Which week of the year is it? The Week Number Calculator answers that question instantly for any date, using the international ISO 8601 week-numbering standard that businesses, schools, payroll systems and project planners rely on around the world. Enter a date and you get the week number, the ISO year it belongs to, and the exact Monday-to-Sunday range of that week.

Week numbers matter more than most people realise. Companies close their books on a weekly basis, so a delivery promised in "week 40" needs a real calendar date attached to it. Schools and universities timetable terms by week number. Holiday planners count weeks to a trip. And anyone coordinating across time zones needs the same week number to mean the same seven days for everybody — which is exactly what ISO 8601 guarantees.

The tool also works in reverse. If you know you need week 32 of 2027, the reverse lookup tells you which Monday and Sunday that is, so you can plan backwards from a week number to actual dates. Everything runs locally in your browser: no dates are sent anywhere.

Reverse lookup: week number → dates

Going the other way? Enter a year and a week number and the calculator returns the exact Monday and Sunday that bound that week, plus the ISO shorthand (for example 2026-W40). If that week number does not exist in the chosen year, the tool says so instead of guessing.

How the ISO week number is calculated

The rule behind the number is simple once you know it: a week belongs to the year that contains its Thursday. Equivalently, week 1 is the week containing the first Thursday of the year. The calculator puts that into practice by taking the date you enter, stepping to the Thursday of the same week, and then measuring how many whole weeks separate that Thursday from the first Thursday of the ISO year.

Working with Thursdays is what makes the system robust. A week that starts in late December and ends in early January never gets split across two years — it is assigned wholly to whichever year owns its Thursday. That is why 29 December 2025 already counts as week 1 of 2026: that week's Thursday is 1 January 2026.

Underneath, every date is normalised to midnight UTC before the arithmetic runs. The JavaScript Date object stores a moment as milliseconds since 1 January 1970, and all calculations here use the UTC variants of the date methods, so a device in Auckland and a device in Los Angeles see the same week number for the same calendar date.

Practical examples

Suppose your supplier quotes a lead time of "week 40, 2026". The reverse lookup shows that week runs from Monday 28 September to Sunday 4 October 2026, so you know the earliest realistic delivery date.

Take a date in the middle of the year: 1 October 2026 falls in week 40 of 2026, and that week spans Monday 28 September to Sunday 4 October. Cross-check it against a printed calendar and the week's Monday matches.

Or check a classic edge case: 1 January 2021 is a Friday, and because its week's Thursday (31 December 2020) belongs to the previous year, that date is actually week 53 of 2020 — not week 1 of 2021. It is the kind of trap that trips up manual counting, and exactly what this calculator is for.

Edge cases and things to watch for

The most common confusion is that the ISO year is not always the calendar year. In the last days of December and the first days of January the two can differ, so a date in 2025 can legitimately sit in week 1 of 2026. The tool always shows both the week number and the ISO year it belongs to, which removes the ambiguity.

Some years have 53 ISO weeks rather than 52 — 2020 and 2026 are recent examples. If you request week 53 in a year that only has 52, the reverse lookup reports that no such week exists instead of silently rolling over to the following year.

Finally, remember that not every calendar in the world numbers weeks this way. Systems built around a Sunday-first week — common in the United States and Canada — can label the same seven days with a different number. If you are comparing against a US-style calendar, expect the numbers to differ by one in many weeks of the year.

Where week numbers are used

In Europe the ISO week number is part of everyday business language: invoices are dated by week, production schedules are planned in weeks, and schools issue timetables as "week 36 to week 41". Payroll and accounting teams frequently close periods on week boundaries because a 52- or 53-week year divides more evenly than months do.

Project managers use week numbers to align teams spread across countries, since a week number is unambiguous in a way that "the week of the 12th" is not. Retailers plan promotions around Black Friday week, logistics firms book capacity by week, and software teams often tie sprint numbering to the ISO week for the same reason.