ISO Week Number Calculator
Updated September 2026
Find the ISO week number for any date, along with day of year, day of week, and days remaining in the year. Free online week number calculator.
Time calculations — deadlines, scheduling across time zones, date arithmetic — are surprisingly error-prone when done manually. The Week Number Calculator makes them instant and accurate.
Find the ISO week number for any date, along with day of year, day of week, and days remaining in the year. Free online week number calculator. Results are immediate — no waiting, no loading screens, no account required.
Who uses the Week Number Calculator?
- Project managers referencing weekly sprint numbers and fiscal reporting periods.
- HR professionals planning quarterly and annual scheduling using week references.
- International teams coordinating on ISO week numbers for consistent scheduling across regions.
Common uses and search terms
The Week Number Calculator is frequently used for: week number, iso week, calendar week. Whether you need a quick result or are working through a longer project, the tool handles all common variations without any configuration.
Why use Allio Tools's Week Number Calculator?
Everything runs locally in your browser. No data is sent to a server, logged, or stored anywhere. The tool is completely free with no usage limits, no premium tiers, and no account required. Available in 11 languages and optimised for all screen sizes, the Week Number Calculator works on desktop, tablet, and mobile with no app download needed.
See also:Time Zone Converter · Online Stopwatch · Date Calculator
How the Week Number Calculator works
Enter any date — it defaults to today and calculates automatically on page load.
Click Calculate after changing the date to update the results.
View the ISO week number, day of the year, days remaining in the calendar year, and day of the week.
There's no reverse lookup — you can't enter a week number and get back its date range.
Tips & tricks
- "Days Remaining" is days left in the calendar year (365/366 minus day-of-year), not weeks remaining and not related to the ISO week count.
- There's no reverse lookup — this tool only goes date → week number, not week number → date range.
- The tool calculates automatically on page load using today's date, then recalculates whenever you click Calculate after changing the date.
- Week numbering follows ISO 8601 (Monday-start weeks, week 1 contains the year's first Thursday) — this can differ from US convention, where week 1 is often just the week containing January 1.
Frequently Asked Questions
What is the ISO week number and how is it calculated?
ISO 8601 defines week 1 as the week containing the year's first Thursday. Weeks run Monday–Sunday. This means January 1 might be in week 52 or 53 of the previous year if it falls on a Friday, Saturday, or Sunday.
Why is January 1 sometimes in week 52 of the previous year?
If January 1 falls on a Saturday or Sunday, the Thursday of that week is in December. So that week belongs to the previous ISO year. January 1, 2022 was a Saturday — so it was in ISO week 52 of 2021.
Does this tool show the full date range (start–end) of the selected week?
No — it shows the ISO week number itself, plus day of year, days remaining in the year, and day of week, but it doesn't display the Monday–Sunday date range for that week.
Can I look up which dates fall in a specific week number?
No — there's no reverse-lookup mode. You can only enter a date and get its week number, not enter a week number and get the corresponding date range.
How many weeks are in a year, and does this tool show that?
Most years have 52 ISO weeks; years where January 1 or December 31 falls on a Thursday have 53. This tool doesn't display a "total weeks this year" or "weeks remaining" figure — its "Days Remaining" stat counts days left in the calendar year, not weeks.
Is my data safe when using the Week Number Calculator?
Completely safe. The Week Number Calculator processes everything locally in your browser. No input data, results, or usage information is ever transmitted to or stored on any server.
Does the Week Number Calculator automatically account for daylight saving time?
Yes. The Week Number Calculator uses your browser's built-in time zone data, which includes daylight saving time rules for all supported time zones, keeping results accurate year-round.
How does the Week Number Calculator handle daylight saving time and leap years?
The Week Number Calculator uses your browser's built-in Intl and Date APIs, which automatically account for daylight saving time transitions, leap years, and regional time zone rules. You do not need to make any manual adjustments for these calendar events.
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