ISO Week Date Converter - ISO 8601 Week Calculator
Instantly convert any Gregorian date to ISO 8601 week notation and convert ISO week dates back to calendar dates with full week ranges.
Updated
What is the ISO Week Date Converter - ISO 8601 Week Calculator?
The ISO Week Date Converter is a complete bidirectional tool for working with ISO 8601 week dates. It converts any Gregorian calendar date to its ISO week year, week number, and weekday, and converts any ISO week date back to a Gregorian date with full week context.
Unlike basic week number calculators, this tool handles the tricky edge cases that cause errors in payroll, accounting, and reporting systems.
What you can do
- Date → ISO: Enter
2024-01-01and instantly get2024-W01-1,2024-W01,2024W011, weekday name, day of year, quarter, and whether the year has 52 or 53 weeks. - ISO → Date: Enter
2024-W05-3or2024W053and get2024-02-01plus the entire Monday-to-Sunday range and a visual 7-day week grid. - Year Overview: See every ISO week in any ISO year from 1 to 9999, with Monday and Sunday dates, highlight for the current week, and export to CSV or JSON.
Why ISO 8601 weeks matter
ISO weeks are used for financial quarters, European payroll, logistics, manufacturing planning, and international reporting because every week has exactly 7 days and week 1 is defined consistently. Getting the year boundary wrong by even one day can shift an entire week into the wrong year.
This converter solves that with:
- UTC-based calculation - no timezone drift
- Year-boundary warnings when calendar year ≠ ISO year (e.g.
2023-01-01→2022-W52-7) - 53-week year detection - automatically shows when a year has 53 weeks
- Multiple input formats -
YYYY-Www-D,YYYY-Www,YYYYWwwD,YYYYWww - Add/subtract weeks calculator for forward and backward planning
- Copy and export - one-click copy for all formats, CSV/JSON download for yearly tables
Perfect for developers, accountants, HR, project managers, and anyone working with week-based calendars.
How it works
The ISO 8601 rules
- Weeks start Monday - Monday = 1, Sunday = 7
- Week 1 is the week with the first Thursday of the year. Equivalent definitions: the week containing Jan 4, or the first week with at least 4 days in the new year.
- ISO year can differ from calendar year for dates near Jan 1 and Dec 31.
- Most years have 52 weeks. A year has 53 weeks if Jan 1 is a Thursday, or Jan 1 is a Wednesday in a leap year.
Date → ISO conversion
Given a Gregorian date Y-M-D:
- Find the ISO weekday:
Mon=1 ... Sun=7 - Find Thursday of that week:
date + (4 - weekday)days. Its year is the ISO year. - Find Jan 4 of that ISO year. Its weekday tells you where week 1 starts:
week1Monday = Jan4 - (weekday_Jan4 - 1)days - Calculate days between
week1Mondayand your date:diffDays = (date - week1Monday) / 86400000 isoWeek = floor(diffDays / 7) + 1
The tool then builds YYYY-Www-D, YYYY-Www, and compact YYYYWwwD formats and computes Monday and Sunday of that week.
ISO → Date conversion
Given isoYear, isoWeek, isoWeekday:
- Find Jan 4 of
isoYearand its weekday week1Monday = Jan4 - (weekday_Jan4 - 1)daystarget = week1Monday + (isoWeek - 1) * 7 + (isoWeekday - 1)days
This returns the exact Gregorian date. The tool then expands it to the full 7-day week.
Year overview
weeksInIsoYear(year) is determined by converting Dec 28 of that year — Dec 28 is always in the last week. If its week number is 53, the year has 53 weeks.
For each week 1..weeksCount, the tool computes Monday (weekday=1) and Sunday (weekday=7) using the ISO→Date algorithm.
How to use
Mode 1 - Date → ISO: Pick a date, see ISO year, week, weekday, all string formats, week range, day of year, quarter, leap year info, and use the weeks offset field to add or subtract weeks.
Mode 2 - ISO → Date: Enter ISO year, week (validated against 52/53), weekday, or paste a quick string like 2024-W05-3 and click Apply. View the resulting Gregorian date and the Monday-to-Sunday grid.
Mode 3 - Year Overview: Enter an ISO year to list all weeks, see first Monday and last Sunday, copy the ISO week list, or download CSV/JSON.
