Free Online Week Number Calculator

1. Pick a date with the date picker.
2. Read the ISO 8601 week number (the European/business standard).
3. Cross-check against the US and Middle East week numbers if you need to.
4. Note the day-of-year, quarter and weekday for additional context.

Different parts of the world number weeks differently, and the convention isn't always specified when someone hands you a "week 23" reference. ISO 8601 — the standard outside the US — starts weeks on Monday and counts week 1 as the one containing the first Thursday of the year. The US system starts weeks on Sunday and labels week 1 as the one containing January 1. The Middle East system also anchors week 1 to January 1 but starts weeks on Saturday.

Payroll administrators reconcile weekly run reports against ISO weeks; engineering teams plan sprints by ISO week; logistics planners coordinate with retailers using US-style weeks. This tool computes all three at once so you can match whichever counterparty you're talking to. It also reports the day-of-year (1–366), the calendar quarter and the weekday name — useful when slotting an event into a project plan or fiscal calendar.

All calculations are local. Use the week number for project tracking, payroll periods, ISO date strings (e.g. 2026-W23) and rota planning.