What ISO 8601 means
ISO 8601 is the global standard for representing dates and times. It defines a format that reduces ambiguity across countries and software systems. Instead of writing dates like 03/04/2024, which can mean March 4 in the United States and April 3 in many other regions, ISO 8601 uses the unambiguous YYYY-MM-DD format. It also defines week dates, which is where ISO week numbers come from.
A week date includes three pieces: the ISO week year, the ISO week number, and a day of week. The standard format is YYYY-Www-D, such as 2024-W09-1 for Monday of week 9. Most people do not need the day component in daily use, but the week number and week year are powerful for planning. That is why ISO week calculators exist: they convert a normal date into the ISO week number you can use for operations, project management, and reporting.
ISO 8601 also standardizes time zones, date separators, and how to represent partial dates. This consistency is essential for modern systems. When you search "week number today" or "ISO week calculator," you want the same result no matter where you live, which is exactly what ISO 8601 delivers. WeekYear follows the standard precisely to keep your answers accurate.
The ISO week number rules
The ISO week system has a few simple rules that are easy to remember but crucial to implement correctly:
- Weeks start on Monday.
- Week 1 is the week that contains January 4.
- The ISO week year is based on the week that contains a Thursday.
The Thursday rule is the secret that makes the system consistent. If you take any date, move to the Thursday of the same week, and read that Thursday's year, you have the ISO week year. This removes ambiguity at year boundaries, because the Thursday is always in the same week and always in one year.
When you run a date through an ISO week calculator, it often uses a trick: shift the date to Thursday, compute which week it is relative to the first Thursday of the ISO year, and then derive the week number. That approach avoids off-by-one errors. It also ensures that weeks are full Monday-to-Sunday blocks, even when they overlap January or December.
| Rule | Meaning |
|---|---|
| Week starts Monday | ISO week boundaries always fall on Monday and Sunday. |
| Week 1 contains Jan 4 | Week 1 is the first week with at least four days in January. |
| Thursday decides week year | The ISO week year is the year that contains the week's Thursday. |
These rules are the reason week numbers sometimes look surprising at the end of December. A late-December date can belong to week 1 of the next ISO week year, while a January 1 date can belong to the last ISO week of the previous year. The output is correct even if it feels counterintuitive at first.
ISO week year vs calendar year
The ISO week year is not always the same as the calendar year. It is a label based on which year the week belongs to, and the week is determined by the Thursday rule. This means early January dates can be part of the previous ISO week year, and late December dates can belong to the next ISO week year.
For example, if January 1 falls on a Friday, Saturday, or Sunday, it is still part of the last ISO week of the previous year. In that case, the ISO week year will be the previous year, and the ISO week number might be 52 or 53. That is not an error. It is the expected behavior of ISO 8601.
This is why business systems that rely on week numbering should always store both the ISO week number and the ISO week year, not just a week number by itself. A label like "week 1" is ambiguous without the ISO year. WeekYear always displays the week code (like 2024-W01) so there is no confusion when you copy or share the result.
ISO week dates are built for clarity across borders. They trade a little intuition around New Year for a lot of consistency the rest of the year.
Edge cases and 53-week years
Most years contain 52 ISO weeks, but some years contain 53. The rule is precise: a year has 53 ISO weeks if January 1 falls on a Thursday, or if it is a leap year and January 1 falls on a Wednesday. In those cases, the calendar alignment creates an extra full week.
Recent examples of 53-week ISO years include 2015, 2020, and 2026. In those years, the ISO week count runs from 1 to 53, and the final week usually spans into early January of the next calendar year. If you are planning payroll, inventory, or sprint cadence, you need to account for this extra week.
Edge cases also show up when you compare ISO week numbers to regional week numbering. For instance, some countries treat Sunday as the start of the week. That shifts the boundaries and can make a week number appear off by one if you use a non-ISO method. When accuracy matters, always specify ISO 8601 and use a reliable ISO week calculator.
