Age in Weeks Calculator

A week is exactly seven days, making it one of the most consistent time units in common use. Unlike months and years, which vary in length, weeks never change. This consistency makes weeks ideal for planning, scheduling, and tracking progress over medium time spans. In pediatric medicine, the first year of life is often described in weeks rather than months because developmental changes happen rapidly. A 6-week-old infant has different capabilities than an 8-week-old, even though both would be described as 1 month old in casual conversation. Key early milestones include 2 weeks (umbilical cord detaches), 6 weeks (first social smile), 12 weeks (improved head control), and 16 weeks (beginning to roll). Pregnancy duration is universally measured in weeks. A full-term pregnancy is 40 weeks from the last menstrual period (LMP), divided into three trimesters: weeks 1-12, 13-26, and 27-40. Premature births are categorized by gestational week: extremely preterm (before 28 weeks), very preterm (28-32 weeks), and moderate to late preterm (32-37 weeks). The seven-day week has ancient origins, likely connected to the approximate quarter-cycle of the Moon (29.5 days / 4 = 7.4 days). The Babylonians used a seven-day cycle, which was adopted into Jewish, Christian, and Islamic traditions. The names of weekdays in English derive from Norse mythology and Roman planetary associations. ISO 8601 defines a week-based date system where years are divided into numbered weeks (W01-W52 or W53). Week 1 is the week containing the first Thursday of January. This system is widely used in European business planning, manufacturing schedules, and software date handling. Some fiscal calendars operate on a 52/53 week year basis rather than calendar months.