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ISO Week Calculator

Calculate ISO 8601 week numbers following international standards

ISO Week Results

ISO Week String

2026-W29

Week Start (Monday)

13/7/2026

Week End (Sunday)

19/7/2026

ISO Week Number

29

ISO Year

2026

Day of Week

6

What Is an ISO Week?

An ISO week is a standardized 7-day period defined by the ISO 8601 international standard. Every ISO week runs from Monday to Sunday, and weeks are numbered sequentially throughout the year (1 through 52, occasionally 53). ISO Week 1 is the week containing the year's first Thursday — guaranteeing that Week 1 always includes January 4 and that every week in the year has the majority of its days in the same calendar year.

This ISO Week tool supports two operations: converting a calendar date to its ISO week number and year, and converting an ISO year + week number to the corresponding Monday–Sunday date range. Both operations are commonly needed in business reporting, manufacturing, and software development.

The ISO week year can differ from the Gregorian calendar year for dates near January 1 or December 31. December 29–31 may belong to ISO Week 1 of the next year, and January 1–3 may belong to ISO Week 52 or 53 of the previous year. This is why ISO week dates always include the ISO year: 2026-W01, never just W01.

ISO Week Calculation Algorithm

The ISO week number is computed by shifting the target date to the Thursday of its ISO week, then counting complete weeks from the first Thursday of the year.

ISO Week Number (Jan 4 Method)

jan4 = new Date(year, 0, 4); weekStart = jan4 − ((jan4.getDay()+6)%7) + (week−1)×7 days

Where:

  • dayNum= (date.getDay() + 6) % 7 — ISO day of week (0=Mon, 1=Tue, ..., 6=Sun)
  • firstThursday= date − dayNum + 3 days — the Thursday of the current ISO week
  • isoWeek= 1 + ceil((firstThursday − firstThursdayOfYear) / (7 days))
  • isoYear= Adjusted calendar year — previous year if week > 50 and month = January, next year if week = 1 and month = December
  • jan4= January 4 of the target year — always in ISO Week 1, used as anchor for reverse conversion

Converting ISO Week to Calendar Dates

To find the Monday–Sunday date range for a given ISO year and week number:

  1. Find January 4 of the ISO year (which is always in ISO Week 1)
  2. Compute the day of week of January 4: dayOfWeek = (jan4.getDay() + 6) % 7
  3. Find the Monday of ISO Week 1: weekStart = jan4 − dayOfWeek
  4. Advance to the target week: targetMonday = weekStart + (week − 1) × 7 days
  5. The Sunday = targetMonday + 6 days

This method correctly handles the cases where ISO Week 1 starts in late December of the previous calendar year.

How to Use This Calculator

  1. Date to ISO Week: Enter any calendar date in the date picker. The result shows the ISO week number, ISO year, ISO week string (YYYY-Www), ISO day of week (1=Monday to 7=Sunday), and the Monday–Sunday date range of the week.
  2. ISO Week to Dates: Switch to 'Week to Date' mode, enter an ISO year and week number (1–53), and the calculator displays the corresponding Monday and Sunday calendar dates.

Real-World Applications

ISO week numbers are the backbone of European business calendars. German companies routinely refer to delivery schedules, project milestones, and production targets by "KW" (Kalenderwoche) number. A supplier in Germany saying "delivery in KW 24" means the week starting June 8, 2026 — without an ISO week calculator, you'd need to work this out manually.

Financial analysts working with Eurozone data, Nielsen retail sales data, or ILO labor statistics often receive datasets indexed by ISO week. Translating these week numbers to calendar dates for plotting and visualization is a standard data engineering task.

Software developers building calendar applications, scheduling systems, or time-tracking tools need to implement ISO week arithmetic. This calculator serves as a quick manual verification tool when testing ISO week functions in Python (isocalendar()), JavaScript (manual calculation), or SQL (EXTRACT(WEEK FROM date)).

Worked Examples

Find ISO Week for December 30, 2025

Problem:

What ISO week does December 30, 2025 fall in?

Solution Steps:

  1. 1December 30, 2025 is a Tuesday (getDay() = 2, ISO dayNum = (2+6)%7 = 1)
  2. 2Thursday of this week: Dec 30 − 1 + 3 = Dec 30 + 2 = January 1, 2026
  3. 3Since the Thursday (Jan 1, 2026) is in 2026, isoYear = 2026
  4. 4January 1, 2026 is the first Thursday of 2026 → ISO Week 1

Result:

December 30, 2025 is in ISO Week 2026-W01 (the first week of ISO year 2026).

