Week Number

Week Number Calculator - ISO Week & Day of Year

Quickly find the ISO week number for any date with our Week Number tool. Also shows day of year, days remaining, quarter, and day of week.

Select any date using the calendar picker to instantly see its ISO week number along with additional temporal information. The display shows which week of the year it is (1-53), the day number within that year (1-366), how many days remain until year end, which fiscal quarter it falls in, and the day of the week name. All calculations follow the ISO 8601 standard where weeks start on Monday and the first week contains January 4th. Tuttilo processes the date locally in your browser using standard date calculation methods.

Manufacturing facilities schedule production runs using ISO week numbers to align with supplier shipment calendars and avoid confusion across international teams. Project managers working with European clients adopt ISO week numbering to match their scheduling conventions accurately. Retail analysts aggregate sales data by calendar week rather than month to spot trends independent of varying month lengths. Software release cycles planned on weekly sprints reference ISO weeks for clear communication across global development teams.

Remember that ISO weeks can span two calendar years—week 1 of 2026 might include days from late December 2025. Some years have 53 weeks instead of the usual 52, occurring when January 1st falls on a Thursday or leap years where it falls on Wednesday. Use consistent week numbering systems within your organization to avoid confusion between ISO and US conventions where weeks start Sunday. Export data with both week number and year to prevent ambiguity in historical records spanning multiple years.

All processing happens directly in your browser. Your files never leave your device — no server uploads, no cloud storage, no data retention. The tool works offline once loaded, requires no registration, and is completely free with no usage limits.