Week Number Calculator
Find the ISO week number for any date, or look up the date range covered by a specific week number and year.
Or look up the date range for a week
Jan 1 – Jan 7, 2024
Monday through Sunday
Recommended Settings
Pro Tips
- •This uses the ISO 8601 week numbering standard, where week 1 is the week containing the year's first Thursday, and weeks run Monday to Sunday
- •Because of this rule, the last few days of December or the first few days of January can belong to a week number from the adjacent year
- •Most years have 52 weeks, but years where January 1 falls on a Thursday (or it's a leap year starting on Wednesday) have 53
- •This differs from the US convention some calendars use, where weeks start on Sunday
Most Popular
Most users check the current week number for sprint planning or timesheet purposes
When to Use This Tool
Reference sprints or milestones by ISO week number instead of specific dates.
Identify which week number a timesheet or payroll period corresponds to.
Decode or generate week-based date codes used in manufacturing and logistics.
Reference academic terms or fiscal periods that are organized by week number.
How It Works
Find the Thursday of the week containing your chosen date, per the ISO 8601 rule
Count how many weeks have elapsed since the first Thursday of that Thursday's year
For the reverse lookup, find the Monday that starts week 1 of the given year, then count forward to the requested week
100% Private
Files never leave your device. All processing happens locally in your browser.
Lightning Fast
Powered by Client-side ISO 8601 week date calculation for optimal performance on modern browsers.
Open Source
Built with verified, open-source libraries. Fully transparent.
Frequently Asked Questions
What is ISO week numbering?
It's an international standard (ISO 8601) where weeks run Monday to Sunday, and week 1 of a year is defined as the week containing that year's first Thursday.
Why does a date near January 1 sometimes show a week number from the previous or next year?
Because ISO weeks are defined by which year contains 'their' Thursday, a date like December 31 can technically fall in week 1 of the following year if that week's Thursday is in January.
How many weeks are in a year?
Most years have 52 weeks, but some years have 53, depending on which day of the week January 1 falls on and whether it's a leap year.
Is my data sent to a server?
No. All calculation happens entirely in your browser.
Does this match the week numbers in my calendar app?
Most calendar apps that show week numbers use the ISO 8601 standard by default, but some regional settings (particularly in the US) use a different, non-ISO week numbering convention.