ISO week numbers are part of the ISO 8601 standard that labels weeks consistently worldwide. Week 1 is the week with the year’s first Thursday (or, equivalently, the week containing January 4), and weeks start on Monday. This system sometimes creates a Week 53 and can assign late-December dates to the next year’s Week 1, or early-January dates to the previous year’s Week 52/53.

If you schedule projects by week, work with multinational teams, or rely on ERP/BI dashboards, understanding ISO week dates helps avoid confusion at year boundaries and keeps reporting aligned.

What are ISO week numbers?

ISO week numbers (also called the ISO 8601 week date or week-based year) assign every Monday–Sunday block a number from W01 to W52 (sometimes W53). The system is designed to be unambiguous across countries and software.

The three core rules

• Weeks start on Monday.
• Week 1 (W01) is the week that contains the year’s first Thursday — equivalently, the week that contains January 4 or has at least four days in the new year.
• Week dates can cross calendar-year boundaries. A week may start in late December and still be Week 1 of the next ISO week year.

ISO week year vs. calendar (Gregorian) year

The ISO week year (the year used with the week number, like 2025-W01) can differ from the familiar Gregorian year on a few days around New Year:

• Late December dates (e.g., Dec 30–31) may belong to the next ISO week year’s Week 1.
• Early January dates (e.g., Jan 1–3) may still belong to the previous ISO week year’s Week 52 or Week 53.

This is intentional: the week-based year ensures that every ISO week is a complete Monday–Sunday block.

Why some years have Week 53

Most years have 52 ISO weeks (52 × 7 = 364 days). Because calendar years have 365 or 366 days, the extra day(s) shift the week pattern slightly each year. In some years, that shift produces a Week 53 (W53).

The quick test for Week 53

A Gregorian year has Week 53 if and only if:

• January 1 is a Thursday, or
• It’s a leap year and January 1 is a Wednesday.

Equivalent formulations you may see:

• The year contains 53 Thursdays.
• December 31 falls on a Thursday (in common years) or a Friday (in leap years).

Recent and upcoming examples of Week 53 years

• 2015: Jan 1 was Thursday → had W53.
• 2020 (leap year): Jan 1 was Wednesday → had W53.
• 2026: Jan 1 will be Thursday → will have W53.
• 2032 (leap year): Jan 1 will be Thursday? Actually, 2032 starts on Thursday? 2032 is a leap year starting Thursday means the prior condition holds via Dec 31 Friday of 2032; in any case, leap years starting Wednesday also have W53.

Across the 400-year Gregorian cycle, 71 out of 400 ISO week years have 53 weeks; the other 329 have 52. That’s roughly one 53-week year about every 5 to 6 years.

Year boundaries: how weeks cross from December to January

Because Week 1 is defined by the first Thursday/January 4 rule, the start of the ISO week year can fall in late December. Here are common edge cases:

• December dates in next year’s Week 1: When the first Thursday of the new year falls on January 1–3, the Monday of that week is in late December of the previous calendar year. Example: 2025-W01 runs from Monday 2024-12-30 through Sunday 2025-01-05. So 2024-12-31 is written as 2025-W01-2 (Tuesday).
• January dates in last year’s week: If Jan 1–3 don’t make up a majority of a week in the new year, they remain in the prior ISO week year. Example: 2016-01-01 fell on a Friday and is labeled 2015-W53-5.

So, to the question “Does the week number reset on January 1?” — not necessarily. It resets on the Monday of the week that contains the year’s first Thursday (the week containing January 4).

How to read and write ISO week dates

The ISO 8601 week-date format encodes week-based years and days as:

• YYYY-Www for a week (e.g., 2026-W53).
• YYYY-Www-D for a specific weekday, where D is 1–7 for Monday–Sunday (e.g., 2025-W01-1 for Monday of Week 1).

In many European contexts you’ll also see CW (short for “calendar week”), e.g., CW 36 meaning W36 of the current ISO week year.

Practical benefits for planning and international coordination

Why organizations adopt ISO week numbering:

• Consistency across borders: Monday-start weeks and a clear Week 1 rule avoid US vs. EU differences.
• Predictable sprints and releases: Two-week sprints and weekly releases line up cleanly, even at year-end.
• Roll-up reporting: Sales, production, and utilization metrics aggregate to weeks without partial weeks at year-end.
• Quarter planning: Quarters typically span 13 ISO weeks. When a 53rd week appears, teams can treat it as a buffer, catch-up, or special period.
• ERP and supply-chain alignment: Systems like SAP commonly reference calendar week (CW) for delivery windows and lead times.

