Week Number Today
Today's ISO 8601 week number — which week of the year we're currently in, with the exact Monday–Sunday date range.
Any date → ISO week number
Pick any Gregorian date to find its week number.
What is the ISO week number?
The ISO 8601 week number is a standardized way of identifying which week of the year it is. Each year has either 52 or 53 weeks, numbered from W01 to W52 (or W53).
Under ISO 8601:
- Weeks start on Monday and end on Sunday.
- Week 1 of any year is the week containing the year's first Thursday (equivalently, the week containing January 4th).
- This means the early days of January can belong to the last week of the previous year, and late December days can belong to week 1 of the following year.
ISO week-dates are written as YYYY-Www-D — for example, 2026-W17-4 means "Thursday of week 17 of 2026."
Why use week numbers?
Week numbers are the lingua franca of European business, manufacturing, and project management. Agencies plan "we'll deliver in W24." Factories schedule production in weekly batches. Schools and universities publish academic calendars by week. Using a single number avoids ambiguity about exactly which days are covered.
ISO vs US week numbering
The United States and several other countries traditionally start the week on Sunday, not Monday, and week 1 is typically the week containing 1 January. This can give a different week number than ISO 8601 for the same date — most commonly in January and at year-end.
This page uses the ISO 8601 system, which is the international standard and the most common convention in spreadsheets, programming languages, and business software worldwide.
Which years have 53 weeks?
A year has 53 ISO weeks when either:
- 1 January falls on a Thursday, or
- 1 January falls on a Wednesday in a leap year.
Recent and upcoming 53-week years: 2020, 2026, 2032, 2037, 2043, 2048.
ISO 8601 vs US vs Middle Eastern week systems
There are three major week-numbering conventions in active use, and they can produce different week numbers for the same date:
| System | Week starts on | Week 1 rule | Used in |
|---|---|---|---|
| ISO 8601 | Monday | Contains first Thursday | Europe, international standard |
| US | Sunday | Contains 1 January | United States, Canada, Latin America |
| Middle Eastern | Saturday | Contains 1 January | Much of the Middle East |
This page uses the ISO 8601 standard, which is also what =ISOWEEKNUM() returns in Excel and what most programming languages produce by default.
53-week years — when do they happen?
Most years have 52 weeks, but some have 53. This affects payroll systems, project planning, and anyone budgeting weekly expenses across a year. A year has 53 ISO weeks when either:
- 1 January falls on a Thursday (that Thursday becomes W01 Thursday), or
- 1 January falls on a Wednesday in a leap year.
53-week years, past and future
2009, 2015, 2020, 2026, 2032, 2037, 2043, 2048, 2054, 2060, 2065.
Business quarters and week numbers
In many industries, quarters roughly map to weeks:
- Q1 — weeks 1–13
- Q2 — weeks 14–26
- Q3 — weeks 27–39
- Q4 — weeks 40–52 (or 53)
Retail planning calendars, however, often use a different system called the 4-5-4 calendar, which divides the year into months of 4, 5, and 4 weeks. The National Retail Federation publishes an annual 4-5-4 calendar that many retailers align to.
When the week ends: fiscal year implications
Companies that close their books weekly (common in payroll and time-tracking) often tie fiscal periods to ISO week numbers. A 52-week fiscal year ends in week 52; a 53-week year ends in week 53, sometimes called a "long year." Multi-year comparisons must account for this extra week or risk a misleading year-on-year comparison.