Week Number Calculator
Find the ISO 8601 week number for any date, see your progress through the year.
Disclaimer: The results provided by this tool are estimates for informational purposes only. Actual values may vary. Please verify important calculations independently.
How ISO week numbers are assigned
ISO 8601 defines a week as a Monday-through-Sunday block and assigns week 1 of any year to the week containing that year's first Thursday. Equivalently, week 1 is the week containing January 4 — a rule that is easier to verify manually. Because the week must include a Thursday from the new year, January 1 itself sometimes falls in the final week of the previous ISO year. January 1, 2023 was a Sunday and belongs to ISO week 52 of 2022, not week 1 of 2023. The tool reports both the ISO week number and the ISO week year to avoid this ambiguity.
Most ISO years contain 52 weeks totaling 364 days, leaving one or two days that get absorbed by the neighboring year's first or last week. A 53-week ISO year occurs when January 1 falls on a Thursday, or on a Wednesday in a leap year — roughly every five to six years. Recent and upcoming 53-week years include 2015, 2020, and 2026. These long years matter for retail and finance because many companies use ISO weeks or a variant (4-4-5 calendars) for reporting, and a 53-week year adds an entire extra reporting period that must be planned for.
Where week numbers are used in practice
European business culture leans heavily on week numbers. Meetings and deadlines are routinely scheduled as 'week 34' or 'KW 42' in Germany rather than as specific dates, which gives some flexibility within the week while still pinpointing a period. Manufacturing, logistics, and retail planning rely on weekly cadences for production runs, shipping windows, and promotional calendars. Semiconductor and consumer-electronics date codes often encode the year and week of manufacture as a four-digit stamp like 2442 — meaning week 42 of 2024 — printed directly on the component.
Software teams use ISO weeks for sprint tracking and release naming; a release tagged 2025-W14 sorts cleanly and is unambiguous across languages. The year-progress percentage complements the week number by showing how much of the year has elapsed. At the end of ISO week 26 in a common year, roughly half the year is gone, which is a useful pacing check for annual goals. Because ISO weeks are locale-agnostic, they help international teams avoid confusion caused by different weekend conventions — some regions treat Sunday as the first day of the week, but ISO weeks always start on Monday.