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What it does
Looks up the ISO 8601 week number, day-of-year, quarter, and weekday for any date. Handles the tricky year-boundary cases correctly: a date near Jan 1 or Dec 31 can belong to a week number from the adjacent calendar year, and this tool gets that right instead of just counting weeks from Jan 1.
How to use it
Paste a date like 2026-08-06 (or a full ISO datetime) into the input, or leave it empty to use today. You get the ISO week, the ISO week-year, day-of-year, weekday name with its ISO number, quarter, days remaining in the year, and the Monday-Sunday date range of that week.
Why this one
Most week-number calculators get the year boundary wrong: they'll happily tell you Jan 1 is week 1 of its own calendar year even when ISO 8601 says otherwise. This one implements the real ISO 8601 rule (week 1 is the week containing the year's first Thursday), runs entirely on your device, and has no ads or rate limits.
FAQ
- Why does Jan 1 sometimes show a week number from the previous year, like W53?
- Per ISO 8601, a week belongs to whichever year contains that week's Thursday. If Jan 1 falls on a Friday, Saturday, or Sunday, that week's Thursday is still in December, so the date counts as the last week (W52 or W53) of the previous year.
- What date formats are accepted?
- ISO 8601 dates like 2026-08-06, or full datetimes like 2026-08-06T21:00:00Z. Only the calendar date is used: time-of-day and offset are ignored so results stay deterministic.
- Is the calculation done in my local time zone?
- No, everything is computed in UTC. This keeps week and day numbers consistent regardless of where you or your server are located.
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