Week Number Calendar
Browse a calendar with ISO week numbers built in, or jump straight to a date or a week number — for whenever someone says "let's meet in week 34."
Related tools
"Let's ship it by week 34" only works as a plan if everyone agrees on what week 34 actually is — and plenty of tools disagree on where a week starts and which week counts as the first one. This calendar uses the ISO 8601 standard, the one used across most of Europe and in business and project planning generally, and lets you move between a date and its week number in either direction.
What "ISO week number" actually means
ISO 8601 — the same international standard behind the YYYY-MM-DD date format — defines two rules that fix most week-number confusion at once: every week starts on Monday, not Sunday, and week 1 of a year is whichever week contains that year's first Thursday. Both rules exist for the same reason: to guarantee every week belongs to exactly one calendar year with no ambiguous partial weeks at the boundaries, which is also why a handful of dates in late December or early January end up in a week number that seems to belong to "the wrong year."
Why week 1 sometimes starts in December
Because week 1 is defined as the week containing the year's first Thursday, and weeks run Monday to Sunday, week 1 can start as early as December 29 of the previous year. For example, if January 1st falls on a Tuesday, Wednesday, or Thursday, the Monday that starts that week is still in December — but the week itself is week 1 of the new year, not the last week of the old one. (If January 1st falls on a Friday, Saturday or Sunday, it's the other way round: those first days belong to the last week of the previous year.) This is exactly the kind of edge case that causes disagreements when people say "week 1" without specifying the year.
Why some years have a week 53
Most years have 52 ISO weeks, but a year has 53 whenever it starts on a Thursday, or whenever it's a leap year that starts on a Wednesday — both cases where the extra day(s) form a whole additional week rather than folding into week 52 or the next year's week 1. This calendar handles that automatically: entering a week number a given year doesn't actually have shows an error rather than silently rounding to the wrong date.
Where week numbers actually get used
Week numbers show up constantly in contexts where "the 14th" is ambiguous without a month, but "week 34" is unambiguous for an entire year at a glance: project and sprint planning, payroll and timesheets (many companies pay biweekly by ISO week), manufacturing date codes stamped on products, school and academic term schedules across much of Europe, and shipping or delivery estimates quoted as "arrives week 22." Once you're used to thinking in week numbers, they're a genuinely faster way to reason about a whole year than juggling twelve separate months.
Frequently asked questions
Does the week start on Monday or Sunday?
Monday, under the ISO 8601 standard this calendar uses. That's different from the US convention, where calendars and some software often start the week on Sunday — which is the single most common source of "our week numbers don't match" confusion.
Why does my phone's calendar show a different week number than this one?
Some calendar apps use a simpler, non-ISO counting method (for example, just counting Sundays since January 1st) rather than the ISO 8601 rule based on the year's first Thursday. The two methods can disagree by one, especially in early January or late December — check your app's settings for an "ISO 8601" or "start week on Monday" option if you need them to match.
Is my data uploaded anywhere?
No — every calculation happens locally in your browser using plain date arithmetic. Nothing you enter or click here is sent to a server.
What happens if I enter a week 53 for a year that doesn't have one?
The calculator shows an error telling you how many weeks that year actually has (52 or 53) instead of silently returning a date in the wrong week — most years top out at 52.
Can I use this to find out what week a past or future date falls in?
Yes — the "Jump to a date" field accepts any date, past or future, and immediately shows its ISO week number and navigates the calendar there.