Week Calculator
About this Tool
The Week Calculator on AllThatConverter converts a date to its ISO 8601 week number (weeks start on Monday) or looks up the Monday–Sunday range for a given year and week. It handles year-end and year-start boundaries correctly, and also shows whether that ISO year has 52 or 53 weeks.
Processed safely on your device
Everything happens in your browser on this device. Your files are never sent to our servers—so you can use it with peace of mind.
How to Use
Choose 'Date → Week' or 'Week → Date' at the top.
In Date → Week mode, enter a date to see the YYYY-Www ISO week and that week's Monday–Sunday range.
In Week → Date mode, enter a year and week number to see the Monday-through-Sunday dates for that week.
Key Scenarios & Use Cases
Labeling weekly reports and sprint schedules
Look up the ISO week number and the exact start and end dates to use in project or work reports.
Checking which week Dec 31 or Jan 1 belongs to
Avoid confusion when a calendar date falls in the previous or next ISO week-year.
Planning around 53-week ISO years
See whether a year has 53 ISO weeks and what date range the last week covers.
Frequently Asked Questions (FAQ)
ISO 8601 weeks start on Monday and end on Sunday. Week 1 is the week that contains January 4.
Yes. For example, 31 December 2024 is a Tuesday and belongs to 2025-W01. This tool handles those boundaries automatically.