Week Number Calculator (ISO 8601)
The ISO week number for any date (including the current week) with the week's Monday-to-Sunday range shown.
- Week runs
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- Weekday
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- Weeks left in year
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- ISO weeks this year
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Note: this date's ISO week belongs to a different year than its calendar date, normal for dates around New Year.
Free forever, no sign-up and no limits, and this tool installs on its own so you can keep it on your home screen.
Today's ISO week number is on screen the moment the page opens; pick any other date to get its week, the exact range it covers, and how many weeks the year has left. Weeks follow the ISO 8601 standard used across European business, with the US Sunday-start count one tap away.
How to use it
- Read this weekThe current week number loads by default, for many visits that is the whole job.
- Or pick a dateAny date, any year: delivery weeks, planning weeks, an old invoice.
- Check the rangeThe week's exact Monday–Sunday span is shown, which settles which days 'week 43' actually means.
The calendar unit business actually runs on
Across European industry, dates are commitments in week numbers: delivery in week 12, production freeze week 30, campaign launch week 45. Weeks work where months fail because every one is exactly seven days, Monday to Sunday, with no short February to distort a schedule. The cost of that regularity is that nobody carries the mapping between weeks and dates in their head, hence this page.
The Monday–Sunday range under the answer is deliberately prominent. Most week-number disputes are really range disputes, "week 43" is only useful once both sides agree on which Monday it starts.
The New Year seam
The one place ISO weeks surprise people is the year boundary. Week 1 is the week holding the year's first Thursday, so up to three days of early January can belong to the old year's final week, and the last days of December can belong to the new year's week 1. Payroll and reporting systems that join data on week numbers hit this seam every January; the tool prints the week-year explicitly and warns whenever it differs from the calendar year, because that mismatch is precisely the bug worth catching.
ISO versus everyone else
If a US colleague's planner disagrees with yours by one week, neither calendar is broken. American conventions often start weeks on Sunday and number from 1 January. The two systems agree most of the year and drift at the edges. When week numbers cross the Atlantic, saying "ISO week 47" instead of "week 47" costs four letters and prevents the whole argument.
The toggle above the result switches between the two counts, and a note appears whenever they disagree for the date you picked, which is the moment the difference actually costs money. The stats also say whether the year has 52 or 53 ISO weeks, a figure worth checking before building a yearly plan on week numbers: a 53-week year breaks any schedule that assumed 52.
Questions people ask
What week number is it now?
It is the first thing the tool shows, no clicking needed. Week numbers run 1 to 52 or 53, starting on Mondays, and roughly track month × 4.3 if you want a sanity check.
How is week 1 decided?
ISO 8601 defines week 1 as the week containing the year's first Thursday, equivalently, the week of 4 January. The practical effect: 1–3 January can belong to the previous year's last week, and 29–31 December can belong to week 1 of the next year. The tool flags when that applies to your date.
Why does my date show a different year's week?
Because ISO weeks have their own year, the week-year, which shifts at the boundary. 1 January 2021 was a Friday, so it sat in week 53 of 2020. It looks like a bug and is the standard working exactly as specified, the flag under the result explains it whenever it happens.
Do all countries number weeks the same way?
No. Europe, and ISO 8601, start weeks on Monday with the first-Thursday rule. North American calendars often start weeks on Sunday and call the week containing 1 January week 1, which can differ by one from the ISO number. This tool computes both: it opens in ISO, and the US toggle switches to Sunday-start counting. Whenever the two disagree for your date, a note under the result says so.
Which years have 53 weeks?
Years that start on a Thursday, and leap years that start on a Wednesday, about every five to six years. 2020 had 53; 2026 has 53. The 'weeks left' figure accounts for it automatically.
Why do businesses plan in week numbers at all?
Weeks are the one calendar unit with constant length, which makes them better than months for production schedules and delivery promises. 'Week 47' is unambiguous across languages and time zones in a way '20 November-ish' is not, provided everyone uses the same standard, which is what ISO is for.