Free Week Number Calculator Online

Week Number Calculator: Find the ISO-8601 Week Number for Any Date

A week number calculator answers a deceptively tricky question: which numbered week of the year does a given date fall in? This tool follows the ISO-8601 standard specifically, where weeks run Monday through Sunday and the year's first week is defined as whichever Monday-to-Sunday span contains that year's first Thursday — not simply "the week containing January 1st," which is a common but incorrect shortcut that produces a different answer several times each decade.

The interface itself is intentionally plain: a text field on the left where you type a date, and a results pane on the right that fills in automatically as you type, listing the parsed date, its ISO week string, its day-of-year count, its calendar quarter, and whether that year is a leap year. Everything computes locally once you pause typing — there's a short debounce so it doesn't recalculate on every keystroke — and the same calculation is also exposed as a metered API endpoint for software that needs the identical week number programmatically.

How to Look Up a Week Number

Using the calculator takes one step, with results appearing automatically:

1. Type a date into the left-hand pane using the "YYYY-MM-DD" format, either directly or by pasting one in.
2. A short pause after you stop typing — real-time auto-convert is on by default, though it can be switched off in favor of a manual button — triggers the calculation.
3. The right-hand pane fills with a plain-text summary: the parsed date, the ISO week written as a "2026-W01"-style year-and-week string, the day-of-year count (1 through 365 or 366), which calendar quarter the date falls in, and a Yes/No leap-year flag.
4. A copy button next to the pane sends that whole text block to your clipboard in one click, and a save button downloads it as a plain text file.
5. Entering a date that doesn't exist on the calendar, like February 30th, returns a clear message saying so rather than a wrong or blank week number.

Where Week Numbers Actually Get Used

ISO week numbering shows up in more everyday contexts than most people expect:

• Payroll and timesheet reporting. Many companies, especially outside the US, log hours and generate reports by week number rather than by calendar date range, so "Week 27" is a standard label on a pay stub or timesheet system.
• Agile and sprint planning. Two-week sprint cycles are frequently named or tracked by the ISO week their sprint boundary falls on, giving teams a consistent numbering scheme that doesn't drift depending on which day of the month a sprint happens to start.
• Manufacturing and food "week codes." A lot code stamped on packaging or a product's date-of-manufacture code often encodes the ISO week and year rather than a full calendar date, since it's shorter and standardized across suppliers.
• Invoicing and purchase-order numbering. Some accounting systems, particularly at European businesses, build week numbers directly into document reference numbers as a compact, sortable identifier.
• Statistical and academic reporting. Public health dashboards, weather services, and research datasets commonly bucket data by ISO week so figures stay comparable across different years' calendars.

Why January 1st Doesn't Always Fall in Week 1

This is the single most common source of confusion around ISO week numbering, so it's worth walking through with real examples. January 1, 2026 happens to land on a Thursday, and since the ISO rule defines week 1 as the week containing that year's first Thursday, January 1, 2026 sits squarely in 2026-W01 — the intuitive case where the new year and its first ISO week line up. But January 1, 2027 lands on a Friday, and a Friday's Monday-to-Sunday span starts three days earlier, in late December 2026 — so instead of kicking off a fresh week 1, that date actually falls in the final week of the previous ISO year, 2026-W53. That also makes 2026 one of the years with a 53rd ISO week rather than the more typical 52, since its calendar spans just enough extra days to need one.

Where January 1st lands	ISO week assignment
Monday, Tuesday, Wednesday, or Thursday	Falls in week 1 of that same calendar year
Friday, Saturday, or Sunday	Falls in the final week (52 or 53) of the PRIOR calendar year's ISO week-year

The reverse situation happens at the other end of the year too: the last few days of December can belong to week 1 of the FOLLOWING year's ISO week-year if December 31st happens to fall early in that Monday-to-Sunday span. This is exactly why the calculator reports a separate "ISO year" alongside the week number rather than assuming it always matches the calendar year printed on the date you typed — for a handful of dates every year, the two genuinely differ.

Common Mix-Ups With Week Numbering

A few habits carry over incorrectly from other systems. Some scheduling software and US-centric calendars count weeks starting Sunday rather than Monday, which shifts where a week boundary falls compared with the strict ISO-8601 rule this tool applies. Assuming every year has exactly 52 weeks is another common slip — the ISO calendar actually alternates between 52-week and 53-week years depending on where January 1st and leap days fall, so a "week 53" showing up in a report isn't a bug. And treating "the week of January 1st" as automatically week 1 skips the actual rule entirely, which is precisely the mistake the first-Thursday definition exists to prevent.

