Which week of the year is this?
ISO 8601 is the reason 1 January can belong to the previous year's week 52 — this shows the number the calendar apps and payroll systems actually use.
How Week Number Calculator works
Dates are computed from your device's clock and timezone settings, both of which stay on the device. Nothing about when you opened the page is reported. Daylight-saving rules come from your browser's own database, so the conversion follows your region rather than a server's guess.
That is the whole architecture: static files, one script, no upload endpoint to leak or log. Close the tab and the work is gone.
Good to know
- Works offline once loaded, including on flights and in locked-down networks.
- No file size limit beyond your device's own memory.
- Free to use, with no account and no usage counter.
- Results are produced locally, so they are not logged or shared with anyone.
Questions
Why does 1 January sometimes belong to week 52?
ISO weeks follow the first Thursday. If 1 January falls on a Friday, the first week of the new year starts on 4 January, and the earlier days belong to the old year's last week.
Is this the same as the US week number?
No. The US convention starts weeks on Sunday and starts counting on 1 January, which can put the two systems one week apart. This page reports ISO.
Which countries use ISO weeks?
Most of Europe, and most payroll and logistics systems everywhere. It is the numbering used in ISO date format, which is what `2026-W46` means.
Can I go the other way, from week to date?
Pick the Monday shown in the result — the week span is printed for exactly that reason.
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