 June 2026. From 15 June 1990 to 14 June 2026 is exactly 13,148 calendar days (nine leap days inclusive). Dividing 13,148 by 7 gives 1,878 complete weeks with a remainder of 2 days, so the age in weeks is 1,878 weeks and 2 days. The exact age is 35 years, 11 months, 30 days; the next birthday is one day away (so 0 weeks until); and 15 June 1990 was a Friday.
Frequently asked questions
How does this calculator define a week?
A week is seven calendar days, the definition given by ISO 8601 §3.4.4 and the US National Institute of Standards and Technology. The first seven days after birth are week zero — you only complete week one on the seventh day after birth. So a newborn aged six days is zero weeks, and a baby aged eight days is one week and one day.
Why use age in weeks instead of months?
For infants under one year, weeks are the conventional unit in paediatric medicine and child development. UK guidance from the NHS, the American Academy of Pediatrics, and the WHO all describe milestones (feeding patterns, sleep, motor development) in weeks. Pregnancy is similarly measured in weeks plus days from the last menstrual period. For adults, weeks are mostly a curiosity — but a useful one for milestone counting (1,000 weeks lived ≈ 19 years, 6 months).
How are leap years and leap-day birthdays handled?
Total days is a true calendar-day count, so every leap day between the two dates is included automatically. For someone born on 29 February, the birthday anniversary clamps to 28 February in non-leap years (matching date-fns, dayjs, PHP DateInterval, and the convention recognised by the UK Births and Deaths Registration Act 1953 for Scotland). The leap-day clamping only affects the "weeks until next birthday" output — total weeks lived is unchanged because it depends only on the absolute day count.
Why does the total week count differ from years × 52?
Because a Gregorian year is 365.2425 days on average, which is 52.1775 weeks — not 52. So over a 35-year span, you actually live about 1,827 weeks rather than 35 × 52 = 1,820. The calculator counts real calendar days then converts, so the result is accurate to the day. If you divide total weeks by 52.1775 you should get a number very close to your age in years.
Can I use a future reference date for milestone planning?
Yes. Set the reference date to a future day and the calculator will report the age in weeks you will reach on that date. This is useful for planning round-number celebrations (1,000 weeks, 2,000 weeks) or for working out how many weeks until a specific date if you also know your date of birth. The reference date must be on or after the date of birth — if you swap them, the calculator returns an error rather than a negative count.
Does this calculator handle BCE dates or year zero?
The calculator accepts years from 1 to 9999 in the Common Era. Year zero does not exist in the standard calendar (the year before 1 CE is 1 BCE), and BCE dates are not supported. For historical or astronomical calculations involving very early dates, use a specialist Julian Day Number tool instead and then divide the result by seven yourself.