ISO Week Number Calculator
Find the ISO 8601 week number for any date, or look up week date ranges.
Week Number → Date Range
Tentang alat ini
The ISO Week Number Calculator implements the ISO 8601 standard for week numbering. ISO weeks start on Monday and the first week of the year is the week that contains the year's first Thursday.
This means week 1 of a year may start in December of the previous year, and the last days of December may belong to week 1 of the next year. The tool also supports reverse lookup: enter a year and week number to get the Monday–Sunday date range.
Cara penggunaan
- Select a date in the top section to find its ISO week number.
- The week number, ISO year, and Monday–Sunday range are shown instantly.
- Use the Week Number → Date Range section to look up dates for a specific week.
- Enter a year and week number (1–53) to get the corresponding Monday–Sunday range.
Kasus penggunaan
Businesses that report on a weekly cadence use ISO week numbers for consistent reporting. Developers implementing scheduling systems need to convert between week numbers and calendar dates. Supply chain teams reference purchase orders by ISO week.
Pertanyaan umum
- Why might week 1 start in late December? — ISO 8601 week 1 is defined as the week containing January 4th. If January 1 is a Friday, Saturday, or Sunday, that week belongs to the last week of the previous year.
- Can a year have 53 weeks? — Yes. Years that start on Thursday, or leap years starting on Wednesday, have 53 ISO weeks.
- Is ISO week numbering the same worldwide? — ISO 8601 is international, but some regions (notably the US) use a different system where weeks start on Sunday and week 1 is the week containing January 1.