ISO Week Number Converter
Convert any Gregorian date to its ISO-8601 week number, week-year, weekday, and Monday-Sunday range—or convert an ISO week back to dates. Free and private.
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What is ISO Week Number Converter?
The ISO Week Number Converter is a free browser-based tool for translating Gregorian dates into ISO-8601 week-year and week values, or converting an ISO week back into its Monday-Sunday date range.
Key Benefits
- Use one international week convention across teams and calendars
- Avoid mistakes around New Year week-year boundaries
- See each week’s exact Monday and Sunday dates
- Keep dates private with fully client-side calculations
Common Use Cases
- •Labeling software sprints and release windows by ISO week
- •Converting an ISO week from a report into calendar dates
- •Checking whether a year has an ISO week 53
- •Reconciling calendar dates across international teams
How to Use the ISO Week Number Converter
- Choose a direction: Select Date to ISO week or ISO week to dates depending on the value you already have.
- Enter the date or week: Pick a Gregorian date, or type an ISO week-year and week number.
- Read the ISO result: Review the ISO week code and the exact Monday and Sunday that bound the week.
- Copy the result: Copy the localized summary into a sprint plan, release note, or calendar.
Key Features
- Convert Gregorian dates to ISO-8601 week numbers
- Convert ISO week-year and week back to a date range
- Show the exact Monday and Sunday of each week
- Validate 52-week and 53-week ISO years
- Handle New Year boundaries with UTC-safe arithmetic
Frequently Asked Questions
What is an ISO week number?
ISO 8601 numbers weeks from Monday through Sunday. Week 1 is the week containing January 4, which is also the first week with at least four days in the new year.
Why can January 1 belong to the previous ISO year?
If January 1 falls before the Thursday of ISO week 1, it remains part of the final ISO week of the previous week-year.
Which ISO years have week 53?
An ISO week-year has 53 weeks when it starts on a Thursday, or when a leap year starts on a Wednesday. The converter validates this automatically.
Are my dates uploaded?
No. Date and week calculations run entirely in your browser using UTC calendar arithmetic.