Week Number Calculator

Ask a European project manager what date a delivery is due and they might answer “W23.” Ask a factory planner when a production run starts and they'll say “CW18.” Week numbers are a compact, unambiguous shorthand used across manufacturing, logistics, agile software development, and financial reporting — yet many people have no idea which week number the current date falls in. This Week Number Calculator answers that instantly for any date, along with the Monday start and Sunday end of that week, day of year, quarter, and a full year-at-a-glance table of all 52 or 53 weeks.

The ISO 8601 Standard — Why Monday and Thursday Matter

ISO 8601 defines two rules that make week numbers unambiguous worldwide. First: every week starts on Monday. Second: Week 1 is the week that contains the year's first Thursday. The Thursday anchor means that the first week always contains at least four days of the new year. A week containing December 31 but whose Thursday falls in January will be called Week 1 of the following year — this is why some December dates technically belong to the next ISO year, and some January dates belong to the previous ISO year.

Long Years: When the Calendar Has 53 Weeks

Most ISO years contain 52 weeks, but roughly every 5–6 years a “long year” with 53 weeks occurs. This happens when January 1 falls on Thursday (a common year), or when January 1 of a leap year falls on Wednesday. Recent long years were 2015 and 2020; the next will be 2026. If you use week numbers in annual planning, awareness of long years prevents the mistake of ending a 52-week plan before December 31. The calculator flags long years explicitly in the Date Details table.

The Year-at-a-Glance Table

The scrollable “All Weeks of the Year” table lists every ISO week of the selected year with its Monday start and Sunday end dates. Your selected date's week is highlighted with an arrow. This is the fastest way to look up what dates a given week number covers — useful for scheduling sprints, verifying invoice periods, planning media campaigns by week, and confirming fiscal week boundaries. To count working days within a specific week range, combine this with the Business Days Calculator.

Where Week Numbers Appear in Professional Tools

• Jira and Linear sprints: Two-week sprints are named by week number — “Sprint W21–W22” is unambiguous regardless of the year or month.
• SAP and ERP systems: Procurement orders, production plans, and material requirements planning all use week numbers as the planning granularity.
• Google Ads and Meta campaigns: Weekly performance breakdowns in advertising platforms use ISO week numbers to group data.
• Retail and fashion: Seasonal merchandise planning and “drop” scheduling is done entirely by week number — “W47 Black Friday.”
• Global logistics: Shipping windows and production lead times in international supply chains are universally expressed in weeks.

Day of Year and Quarter

Alongside the week number, the calculator shows the day of year (1–365/366) and the calendar quarter (Q1–Q4). Day of year is used in scientific computing, military scheduling, and financial derivatives. Quarter is the standard slice for company earnings, GST filings, and investment analysis. To find the exact day of the week for any date, use the Day of Week Calculator.

Privacy

The Week Number Calculator runs entirely in your browser. No date you enter is transmitted to any server. Safe for sensitive project planning dates, financial reporting periods, and payroll scheduling data.