Get the ISO 8601 week number for any date.
Tip: use samples, upload, copy, download, and send-to actions inside the workspace where available.
Week Number Calculator is a free, browser-based tool that helps you get the job done quickly. Get the ISO 8601 week number for any date. It's built for speed and privacy: Everything runs locally in your browser — your data is never uploaded to a server. No sign-up, no installs, and no daily limits.
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Date Calculator: Add or subtract days, weeks, months, or years from a date.
Open toolDate Difference: Difference between two dates in years, months, days, and weeks.
Open toolISO Date Converter: Convert any date to ISO 8601, Unix, UTC, and locale formats.
Open toolW01 starts on 2025-12-29 and W53 ends on 2027-01-03, so week-year 2026 spans 371 days — note how it reaches back into 2025 and forward into 2027.
The ISO-8601 rule. Weeks run Monday to Sunday, and week 1 is the week that contains the year’s first Thursday — equivalently, the week containing 4 January, or the first week with at least four days in the new year. Because a week is never split, the days at either end of a year can belong to a neighbouring week-year: 1 January 2021 is a Friday inside 2020-W53, and 28 December 2026 is a Monday that opens 2026-W53. Always quote the week-year with the week number; a bare “W53” is ambiguous.
52 or 53 weeks? A week-year has 53 weeks exactly when 1 January falls on a Thursday, or when it is a leap year whose 1 January falls on a Wednesday — the compact test used here is “1 January is a Thursday, or 31 December is a Thursday”. That happens in 2004, 2009, 2015, 2020, 2026, 2032 and so on, roughly 71 times in 400 years. Everything else has 52 weeks, which is why 52 × 7 = 364 leaves one or two days over each year and the boundary keeps drifting.
The US / broadcast convention is different. Weeks start on Sunday and week 1 is simply the week containing 1 January, so the first week is usually a stub of one to six days and a year can show 53 or even 54 numbered weeks. Retail and TV scheduling use it; ISO is what finance, manufacturing and every European ERP mean by “week 32”. Mixing the two is the single most common source of off-by-one week reporting bugs, so this tool always shows both.
