Week Number Calculator — ISO 8601 & US Week Numbers

Most years have 52 ISO weeks, but some have 53. A year has 53 ISO weeks when: (1) January 1st falls on a Thursday, or (2) January 1st falls on a Wednesday in a leap year. Upcoming 53-week years include 2026, 2032, 2037, 2043, 2048, 2054, and 2060. In a 53-week year, Week 53 runs at the very end of December and may extend into early January of the following calendar year. The Full Year View tab in this tool shows whether the displayed year has 52 or 53 weeks, and Week 53 rows are highlighted with an orange background so they're easy to spot. The reverse lookup mode also validates that Week 53 only exists in 53-week years.

The two main differences are the start day of the week and the definition of Week 1. ISO 8601 starts weeks on Monday and defines Week 1 as the first week containing a Thursday. The US system starts weeks on Sunday and defines Week 1 as the week containing January 1st (which may be a partial week). As a result, the week numbers can differ — especially near the year boundaries and on Sundays. On a Sunday, the ISO system counts that day as the last day of the previous week, while the US system counts it as the first day of the new week. For international business and most project management software (like Jira, Asana, and SAP), ISO 8601 is the standard. The US system is common in North American consumer software, retail calendars, and school schedules. This tool shows both simultaneously so you can communicate clearly with colleagues using either system.

The ISO 8601 week date notation is a compact way to refer to a specific week or day within a week. The formats are: YYYY-Www for a full week (e.g., 2025-W14 means Week 14 of 2025) and YYYY-Www-D for a specific day within a week (e.g., 2025-W14-3 means Wednesday of Week 14, 2025, because Monday=1, Tuesday=2, Wednesday=3, etc.). This notation is unambiguous globally — anyone can look up 2025-W14 and know it means April 7–13, 2025. It's widely used in project planning, manufacturing schedules, logistics, and payroll systems. Copy the ISO notation from this tool using the "Copy ISO notation" button to paste directly into spreadsheets, emails, or planning documents. Excel and Google Sheets both support ISO week notation in date functions.

The Sprint Planner in this tool generates a labeled sprint schedule from any starting week. Enter the number of sprints and your sprint length in weeks (2 weeks is the most common Scrum sprint length), then click Generate. The tool shows Sprint 1, Sprint 2, etc., each with the ISO week numbers and date ranges it covers. This is useful for: planning sprint start/end dates at the beginning of a quarter or project, communicating sprint timelines to stakeholders in week-number format, checking that sprints don't overlap with major holidays or time-off periods, and aligning sprint reporting cycles with fiscal reporting weeks. ISO week numbers are preferable to calendar dates for sprint planning because they provide a consistent, timezone-independent way to reference time periods in distributed teams.