ISO Week Number Converter — Week ⇄ Date (Mon–Sun)

About This ISO Week Converter

The ISO week date system defines weeks starting on Monday. ISO week 1 is the week containing the year's first Thursday (equivalently, January 4). Because of this, the ISO week-year for dates near New Year’s can differ from the calendar year.

This tool performs all calculations at midnight UTC to avoid daylight-saving surprises across regions. Results are shown in ISO format (YYYY-MM-DD) for easy copying into spreadsheets and documents.

Tip: the Use This ISO Week shortcut fills the week-year and week number based on today.

ISO Week Converter: FAQs

What is ISO week 1?

ISO week 1 is the week with the year's first Thursday (equivalently, the week containing January 4). Weeks start on Monday.

Why does the week-year differ from the calendar year?

Dates near the start or end of a calendar year can belong to the adjacent ISO week-year because ISO weeks are whole Monday–Sunday blocks.

How do you validate week numbers?

The tool computes the number of ISO weeks in the selected week-year (52 or 53) and warns if the week number is out of range.

Is my data private?

Yes—everything runs locally in your browser. No uploads, no tracking.

What Is the ISO Week Date & Why It Matters

The ISO week date (ISO 8601) is a global standard for numbering weeks. In this system, weeks start on Monday, and ISO week 1 is the week that contains the year’s first Thursday (equivalently, January 4). Because ISO weeks are grouped by full Monday–Sunday blocks, the ISO week-year for a date can differ from its calendar year near New Year’s. For example, 2024-12-31 belongs to ISO week-year 2025 (week 1, Tuesday), while 2025-12-31 falls in ISO week-year 2026 (week 1, Wednesday). This converter handles those edge cases automatically and shows the full week range (Monday → Sunday).

Where ISO Weeks Are Used (UK, EU & beyond)

ISO week numbering is common across the UK and Europe for project plans, manufacturing schedules, retail promotions, payroll cycles, school timetables, shipping windows, and agile sprints (e.g., “deliver in W32”). Many UK businesses use Monday-based weeks in reports and spreadsheets so that weekdays align consistently. While the United States often quotes dates in month/day format, global teams, suppliers, and freight forwarders frequently exchange schedules using ISO week numbers to avoid ambiguity across time zones and locales.

Two Essential Conversions

This tool performs both directions, locally in your browser for privacy:

• Date → ISO week: Enter a calendar date to see its ISO week number, week-year, the weekday (Mon=1 … Sun=7), and the week’s Monday–Sunday range.
• ISO week → Date: Provide the ISO week-year, week number (1–52/53), and weekday to get the exact calendar date in YYYY-MM-DD.

Handling Week 53, Leap Years & Edge Cases

Most years have 52 ISO weeks, but some have 53. The converter validates the week number against your chosen week-year and explains if a value is out of range. Leap years are naturally supported: the algorithm works from the ISO definition (the first Thursday rule) rather than counting days month-by-month, which avoids off-by-one errors around February and year boundaries.

Practical Examples

• 2025-01-01 → ISO 2025-W01-3 (Wednesday), with week range Monday to Sunday of that first ISO week.
• 2024-12-31 → ISO 2025-W01-2 (Tuesday): note the week-year is 2025, not 2024.
• ISO 2025-W10-1 → 2025-03-03 (Monday): useful for sprint planning and delivery windows.

Why Teams Prefer ISO Weeks

Monday-based, numbered weeks make it easy to compare schedules year-over-year, roll up weekly KPIs, align pay periods, and communicate timelines with partners across countries. By standardising on ISO week numbers in the UK/EU and with international stakeholders, you reduce misunderstanding and speed up planning.

Tips for Better Reporting

• Always store a week key like YYYY-Www (e.g., 2025-W32) alongside dates in spreadsheets.
• For cross-border projects, publish both the calendar date and the ISO week code in briefs and roadmaps.
• If you export data to BI tools, group by ISO week to keep weeks aligned Monday–Sunday.