The short answer
ISO 8601 — and therefore all week numbering on this site — starts the week on Monday. The Sunday start you see on US calendars is a cultural convention, not a different fact about time: the days are the same, only the printed column order and the week-number bookkeeping change.
Week start by country
| First day | Where | Weekend |
|---|---|---|
| Monday | All of Europe, UK, Australia, New Zealand, China, India, most of Africa and South America; the ISO 8601 standard | Sat–Sun |
| Sunday | United States, Canada, Mexico, Japan, South Korea, Taiwan, Philippines, Brazil (traditional), Israel | Sat–Sun (Israel: Fri–Sat) |
| Saturday | Iran, Afghanistan, and traditionally some other countries with a Friday-centred weekend | Fri (–Sat) |
Several Middle Eastern countries (Saudi Arabia, UAE, Egypt) moved their weekend to Friday–Saturday in the 2010s, so their working week now starts on Sunday even though devices sold there may default either way.
Why the conventions differ
The Sunday start is the older tradition: in the Jewish and early Christian reckoning the Sabbath (Saturday) is the seventh day, making Sunday day one — an ordering preserved in religious calendars and carried into American civil life. The Monday start is the industrial-era view: once the two-day weekend became standard, treating Monday–Friday as the working block with the weekend at the end was simply more practical, and European standards bodies wrote it into national norms (Germany's DIN 1355 in the 1970s) and then into ISO 8601 (1988).
Neither is "correct" — but only one of them comes with a standardised numbering system, which is why software, logistics, and international business default to Monday/ISO.
How the start day changes week numbers
The start day matters most when weeks get numbered. The US convention typically pairs Sunday starts with
the rule "week 1 contains January 1" (Excel's default WEEKNUM). Compare that with ISO:
| ISO 8601 | US convention | |
|---|---|---|
| Week starts | Monday | Sunday |
| Week 1 rule | Contains Jan 4 (majority in new year) | Contains Jan 1 |
| Weeks per year | Always 52 or 53 full weeks | Labels up to 53–54, first/last may be partial |
| Jan 1 belongs to | Sometimes previous year's last week | Always week 1 |
The upshot: in years where January 1 falls on a Friday, Saturday or Sunday, the US number runs one ahead of the ISO number for most of the year. If your team is comparing week numbers and someone is consistently off by one, this is almost always the cause — not a bug, two systems. Today the ISO week is 29 and the US week is 29 (computed live; they may match or differ this year).
Fixing your apps
Every major calendar lets you choose the week start — set it to Monday if you work with ISO week numbers:
step-by-step instructions for Google Calendar, Outlook, and Apple Calendar are in
week numbers in calendar apps, and the spreadsheet equivalents
(WEEKNUM types vs ISOWEEKNUM) are in
week numbers in Excel.