What Week Number Is It Today — How ISO Weeks Work

If you work in project scheduling, supply chain, manufacturing, or broadcasting, you almost certainly track time by week number. "We're in week 14" is more useful than "it's the second week of April" for cross-team coordination. But the moment you need to know exactly what week number today falls in — and whether your colleagues in Germany or your spreadsheet formula agree — it gets complicated.

The Current Week Number tool shows you the ISO week number for today immediately, with no input required. This article explains how the ISO week numbering system works, why January 1 is not always in week 1, and what to do when different systems give different answers.

The Short Answer: ISO 8601

The most widely used week numbering standard is ISO 8601. Under this standard:

• Weeks run Monday to Sunday
• Week 1 is the week containing the first Thursday of the year
• There are 52 or 53 weeks per year
• The ISO week year can differ from the calendar year near January 1

That last point is where most people get caught out. Let's look at it in detail.

Why January 1 Is Not Always in Week 1

Under ISO 8601, week 1 must contain January's first Thursday. This means:

• If January 1 is a Monday, Tuesday, Wednesday, or Thursday — it's in week 1
• If January 1 is a Friday, Saturday, or Sunday — it's in the last week of the previous ISO year

Similarly, late December can fall in week 1 of the next ISO year if December 29, 30, or 31 fall on a Monday, Tuesday, or Wednesday.

Some concrete examples:

This "ISO week year" concept confuses people because when someone says "we're in week 1 of 2021," they might mean January 4 or later — not January 1. Payroll and project systems that use ISO week years need to reference the week year, not just the week number, to avoid ambiguity.

How to Calculate Which ISO Week a Date Falls In

The rule: find the Thursday of the same week (since ISO weeks contain exactly one Thursday), then the week number is determined by how many Thursdays have occurred so far in that year.

In practice, nobody does this by hand — it requires knowing what day of the week a date falls on, then finding Thursday of that week, then counting. The Current Week Number tool handles this for today. For other dates, the Week Number Calculator lets you enter any date.

The mental check that is fast enough to be useful: the week containing January 4 is always week 1. So if you know what day of the week January 4 falls on, you can count forward from there.

ISO Weeks vs US Week Numbering

ISO 8601 is dominant in Europe and international business contexts. The US and some other countries use a different system where:

• Weeks can start on Sunday (instead of Monday)
• Week 1 starts on January 1, regardless of what day of the week it falls on

In Excel and Google Sheets, this matters: WEEKNUM uses the US system by default (week starts Sunday, first week starts Jan 1), while ISOWEEKNUM explicitly uses ISO 8601.

The difference is usually 0 or 1 week, but near the year boundary it can be more — and it can cause two systems to report different week numbers for the same day, which creates real confusion in cross-border scheduling.

Example: January 2, 2022

• ISO week number: Week 52 of 2021 (because Jan 2 is a Sunday, and that Sunday belongs to the week that started Monday Dec 27, 2021)
• US WEEKNUM: Week 2 of 2022 (because Jan 1 is week 1, and Jan 2 is in the week that immediately follows)

If you are working with colleagues or systems in different countries, always confirm which system is being used when week numbers are referenced in planning documents or contracts.

Week Numbers in Practice: Who Uses Them

Manufacturing and supply chain: Production plans, delivery windows, and supplier agreements often reference ISO weeks. "Delivery by week 22" is unambiguous once you know the year and the ISO week year.

Broadcasting: Television and radio schedules use week numbers extensively. A "week 14 schedule" means the programming for the week that falls in ISO week 14.

Project management: Particularly in European companies, sprint planning, milestone tracking, and delivery dates are often expressed in week numbers. Agile sprints running Monday-Friday fit naturally into ISO week structure.

Payroll: Some payroll cycles are weekly by ISO week. Payslips may reference "week 32 pay" rather than a date range.

Agriculture: Crop planning, harvest windows, and planting calendars often use week numbers, particularly in Northern Europe where weather patterns map to ISO week ranges.

Finding the Week Number for a Specific Date

If you need to convert between a date and a week number — say, to confirm that a delivery scheduled for "week 18" corresponds to specific calendar dates — two approaches work:

Using the tool: The Week Number Calculator lets you enter any date and shows the ISO week. Or enter a week number and year to see which dates it spans.

Using a spreadsheet: In Google Sheets or Excel, =ISOWEEKNUM(A1) returns the ISO week number for the date in cell A1. To find the Monday start of a given ISO week, the formula is more complex — the tool is faster.

Mental shortcut: If you roughly know the date ranges, each ISO week runs Monday to Sunday. Week 1 always contains January 4. From there, count forward by 7 days per week.

The 53-Week Year

Most years have 52 ISO weeks. But some years have 53. This happens when:

• January 1 falls on a Thursday (non-leap year), or
• January 1 falls on a Wednesday or Thursday (leap year)

In a 53-week year, there is a week 53 at the end of December. For planning systems, this creates an occasional anomaly where a year has an extra period — some budgeting and scheduling systems handle this by absorbing week 53 into week 52, or by treating it as an exceptional period.

Years with 53 ISO weeks (recent examples): 2009, 2015, 2020, 2026, 2032.

The 53 Week Years Explained article covers this in more detail if you are building a system that needs to handle the edge case.