Reverse: Dates for ISO Week 2026-W24

Problem:

What Monday–Sunday does ISO Week 2026-W24 cover?

Solution Steps:

  1. 1jan4 for 2026: January 4, 2026 (Sunday, getDay()=0, dayOfWeek=(0+6)%7=6)
  2. 2Monday of ISO Week 1: Jan 4 − 6 = December 29, 2025
  3. 3Monday of ISO Week 24: Dec 29 + (24−1)×7 = Dec 29 + 161 days
  4. 4161 days from Dec 29, 2025: Dec 29 + 2 = Dec 31 (+31 Jan +28 Feb +31 Mar +30 Apr +31 May = +3 = June 3) → June 8, 2026

Result:

ISO Week 2026-W24 runs Monday June 8, 2026 to Sunday June 14, 2026.

ISO Week vs Calendar Week at Year Start

Problem:

What is January 1, 2026 in ISO week notation?

Solution Steps:

  1. 1January 1, 2026 is a Thursday (ISO dayNum = 3)
  2. 2The Thursday of this week = January 1 itself
  3. 3First Thursday of ISO year 2026 = January 1 → this is ISO Week 1
  4. 4ISO Year: 2026 (January, and week ≤ 50 so no adjustment needed)

Result:

January 1, 2026 = 2026-W01-4 (Thursday of ISO Week 1, 2026).

Tips & Best Practices

  • ✓Always include the ISO year when writing week numbers — 'W24' alone is ambiguous; '2026-W24' is not.
  • ✓Use this calculator to verify your date library's ISO week implementation against known boundary cases like Dec 29–31 and Jan 1–3.
  • ✓In Excel, use ISOWEEKNUM(date) for ISO week numbers — not WEEKNUM(), which uses a different convention.
  • ✓In Python: date.isocalendar() returns (iso_year, iso_week, iso_weekday) in one call.
  • ✓Years with ISO Week 53: 2004, 2009, 2015, 2020, 2026, 2032, 2037 — useful to know when building year-over-year comparisons.
  • ✓The Monday of any ISO week can be found as: Jan 4 of the ISO year minus its weekday offset, plus (week−1)×7 days.

Related Calculators

ISO Week Calculator

Calculate ISO 8601 week numbers with full week date details

Fiscal Week Calculator

Calculate fiscal week numbers for custom fiscal years

Frequently Asked Questions

ISO week boundaries do not align perfectly with January 1. For dates in the last few days of December or the first few days of January, the ISO week may 'belong' to the adjacent calendar year. This happens because ISO Week 1 is defined by the Thursday rule — if a week's Thursday falls in the new year, the entire week belongs to the new year, even if its Monday is still in December.
An ISO week date is written as YYYY-Www or YYYY-Www-D, where YYYY is the ISO week year, ww is the 2-digit zero-padded week number, and D is the optional ISO day of week (1=Monday to 7=Sunday). Always use the ISO week year, not the Gregorian calendar year. For example: 2026-W01-1 = Monday of the first ISO week of 2026 = December 29, 2025.
Both calculators compute ISO 8601 week numbers. This ISO Week page uses a slightly different implementation algorithm (the Jan 4 anchor method vs. the shift-to-Thursday method) but produces identical results. The ISO Week Calculator page includes additional features like the total number of ISO weeks in a year and the complete week-start/week-end date display.
ISO weeks are less commonly used in North America than in Europe, but they appear in international business contexts, supply chain management for multinational companies, and any software that interoperates with European partners. The US more commonly uses calendar weeks starting on Sunday (WEEKNUM in Excel, WEEK() in MySQL). Always confirm which week numbering system a data source uses before assuming ISO.
Yes — a year has 53 ISO weeks if January 1 falls on a Thursday (e.g., 2026), or if it is a leap year and January 1 falls on a Wednesday (e.g., 2020). In all other years, the last ISO week is Week 52. This means that comparing week 52 data between a 52-week and a 53-week year requires care — week 52 doesn't cover the same relative period in both years.

Sources & References

Last updated: 2026-06-06

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Editorial Note

MyCalcBuddy Editorial Team

This page is maintained as an educational calculator reference.

Source

Formula Source: Standard Mathematical References

by Various

UpdatedLast reviewed: May 2026
CheckedFormula checks are based on standard references and internal QA review.

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