Week Numbers: US vs ISO — What's the Difference?

If you've ever compared week numbers between two systems — a spreadsheet and an ERP, a US calendar and a European one, Excel's WEEKNUM and Google Calendar — you've probably noticed they don't always agree. A date that's week 1 in one system is week 52 in another. This isn't a bug. It's a fundamental difference in how the two most common week numbering standards define "week 1."

The Two Main Systems

ISO 8601 (the international standard) defines weeks as running Monday through Sunday. Week 1 is the week containing the first Thursday of the year — equivalently, it's the first week with at least four days in January. This is the standard used across Europe, in international supply chains, and in most global business systems.

US-style week numbering (used in many North American contexts) defines week 1 as simply starting on January 1, with weeks running Sunday through Saturday (though some US systems use Monday). There's no rule about how many days of the week fall in the new year — week 1 starts when the calendar year starts.

Both systems label weeks 1 through 52 (or 53), but the week that gets label "1" can differ by a full week.

Where They Agree and Where They Don't

For most of the year — roughly February through November — both systems give the same or very similar week numbers for most dates. The disagreements cluster around two points:

Early January: If January 1 falls on a Friday, Saturday, or Sunday, those days belong to the last ISO week of the previous year under ISO 8601. Under the US system, those same days are in week 1 of the new year. The difference can be a full week-number gap.

Late December: December 28–31 can fall in ISO week 1 of the next year, while the US system puts them in week 52 or 53 of the current year.

A concrete example: January 1, 2016 was a Friday. Under ISO 8601, it fell in week 53 of 2015. Under the US system, it was week 1 of 2016. Same date, week numbers from different years.

Why This Causes Real Problems

The mismatch becomes a practical issue whenever two systems exchange week-based data.

Spreadsheets: Excel's WEEKNUM function defaults to the US system (weeks starting Sunday, week 1 on Jan 1). ISOWEEKNUM gives the ISO result. If you're building a report that feeds data from an ISO-week ERP system into Excel using WEEKNUM, the week numbers won't match near year boundaries.

Payroll and HR: Systems that run weekly payroll often use either ISO or US week numbers as keys in their database. If your HR system uses ISO weeks and your spreadsheet reporting uses US weeks, you'll get week-number mismatches in the first and last weeks of the year — exactly the weeks that often include holidays and adjustments.

Supply chain and manufacturing: European suppliers, ISO-standard ERP systems, and international logistics systems overwhelmingly use ISO week numbers. US buyers using US-style week numbering need to be aware of potential off-by-one-week errors when reading delivery schedules near year-end.

Broadcasting: The broadcast calendar (used in US media and advertising) is yet another system — it uses 4-week and 5-week month groupings that don't map to ISO weeks at all.

How to Check the Current Week Number

The easiest way to check is with the current week number tool, which shows today's ISO week number and the Monday-to-Sunday range it covers. If you need the week number for a specific date, the week number calculator lets you enter any date and returns both the ISO week and the date range.

For a quick sanity check: the ISO week number for January 1 is either week 1 of the current year, or week 52 or 53 of the previous year, depending on which day of the week January 1 falls on. If January 1 is a Monday, Tuesday, Wednesday, or Thursday, it's in week 1 of that year. If it's a Friday, Saturday, or Sunday, it's in the final week of the previous ISO year.

Which System to Use

In most international and European business contexts: use ISO 8601. It's the standard across EU countries, international standards bodies, and most enterprise software built outside the US.

In US-specific contexts: check what your specific system or tool uses. Excel defaults to US-style WEEKNUM; Google Sheets offers both. Many US ERP and accounting packages use ISO anyway, so don't assume.

When exchanging week-numbered data between two systems: always verify which standard each is using, and include a sample date with its expected week number in any specification or data handoff. That one check prevents a class of silent errors that are hard to debug after the fact.

How to Convert Between the Two

If you have a US week number and need the ISO equivalent (or vice versa), the safest approach is to convert through the actual date rather than doing arithmetic on week numbers directly.

1. Find the Monday of the US week in question (or the first day of that week under whatever Sunday-start rule applies). 2. Enter that date in the week number calculator to get the ISO week.

Direct arithmetic — "just add 1" — doesn't work reliably because the offset between US and ISO weeks changes across years and is sometimes zero.

ISO Week Year vs Calendar Year

One last wrinkle specific to ISO weeks: the ISO "week year" can differ from the Gregorian calendar year for dates near the year boundary.

December 30, 2024 falls in ISO week 1 of 2025. Its ISO week year is 2025, not 2024. This matters when you're storing or sorting week-labeled data — if you sort by calendar year + ISO week number, dates like December 30 will appear to jump forward into the wrong year grouping.

The correct approach for ISO week data is to store or display the ISO week year (sometimes written as the "YYYY" in ISO 8601's YYYY-Www format) rather than the Gregorian calendar year. Most date libraries that support ISO weeks handle this automatically, but when implementing it yourself, it's a common source of off-by-one-year bugs.

Summary

If you're working with week numbers and things aren't adding up, the first question to ask is: which standard is each system using? That single question resolves most week-number discrepancies.