Project timelines, pregnancy trackers and rental agreements all juggle days and weeks interchangeably, and this tool handles the conversion — including the fractional weeks that come up more often than you'd expect.
The seven-day week is older than almost any other unit still in daily use
Unlike most units on this site, the week has no basis in physical measurement at all — it isn't derived from an astronomical cycle the way the day (Earth's rotation) or the year (Earth's orbit) are. Its seven-day length is generally traced to ancient Babylonian and Jewish traditions, later spread throughout the Roman Empire and much of the world through the influence of Judeo-Christian and, separately, astrological traditions that assigned each day to one of the seven classical "planets" (sun, moon and five visible planets). Notably, the week has survived multiple deliberate attempts to replace it — Revolutionary France briefly imposed a 10-day week in the 1790s, and the Soviet Union experimented with five- and six-day weeks in the late 1920s and early 1930s — both abandoned within years, making the seven-day week one of the most remarkably persistent human conventions in history.
The conversion
1 week = 7 days, so converting days to weeks is simply dividing your day count by 7 — a whole number of weeks plus, often, a fractional remainder (like 10 days = 1.43 weeks, or 1 week and 3 days).
Where this conversion comes up
- Pregnancy tracking — pregnancy is conventionally measured in weeks (up to 40), while some milestones and countdowns are naturally thought of in days, requiring frequent back-and-forth conversion.
- Project management and scheduling — task durations estimated in days often need rolling up into weeks for a higher-level project timeline or sprint planning view.
- Shipping and delivery estimates — some carriers quote delivery windows in business days while others use weeks, and comparing the two requires a quick conversion.
- Fitness and training plans — training cycles are often structured in weeks, while day-by-day workout schedules need converting to see how many full weeks a plan spans.
Frequently asked questions
Why is a week exactly 7 days and not tied to the moon? Although the week is roughly a quarter of the lunar month (about 29.5 days), it isn't precisely astronomically derived the way months and years are — its length appears to be a cultural and religious convention rather than a natural cycle, which is part of why it has stayed fixed at exactly 7 days regardless of any astronomical drift.
What happened to France's 10-day week? The French Republican Calendar (1793–1805) replaced the seven-day week with a 10-day "décade" as part of a broader decimal-time push, but it proved deeply unpopular — workers effectively lost rest days under the new schedule — and Napoleon formally abolished it in 1805.
Does "business days" change this conversion? Yes, importantly — if a figure refers to business days rather than calendar days, weekends are typically excluded, meaning 5 business days equals 1 calendar week, not 5/7 of one; this tool converts calendar days unless you adjust accordingly.
Further reading
Wikipedia — Week — The ancient origins of the seven-day week and history of attempts to replace it.