ISO weeks in Europe vs the USA
ISO week numbering is the default across much of Europe. Calendars and business tools typically start weeks on Monday and display week numbers, especially in logistics, manufacturing, and project management. The ISO week number today is a common reference point, and most European teams will instantly understand a week code like 2024-W35.
In the United States, common calendars often start on Sunday. Week numbers are used less consistently in everyday settings, although they appear in enterprise software, retail planning, and systems that operate internationally. That means American teams may see different week numbers in consumer calendars compared to ISO 8601. When a project spans Europe and the USA, the safest approach is to use ISO week numbers and clearly label them as such.
The practical takeaway: if you are coordinating across regions, always include the ISO week year and the date range. "Week 11" is less clear than "2024-W11 (Mar 11 to Mar 17)." A clear week range eliminates ambiguity and avoids mismatched schedules.
Coding week numbers in Python, JS, and Excel
Most modern programming languages support ISO week numbers, but the implementation details vary. The safest approach is to either use built-in ISO functions or carefully implement the Thursday-based algorithm. Below are accurate, minimal examples for the most common environments.
Python (datetime)
Python's standard library already supports ISO week numbers via date.isocalendar() . It returns the ISO week year, ISO week number, and weekday in a single call.
from datetime import date
today = date.today()
iso_year, iso_week, iso_weekday = today.isocalendar()
print(iso_year, iso_week)JavaScript (UTC-based)
JavaScript does not have a built-in ISO week function, so you need a small helper. Use UTC to avoid time zone shifts that can flip the week number around midnight.
function getISOWeek(date) {
const utc = new Date(Date.UTC(date.getFullYear(), date.getMonth(), date.getDate()));
const day = utc.getUTCDay() || 7;
utc.setUTCDate(utc.getUTCDate() + 4 - day);
const yearStart = new Date(Date.UTC(utc.getUTCFullYear(), 0, 1));
const week = Math.ceil(((utc - yearStart) / 86400000 + 1) / 7);
return { year: utc.getUTCFullYear(), week };
}Excel and Google Sheets
Excel includes ISOWEEKNUM, which returns the ISO week number directly. If that function is not available, use WEEKNUM with return type 21 to force ISO 8601 rules.
=ISOWEEKNUM(A2)
=WEEKNUM(A2,21)If you need the ISO week year in Excel, combine functions to read the Thursday of the week. A practical method is to shift the date to Thursday, then extract the year from that adjusted date.
=YEAR(A2 - WEEKDAY(A2,2) + 4)Accuracy checklist
If you are implementing an ISO week calculator or validating a week number today, use this checklist to avoid common mistakes:
- Use Monday as the first day of the week.
- Define week 1 as the week containing January 4.
- Calculate the ISO week year using the Thursday rule.
- Store both ISO week number and ISO week year in data systems.
- Use UTC for calculations to avoid time zone drift.
When these rules are followed, your week numbers will align with ISO 8601 and match the results shown on WeekYear. That makes cross-team planning and data comparison much easier.
ISO 8601 FAQ
What week number is today?
The ISO week number today is available on the WeekYear homepage. It includes the ISO week year and the Monday to Sunday range, so you can share it without ambiguity.
Why does my calendar show a different week number?
Some calendars start weeks on Sunday or use local rules that do not follow ISO 8601. That can shift the week number by one compared to ISO. Use an ISO week calculator when you need consistent global results.
How many weeks are in a year?
Most years have 52 ISO weeks, but some have 53. The determining rule is whether the year begins on a Thursday, or begins on a Wednesday in a leap year. This creates an extra week at the end of the ISO week year.
Can I plan by weeks using ISO numbers?
Yes. ISO week numbers are excellent for planning by weeks because they are standardized and stable. Use the week code (like 2024-W21) in schedules, tickets, and timelines for clarity.
Summary
ISO 8601 week numbers give you a clean, global way to label time. The rules are simple but strict, and they ensure consistent results even at year boundaries. With the converter above, you can translate any date into an ISO week code instantly. Whether you are searching for the week number today, building a calendar app, or organizing a team roadmap, ISO 8601 is the most reliable standard to follow.