Quick ways to determine the ISO week number

Human-friendly methods

• January 4 rule: Week 1 is the week containing January 4. Find the Monday of that week — that’s the start of W01.
• Thursday rule: Identify the Thursday of the current week; the ISO week year is the year of that Thursday. Then count weeks from the first Monday of W01.
• Majority-of-days rule: The first ISO week is the first week with four or more days in the new year.

Spreadsheet functions

• Excel:ISOWEEKNUM(date) returns the ISO week number; WEEKNUM can be regional and non-ISO depending on the return type.
• Google Sheets:ISOWEEKNUM(date) returns the ISO week number.

Programming APIs

• Python:date.isocalendar() → (iso_year, iso_week, iso_weekday).
• Java (java.time):IsoFields.WEEK_OF_WEEK_BASED_YEAR and IsoFields.WEEK_BASED_YEAR.
• JavaScript libraries: Day.js (ISOWeek), date-fns (getISOWeek), Luxon (weekNumber, weekYear).

Common pitfalls and how to avoid them

• Assuming Jan 1 is Week 1: Not always. Verify using the Thursday/Jan 4 rule.
• Mixing ISO and non-ISO weeks: Some regions/systems start weeks on Sunday. State “ISO weeks” or “CW” in schedules to avoid ambiguity.
• Ignoring time zones: Week boundaries are by local midnight. For distributed teams, specify the time zone (e.g., “ISO weeks in CET”).
• Forgetting Week 53 planning: Plan how to handle W53 years: special sprint, maintenance, or buffer for reporting alignment.
• Quarter comparisons: Some fiscal calendars use 4-4-5 patterns unrelated to ISO weeks. Align definitions before comparing metrics.

Worked examples around New Year

• Example A — Dec 31 belongs to next year’s Week 1: In the 2025 week year, W01 runs Mon 2024-12-30 to Sun 2025-01-05. Thus, 2024-12-31 is 2025-W01-2.
• Example B — Jan 1 belongs to last year’s Week 53: For the 2015/2016 boundary, 2016-01-01 (Friday) is 2015-W53-5. The first true Week 1 of 2016 started on Monday 2016-01-04.
• Example C — A 53-week year: In 2020 (leap year starting on Wednesday), the weeks run from 2020-W01 through 2020-W53.

How ISO week numbering helps teams

The ISO 8601 framework brings operational clarity:

• Scheduling: “Ship in CW10” is exact, regardless of country calendar formats.
• Forecasting: Weekly targets and moving averages align to the same Monday–Sunday windows.
• Compliance and audit: Logs and reports are consistently indexed by week-based year and week number.
• Communications: Stakeholders can reference a week number without confusion over date formats (e.g., 03/04 vs. 04/03).

Summary

ISO week numbers give you a reliable, global way to refer to weeks. Remember the essentials: weeks start Monday; Week 1 contains the first Thursday (January 4); and some years legitimately have Week 53. At year boundaries, dates can belong to the prior or next ISO week year — which is a feature, not a bug. With these rules in mind, your project planning and international coordination become clearer, cleaner, and easier to automate.

FAQ

Is Week 1 always the week of January 1?

No. Week 1 is the week containing the year’s first Thursday (or January 4). January 1–3 may still belong to the prior ISO week year.

How often does Week 53 occur?

In the long run, about 71 out of 400 ISO week years have Week 53 — roughly once every 5–6 years.

Why do ISO weeks start on Monday?

ISO 8601 defines Monday as the first day to avoid regional ambiguity and to keep full Monday–Sunday weeks. This is common in Europe and many global standards.

What’s the format to write an ISO week date?

Use YYYY-Www for a week (e.g., 2026-W53) and YYYY-Www-D for a day (e.g., 2025-W01-1 for Monday of Week 1).

Can December dates belong to the next year’s Week 1?

Yes. If the first Thursday of the new year is Jan 1–3, the preceding Monday is in late December, and those December dates are part of the next ISO week year’s Week 1.

Do all countries use ISO week numbers?

No, but ISO 8601 is widely used in Europe, manufacturing, logistics, and software. Some regions start weeks on Sunday or use different week numbering. State “ISO weeks” when needed.

How do I calculate ISO week numbers in software?

Use built-in tools: Excel/Sheets ISOWEEKNUM, Python’s date.isocalendar(), and Java’s IsoFields.WEEK_OF_WEEK_BASED_YEAR. These APIs handle edge cases correctly.