All calculations run 100% client-side.
Examples
Date to ISO week
Convert January 1st 2024 to its ISO 8601 week notation
ISO week to date
Find the Gregorian date for ISO week 5 day 3 in 2024
Year boundary case
A date where calendar year and ISO year differ
53-week year overview
List all ISO weeks in a 53-week year
Frequently asked questions
What is an ISO week date?
What is an ISO week date?
An ISO week date is defined by ISO 8601 and written as YYYY-Www-D, where YYYY is the ISO year, Www is the week number from W01 to W53, and D is the weekday from 1 (Monday) to 7 (Sunday). For example, 2024-W05-3 means Wednesday of week 5 in ISO year 2024, which is 2024-02-01. The short form YYYY-Www implies Monday.
How is ISO week 1 defined?
How is ISO week 1 defined?
ISO week 1 is the week that contains the first Thursday of the year. That is equivalent to the week that contains January 4, or the first week with at least 4 days in the new year. Because of this, week 1 can start in late December of the previous calendar year, for example 2024-W01 starts on Monday 2024-01-01, while 2021-W01 started on Monday 2021-01-04.
Why does my date show a different ISO year than calendar year?
Why does my date show a different ISO year than calendar year?
This happens at year boundaries. If Jan 1-3 belong to the last week of the previous ISO year, they will show the previous ISO year. For example, 2023-01-01 is a Sunday and its ISO date is 2022-W52-7, so its ISO year is 2022. Similarly, Dec 29-31 can belong to week 1 of the next ISO year.
How many weeks are in an ISO year and when does a year have 53 weeks?
How many weeks are in an ISO year and when does a year have 53 weeks?
Most ISO years have 52 weeks. A year has 53 weeks when January 1 is a Thursday, or when January 1 is a Wednesday in a leap year. In a 53-week year, the extra week is W53 at the end of December. Examples of 53-week years are 2015, 2020, 2026, and 2032.
What formats does this converter accept for ISO week input?
What formats does this converter accept for ISO week input?
The parser accepts YYYY-Www-D like 2024-W05-3, YYYY-Www like 2024-W05 (defaults to Monday), compact YYYYWwwD like 2024W053, and YYYYWww like 2024W05. It is case-insensitive and allows spaces, so 2024 w05 3 also works. Week is validated against the actual number of weeks in that ISO year.
How do I convert an ISO week like 2024-W05-3 back to a normal date?
How do I convert an ISO week like 2024-W05-3 back to a normal date?
Switch to ISO → Date mode, enter ISO year 2024, week 5, weekday 3, or paste 2024-W05-3 into the quick parse field and click Apply. The tool returns 2024-02-01 with long formats, the Monday to Sunday range 2024-01-29 to 2024-02-04, and a 7-day visual grid highlighting the selected day.
What is the difference between ISO weeks and US weeks?
What is the difference between ISO weeks and US weeks?
ISO weeks always start on Monday, week 1 contains Jan 4, and every week has exactly 7 days. US weeks often start on Sunday and week 1 is the week containing Jan 1, so US week numbers can differ by 1 and year boundaries are handled differently. For international business, payroll, and European accounting, ISO is the standard.
Can I get a list of all ISO weeks for a year and export it?
Can I get a list of all ISO weeks for a year and export it?
Yes. Use Year Overview mode. Enter any ISO year from 1 to 9999. The tool shows the number of weeks, first Monday and last Sunday, and a table of every week with its Monday and Sunday dates. You can copy the ISO week list or download it as CSV or JSON for spreadsheets and reporting.
What does the weekday number mean in YYYY-Www-D?
What does the weekday number mean in YYYY-Www-D?
Weekday is 1 to 7 where 1 = Monday, 2 = Tuesday, 3 = Wednesday, 4 = Thursday, 5 = Friday, 6 = Saturday, 7 = Sunday. So 2024-W05-1 is Monday of week 5, and 2024-W05-7 is Sunday of the same week.
Is this converter accurate for leap years and old dates?
Is this converter accurate for leap years and old dates?
Yes. The logic is UTC-based to avoid timezone issues, uses setUTCFullYear to handle years below 100 correctly, validates month lengths including Feb 29, and follows the official ISO 8601 Thursday rule. It supports ISO years 1 to 9999 and correctly handles 52 vs 53-week years.
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