How ISO Week Numbers Compare to Other Week-Counting Systems

ISO-8601 isn't the only week-numbering convention in circulation, and mixing it up with a different one is an easy way to end up with a number that doesn't match a colleague's spreadsheet. Some US government and retail calendars use a "week of year" count that simply treats January 1st as always falling in week 1, regardless of which weekday it lands on, and counts forward from there in plain seven-day blocks — a simpler rule, but one that drifts out of sync with the ISO-8601 numbering this calculator uses by a full week in many years. Spreadsheet programs often make the confusion worse by offering several WEEKNUM-style modes within the same formula, only one of which actually matches the ISO-8601 rule implemented here; the others use a Sunday-start or January-1-anchored scheme instead, and picking the wrong one from a dropdown produces a number that looks perfectly plausible while being off by one for a meaningful share of the calendar.

Retailers and some manufacturers go a step further and use what's often called a 4-4-5 fiscal calendar, which groups weeks into quarters of four, four, and five weeks rather than tracking a continuous ISO week count through the whole year — a completely different system built for consistent quarterly comparisons rather than calendar-date lookups, and not something this calculator attempts to reproduce. If a business report or fiscal document references a week number, it's worth confirming which of these systems it's actually using before comparing it against the strict ISO-8601 figure returned here.

What the API Sends and Returns

The endpoint behind this page takes a single required parameter, a date string in the same "YYYY-MM-DD" format the text pane accepts, and returns four fields: the parsed date echoed back, the ISO week-year, the ISO week number itself, and a combined isoWeekString like "2026-W01" that matches exactly what the results pane displays. Keeping the week-year and week number as separate fields, rather than only the combined string, makes it straightforward for a calling application to sort or group records by week-year first and week number second without parsing the combined string back apart.

As with the other date tools on this site, an unparseable or nonexistent calendar date — a malformed string, or a real-looking date like April 31st that doesn't actually exist — returns a descriptive error rather than a best-guess result, so a script calling the endpoint in bulk can tell a genuine bad input apart from a valid one without sanity-checking every response by hand.

Working Out a Week Number by Hand vs. Using This Calculator

The ISO rule can be applied manually, but a few approaches are more error-prone than others:

• Counting weeks from January 1st on your fingers. Fast for a rough guess, wrong whenever the year in question doesn't happen to start on a Monday, which is most years.
• A paper or wall calendar with printed week numbers. Convenient if the calendar's printer used the correct ISO rule, but some consumer calendars print a simplified, non-standard numbering that won't match official ISO-8601 reporting.
• A spreadsheet's built-in week-number formula. Many spreadsheet programs offer more than one week-numbering mode, and picking the wrong mode from the dropdown produces a plausible-looking but ISO-incorrect result.
• Deriving it from the day-of-year count. Dividing the day-of-year number by seven gets close but doesn't correctly account for the first-Thursday rule at either end of the year, so it drifts by one in exactly the boundary cases this page exists to get right.
• This calculator's API. A payroll system, a manufacturing lot-code generator, or any backend that needs the same ISO week-year and week number for a date can request it as a metered endpoint rather than reimplementing the first-Thursday rule in-house; see the API reference for the request format.

Why did a date on January 1st show up as week 52 or 53 of last year instead of week 1?

That happens whenever January 1st lands on a Friday, Saturday, or Sunday: the ISO rule assigns that date to whichever Monday-to-Sunday span already contains it, and that span belongs to the previous ISO week-year rather than starting a fresh week 1.

Do ISO weeks start on Monday or Sunday?

Monday. This is one of the defining differences between ISO-8601 week numbering and the Sunday-start convention used by some US calendars and spreadsheet functions, and it's the reason the two can disagree on which week a given date belongs to.

Can a year really have 53 weeks instead of 52?

Yes. A calendar year has 53 ISO weeks whenever January 1st falls on a Thursday, or in a leap year, when it falls on a Wednesday — both situations push enough extra days into the year's span to require a 53rd week rather than fitting neatly into 52.

Is the "ISO year" shown here ever different from the calendar year?

Yes, for a small number of dates each year — specifically the very end of December and the very start of January, when the Monday-to-Sunday week a date falls in crosses over the calendar-year boundary. The calculator reports the ISO year separately from the date's own calendar year specifically to surface that difference.

Related Time Tools

Once you know which ISO week a date belongs to, the day of week calculator pins down exactly which weekday it is. For shifting a date forward or backward by a set number of weeks, or finding the next occurrence of a particular weekday, the date pattern calculator handles that arithmetic, and the age on date calculator covers a related but distinct kind of date math: working out someone's age as of a chosen day.