About Week Number
Week Number returns the ISO 8601 week number for a date, formatted as year and week — for example 2024-W24. That standard is the one most business, payroll and logistics systems mean when they talk about weeks, and it is not the same as counting how many weeks have elapsed since the first of January.
The ISO definition is specific: weeks begin on Monday, week one is the week containing the first Thursday of the year, and a date in early January can therefore belong to the final week of the previous year. Because of that, the year shown in the result may differ from the calendar year of the date you entered. That is correct behaviour, not an error, and the tool prints the ISO year alongside the week precisely so it is unambiguous.
This matters most around New Year and when you are aligning with a system that reports in weekly buckets. If your organisation instead numbers weeks from January 1st, the figure here will disagree, and you should follow whichever convention your system expects.
How to use Week Number
- 1 Enter the date in YYYY-MM-DD format.
- 2 Run the tool to see the ISO year and week, such as 2024-W24.
- 3 Use the result when matching a system that follows the ISO 8601 week convention.
Frequently asked questions
Why does the year in the result sometimes differ from my date's year?
Because under ISO 8601 a few days at the start of January can fall into the last week of the previous year, and a few at the end of December into week one of the next. The tool reports the ISO year with the week to make that clear.
Which day does the week start on?
Monday. That is part of the ISO definition and not the Sunday-first convention used in some regions, which is why the week numbers here will not match a Sunday-based calendar.
Is week one always the week containing January 1st?
No. It is the week that contains the first Thursday of the year. Depending on where January 1st lands, week one can begin in late December of the previous year.
How many weeks can a year have?
Either 52 or 53. Long years that begin on certain weekdays gain a 53rd week, which is why the maximum is not fixed at 52.
Free, online, and private
Free to use
Week Number is completely free. There is no account to create, no login, and no hidden charge or usage limit.
Runs online in your browser
It is a free online tool that runs entirely in your browser. Nothing to download or install — open the page and start using it right away.
Your data stays private
All processing happens locally on your device. Your input is never uploaded, stored, or tracked, so it never leaves your browser.
How Week Number behaves in detail
These are the specifics of this implementation — the exact ranges, defaults and fixed settings you are working with.
| Parameter | Value |
|---|---|
| Input | one date as text, default 2024-06-15 |
| Output | an ISO week as YYYY-Wnn, zero-padded — the default gives 2024-W24 |
| Algorithm | ISO 8601, using the Thursday-of-the-week rule, so weeks always run Monday to Sunday |
| Year in the answer | may differ from the year typed — 2024-12-30 reports 2025-W01 |
| Weekday basis | the local year, month and day are re-assembled as a UTC date before the week is counted |
Compiled from the implementation of this tool and checked against it by itoolnet.
What Week Number does not do
Knowing the edges of a tool saves more time than another list of its features. These are the cases where it will not do what you might expect.
- The answer is ISO 8601 only. US-style numbering, where week 1 starts on 1 January, and the casual day-of-year-divided-by-seven count are not offered, so the figure will not match a US wall calendar and near New Year the year shown can be next year.
- Only the week is reported. The weekday number, the day of year, the Monday and Sunday that bound the week and the quarter are all missing, and there is no week-to-date direction on the same page — a separate tool goes the other way.
- The week follows the local calendar date rather than the instant. A date-only entry parsed as UTC midnight lands on the previous day west of UTC, and for a Monday that drops the result back into the preceding ISO week — and into December of the previous year at the start of January.
Tested and documented by itoolnet, the team that builds and maintains this site.