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Week number calculator

Get the ISO 8601 week number for any date, or the dates spanning a given week.

Input
Output
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Week number calculator
Paste one date per line and this tool adds its week number, weekday and ordinal day of the year — or flip the direction and turn a week number back into the first and last day it covers. It handles a single date or a long list at once, useful for a quick lookup or for tagging a whole spreadsheet export.
By default the week number follows ISO 8601: weeks start on Monday and week 1 is the week containing the year's first Thursday, which is why late December sometimes belongs to week 1 of the following year, and early January can belong to week 52 or 53 of the year before. Switch "Numbering standard" to "Sunday-start week" for the simpler convention some spreadsheets and US calendars use instead, where weeks start on Sunday and week 1 always begins on 1 January, even as a short week. The two can disagree by more than one on the same date, so the tool always shows a four-digit week-year next to the week number rather than assuming the calendar year.
Accepted date formats are YYYY-MM-DD, YYYY.MM.DD, DD.MM.YYYY and DD/MM/YYYY; going the other way, a week reference like "2026-W32" or "2026-32" resolves to its Monday-to-Sunday (or Sunday-to-Saturday) span. Choose how output dates are written — ISO, day-first European, or a spelled-out long date — independently of how you typed the input. Lines that aren't a valid date or week are marked instead of breaking the batch, and the tally under the output shows how many lines converted.
Everything runs locally in your browser: the dates you paste, whether project milestones, invoice due dates or a payroll calendar, are never uploaded anywhere. Copy the result, download it as a .txt file, or send it back into the input to adjust the options and recalculate.
FAQ
Why does a date in late December sometimes show week 1 of the next year?
That is correct under ISO 8601: week 1 is defined as the week containing the year's first Thursday, so if 1 January falls on a Friday, Saturday or Sunday, the days before it still belong to the last week of the previous year, and the days right after a late December weekend can already belong to week 1 of the new year.
What's the difference between the two numbering standards?
ISO 8601 weeks start on Monday and always contain a full 7 days, with week 1 anchored to the year's first Thursday. The Sunday-start standard is simpler but less standardized: weeks start on Sunday and week 1 always begins on 1 January, even when that makes it a short week of just a few days.
What formats can I type a week number in?
The reverse direction reads "2026-W32", "2026 W32", "2026-32" and "2026W32" the same way — a four-digit year and a 1–2 digit week number, with or without the letter W.
Can I convert a whole list of dates at once?
Yes. Paste one date (or one week reference) per line and every line is converted independently; the tally shows how many succeeded and how many could not be read.
Is my data uploaded anywhere?
No. The calculation runs entirely in your browser using standard JavaScript date logic — the dates you enter never leave your device.