Week Number Calculator
ISO weeks start on Monday and week 1 is the week with the first Thursday of the year.
Choose the date and week system
Enter a date and choose the week numbering system. ISO week numbers start on Monday and define week 1 as the week containing the first Thursday of the year.
Some US style systems start weeks on Sunday and treat the week containing January 1 as week 1. Choose the system your calendar, report or software expects.
If the week number goes into a database or report, save the selected rule with it. Otherwise the number can be misunderstood later.
What week number means
The result tells which numbered week the date belongs to under the selected rule. The same date can have different week numbers in different systems.
ISO weeks also have a week year. A date in early January can belong to the last ISO week of the previous year. A date in late December can belong to week 1 of the next ISO year.
ISO versus US week rules
ISO 8601 weeks begin on Monday. Week 1 is the week with at least four days in the new year, which is the same as the week containing the first Thursday.
Other week systems may begin on Sunday or use January 1 as the start of week 1. That simpler rule can be easier for local calendars but will not always match ISO reports.
Finding the ISO week for a date
January 1, 2021 was a Friday. Under ISO rules, it belonged to the last week of ISO year 2020 because the first ISO week of 2021 started on Monday, January 4.
Under a January 1 based system, that same date may be counted as week 1 of 2021. The result depends on the rule.
This is why week number calculators should show the method, not only the number. The method explains the difference.
Week-year confusion
The most common mistake is comparing ISO week numbers with spreadsheet WEEKNUM defaults. Many spreadsheet defaults start on Sunday unless you choose the ISO option.
Another mistake is ignoring the week year. Week 53 of one year and week 1 of the next can cross the December and January boundary.
If your report groups by week, store the week year with the week number. Week 1 alone is not enough near New Year.
Week Number Calculator FAQ
What is an ISO week number?
An ISO week number follows ISO 8601 week date rules. Weeks start on Monday, and week 1 is the week that contains the first Thursday of the year.
This rule makes weekly reporting consistent, but it can make early January dates belong to the previous ISO week year.
Why does week 1 sometimes start in December?
Because ISO week 1 is based on the week containing the first Thursday, not strictly January 1. If the first Thursday falls in a week that starts in late December, ISO week 1 starts in December.
That is normal for ISO calendars. It is not a calculator error.
Why is my spreadsheet week number different?
Your spreadsheet may be using a different return type or default week start. Some functions start weeks on Sunday. Some have an ISO option that starts on Monday.
Check the function setting before comparing results. In many tools, the ISO option must be selected explicitly.
Can a year have 53 weeks?
Yes. Some years have ISO week 53. This happens because 52 weeks cover 364 days, and calendar years have 365 or 366 days.
The extra day pattern sometimes creates a 53rd week under ISO rules.
This is normal and should not be forced back to week 52. Weekly reports need to allow the extra week when it appears.
Should I use ISO or US week numbers?
Use the system expected by the report, calendar or audience. ISO is common for international reporting and many business systems. US style week numbering may be used in local calendars.
The calculator can show the result, but the correct choice comes from the context that will use the week number.