Week Number Calculator — What Week Is It?
Find the current ISO week number, look up the week number for any date, and browse the full year calendar with week numbers. Used for project planning, payroll, and reporting.
What Is a Week Number?
A week number identifies which week of the year a date falls in. The ISO 8601 standard — used internationally for business, logistics, and government — defines week 1 as the week containing the first Thursday of the year (equivalently, the week with January 4th). ISO weeks run Monday to Sunday. The year has either 52 or 53 weeks depending on which day January 1 falls on.
Week Numbers in Business and Payroll
Week numbers are widely used in manufacturing (production schedules), retail (sales reporting), project management (sprint planning), and payroll processing (biweekly pay periods). Many enterprise systems — SAP, Oracle, Excel — use ISO week numbers as a standard reference. In the US, fiscal calendars often use a different week numbering (starting Sunday or Saturday) that may differ by 1 from the ISO standard.
How to Calculate the Week Number
ISO week number formula: count days from the nearest Thursday, then divide by 7. In practice: find January 1, count how many days until the first Monday, then count 7-day blocks. The calculator above does this instantly for any date. In Excel: =ISOWEEKNUM(date). In Python: date.isocalendar().week. In JavaScript: there's no built-in, which is why this calculator exists.