# ISO Week Number Find the ISO-8601 week number for any date — plus the week’s Monday-to-Sunday range and the day of the year. Runs entirely in your browser. Week runs Mon, 20 Jul 2026 → Sun, 26 Jul 2026. ### How it works Under ISO 8601, weeks start on **Monday** and week 1 is the week containing the year’s first Thursday. The number is found by taking the Thursday of your date’s week and counting weeks from the first Thursday of that ISO year — which is why early-January or late-December dates can belong to the previous or next year’s week count. All computed locally. --- *Source: [ocxly.com/week-number.html](https://ocxly.com/week-number.html) — OCXLY, free 100% client-side privacy-first tools. Free ISO week number calculator. Find the ISO-8601 week number for any date, with the Monday-to-Sunday range and day of the year, entirely in your browser.*