Free Online Week Number Calculator
Find the ISO week number and date range for any date.
ISO 8601 weeks start on Monday and week 1 is the week that contains the first Thursday of the year. Near the end of December and the start of January, the ISO week-year can differ from the calendar year, so a date such as December 31 may belong to week 1 of the following year.
About the Week Number Calculator
Week numbers are used in planning, payroll, logistics, manufacturing, and project management, but there is more than one way to count them. The most widely used standard is ISO 8601, which defines a week as beginning on Monday and assigns week 1 to the week that contains the first Thursday of the year. That rule ensures every week has exactly seven days and keeps the numbering stable across year boundaries. This calculator returns the ISO week number, the correct ISO week-year, the Monday and Sunday that bound the week, the day of the year, and the calendar quarter.
Why the ISO week-year matters
Most calendar years begin and end in the middle of an ISO week. December 31 can fall in week 1 of the next year, and January 1 can fall in the final week of the previous year. That is why the week-year is reported separately from the ordinary calendar year. If you are storing a date as year plus week, using the calendar year with an ISO week number can produce an invalid or ambiguous value. Always pair an ISO week with its ISO week-year.
Week start and week end
The calculator finds the Monday on or before the selected date and adds six days to find Sunday. The result is shown in a readable long-date format, so you can confirm that the range matches the week you have in mind. Some organisations use Sunday as the first day of the week instead. Those calendars follow a different convention and can produce different week numbers, so it is worth checking which standard a report or system expects.
Day of year and quarter
The day of the year counts from January 1, so January 1 is day 1 and December 31 is day 365 in a common year or day 366 in a leap year. The quarter is the month divided into four groups of three: January to March is Q1, April to June is Q2, July to September is Q3, and October to December is Q4. These companion values are useful for reporting and for cross-checking the week number against a fiscal calendar.
Where week numbers are used
Manufacturing schedules, retail replenishment, payroll batches, epidemiological reports, and agile sprint retrospectives often refer to weeks rather than exact days. A supplier might promise delivery in week 38, a hospital might publish influenza statistics by week, and a logistics team might group shipments by week-year. Always state the numbering convention alongside the value, because a Sunday-based retail calendar can disagree with ISO by a whole week. When exporting data, store the week-year and week number together, and avoid deriving one from the other without applying the same rule.
Frequently asked questions
Can week 1 start in the previous calendar year?
Yes. If January 1 falls late in the week, week 1 begins in late December of the previous year. The week-year is still the new year.
How many weeks are in a year?
An ISO year has 52 or 53 weeks. A year has 53 weeks when it starts on a Thursday, or when it is a leap year that starts on a Wednesday.
Does this use US or European week numbering?
It uses ISO 8601, which is the Monday-based standard common in Europe and in international business. It is not the same as the US Sunday-based convention.