ISO Week Number Calculator
Find the ISO 8601 week number for any calendar date — the standard week numbering used across Europe, in business, and in project planning.
ISO 8601 week numbering exists because plain calendar dates don't map cleanly onto the concept of a "week" that businesses and project planners actually care about — sprint planning, shipping schedules, and financial reporting are often organized by week number rather than by specific dates, and having a single international standard for what "week 23" means prevents the ambiguity that comes from different countries defining weeks differently.
The standard itself has a specific, slightly unintuitive rule at its core: week 1 of any year is defined as the week containing that year's first Thursday, and weeks always run Monday through Sunday rather than the Sunday-through-Saturday convention common in the United States. That rule is precise enough to calculate correctly, but genuinely awkward to do reliably by hand, especially around the boundary between one year and the next, where the "ISO week year" can actually differ from the calendar year for a few days in either direction.
This tool handles that calculation directly: enter any date, and it returns the correct ISO week number along with the ISO week year, computed the same way software systems that rely on ISO 8601 dates handle it internally — which matters if you're cross-checking against a system that uses week numbers for scheduling, invoicing, or reporting.
- Pick any date.
- Press Find week number.
- See which ISO week (1–53) that date falls in, and which ISO year it belongs to.
The ISO 8601 standard defines week 1 as the week containing the year's first Thursday, so the first few days of January can belong to the last week of the previous year, and vice versa in late December.
ISO 8601 weeks always start on Monday, which is different from the US convention of Sunday-start weeks.