Date Calculator
Add days, weeks or months to a date, or count the gap between two dates.
Date & Time Calculators
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Week Number Calculator
The ISO week number for a date, and back again.
Week Number Calculator tool
Used when working the other way.
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ISO week
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That week runs
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Day of the year
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Quarter
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Weeks left in the year
| Measure | Value |
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ISO weeks start on Monday, and week 1 is the week containing the first Thursday of the year. That means early January can fall in week 52 or 53 of the previous year, which is correct rather than a bug.
About the Week Number Calculator
Week Number Calculator gives the ISO 8601 week number for any date, and works backwards from a week number to the dates it covers. ISO weeks start on Monday, and week 1 is the week containing the first Thursday of the year -- equivalently, the week containing 4 January.
That definition produces the behaviour people report as a bug and which is in fact correct: the first days of January can belong to week 52 or 53 of the previous year, and the last days of December can belong to week 1 of the next. A date's ISO year is therefore not always its calendar year, which is why both are shown.
Only some years have 53 weeks -- those whose 1 January falls on a Thursday, or on a Wednesday in a leap year. Asking for week 53 of a 52-week year returns an explanation rather than a wrong date, which is the kind of mistake that silently shifts a whole reporting calendar.
The page shows its working, not just its answer. Where there is a formula it is printed with your own numbers substituted in, so you can check the result rather than take it on trust -- which is the difference between a calculator you can use for something that matters and one you cannot.
Everything runs inside your browser. What you type is never uploaded, never logged and never stored, and the calculator keeps working if your connection drops after the page has loaded.
Looking for something this does not cover? FreeToolsHub.org is another hub of free online tools that need no account, and it is worth a look if week number calculator is only part of what you are working on.
How to use the Week Number Calculator
- Choose work outThe fields below change to match what you pick, so only the figures that matter to your case are asked for.
- Enter date, week number and yearDecimals are fine, and thousands separators are accepted and ignored.
- Read the iso week, that week runs and day of the yearThe figures update as you type. The Calculate button is there if you would rather press something, but nothing waits for it.
- Copy or check the resultCopy the summary with the copy button, or open the table to check it row by row.
Features
- Shows iso week, that week runs, day of the year, quarter and weeks left in the year together, so you are not running it twice
- A results table you can read across
- Updates as you type, with no button to press and no page reload
- Copy the result in one click
- Free, no sign-up, and it runs entirely in your browser
Who uses this tool
The week number calculator is used by writers, developers, students, marketers and anyone else who needs the job done once without installing software. Common cases include:
- Finding the week number for a reporting calendar
- Converting a week number into its date range
- Checking which quarter a date falls in
- Working out the day of the year
Frequently asked questions
Why is 1 January sometimes in week 53?
Because ISO week 1 is the week containing the first Thursday. If 1 January falls on a Friday, Saturday or Sunday, it belongs to the final week of the previous ISO year.
Do all years have 52 weeks?
No. Years whose 1 January is a Thursday, or a Wednesday in a leap year, have 53. The calculator refuses week 53 for years that do not have one.
Is this the same as the American week numbering?
No. Some US systems start weeks on Sunday and count week 1 from 1 January regardless. This is the ISO standard, which is what Europe, business software and most calendars use.
Related tools
If the week number calculator is not quite what you need, these other free tools solve closely related problems, and there are 13 more in date & time calculators.
Day of the Week Calculator
The weekday for any date, past or future.
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Business Days Calculator
Count working days between dates, or add them to one.
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Days Until Calculator
Countdown to any date in days, weeks and working days.
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Leap Year Calculator
Check any year for a leap year, with the rule explained.
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Checked and rebuilt on 2026-10-14. The method this calculator uses is described in the section above, and corrections go through the contact page.