Enter any calendar date and the calculator returns its ISO 8601 week number, plus the four-digit ISO week-numbering year that owns it. ISO weeks always run Monday through Sunday, and Week 1 is the week that contains the first Thursday of the calendar year. That rule means a January date can fall in the previous ISO year, and a late-December date can already belong to next year. Switch the form to the reverse mode to convert an ISO week back to its Monday-Sunday range. Pair the result with the weekday calculator when you also need the day name.
Common use cases
- Filing weekly status reports. European HR systems, SAP, and many engineering trackers tag tickets and timesheets by ISO week. Paste the date you logged work and the calculator returns the exact W## label your reporting tool expects.
- Planning sprints and releases. Sprints often map to a single ISO week or a fixed pair of weeks. Look up the week number for a planned release date, then use the reverse mode to see which Monday opens the sprint and which Sunday closes it.
- Cross-referencing year-end logs. Audit logs and finance exports near January 1 frequently mix ISO years with calendar years. Convert each boundary date here to confirm whether 2025-12-29 sits in W1 of 2026 or W52 of 2025 before you reconcile totals.
- Scheduling weekly recurring events. When a recurring meeting must skip a holiday week, find the ISO week for the holiday, then add one with the date add or subtract tool to land on the next valid Monday.
How it works
The calculator parses your date as a year-month-day value, builds a Date in UTC, and runs the ISO 8601 week algorithm. ISO 8601 anchors every year on the Thursday rule: the week that contains January 4 is Week 1, and weeks always start on Monday. Some calendar years contain 52 ISO weeks, and exactly 71 of every 400 years contain 53. The output is the week number paired with the ISO week-numbering year, which can differ from the calendar year.
- Pick the input mode. Choose "Date to week" to convert a calendar date into its ISO week. Choose "Week to range" to convert an ISO week and year back to its Monday-Sunday span.
- Enter the date or week. In date mode, paste any YYYY-MM-DD value. In week mode, enter a four-digit ISO year and a week number from 1 to 52, or 53 in long years.
- Read the ISO week label. The result is W## of the ISO week-numbering year, for example W01 of 2026. The ISO year is shown explicitly because it can disagree with the calendar year at year-end boundaries.
- Inspect the Monday-Sunday range. In reverse mode the calculator shows the Monday that opens the week and the Sunday that closes it, with both dates in YYYY-MM-DD form so you can paste them straight into a planner.
Worked examples
New Year Thursday lands in Week 1
Convert 2026-01-01, which is a Thursday.
Result: ISO week 1 of 2026.
Any year that starts on a Thursday puts January 1 in Week 1, because the Thursday rule is satisfied immediately.
Late December date jumps to next ISO year
Convert 2025-12-29, the Monday after Christmas.
Result: ISO week 1 of 2026.
Once the Thursday in this Monday-Sunday span (2026-01-01) lands in 2026, the whole week belongs to ISO year 2026.
A 53-week long year
Convert 2026-12-31, a Thursday.
Result: ISO week 53 of 2026.
Years that start on a Thursday or whose preceding year is a Thursday-leap year have 53 ISO weeks. The next 53-week year after 2026 is 2032.
January 1 sits in the previous ISO year
Convert 2023-01-01, a Sunday.
Result: ISO week 52 of 2022.
When January 1 falls on Friday, Saturday, or Sunday, it belongs to the last ISO week of the previous year.
Edge cases & gotchas
- Calendar year vs ISO week-numbering year. The ISO week-numbering year can differ from the calendar year by up to three days at each boundary. Storing only the week number without the ISO year is a common bug: 2025-12-29 logged as "Week 1" is ambiguous between ISO year 2025 (a 53-week year) and ISO year 2026. Always store the year alongside the week.
- 53-week years. A long ISO year contains 53 weeks instead of 52. Long years occur when January 1 is a Thursday, or when it is a Wednesday and the year is a leap year. Hard-coding "1..52" in a dropdown rejects valid weeks for years like 2009, 2015, 2020, and 2026.
- US "week 1 starts January 1" convention. Some US payroll, broadcast, and retail systems define Week 1 as the week containing January 1, with weeks starting on Sunday. That is not ISO 8601 and the two conventions disagree by a week several times per decade. This calculator implements the ISO 8601 rule only.
- Fiscal weeks and 4-4-5 calendars. Retail uses 4-4-5 or 5-4-4 fiscal calendars, where each quarter is divided into 13 weeks following a fixed pattern. Those week numbers are not ISO weeks and do not align with the dates returned here. Use a fiscal-calendar tool for those domains.
Frequently asked questions about Week Number Calculator
How is ISO Week 1 defined?
ISO 8601 defines Week 1 as the week that contains the first Thursday of the calendar year, equivalent to the week that contains January 4. Weeks start on Monday and end on Sunday.
Why does Week 1 sometimes start in December?
When January 1 falls on a Tuesday, Wednesday, or Thursday, the Monday that opens Week 1 is in the previous calendar year. For example, ISO Week 1 of 2026 starts on Monday 2025-12-29.
Can a year have 53 ISO weeks?
Yes. A long ISO year has 53 weeks. It happens when January 1 is a Thursday, or when January 1 is a Wednesday and the year is a leap year. About 71 of every 400 years are long.
Is ISO week the same as the US "week of year"?
No. Many US tools number weeks starting from the week containing January 1, with Sunday as the first day. ISO 8601 uses Monday-Sunday weeks anchored on the first Thursday, so the two conventions disagree several times per decade.
How do I store an ISO week in a database?
Store the ISO week-numbering year and the week number as two integers, or as a single text field formatted YYYY-W## (for example 2026-W01). Storing only the week number loses the year boundary information.
Does the week number depend on my time zone?
The calculator runs in UTC, so the same YYYY-MM-DD always returns the same ISO week. If you start from an instant such as a Unix epoch, convert it first with the Unix timestamp tool to pin the date in UTC.
What does W01 mean in YYYY-W## form?
The W## suffix is the ISO 8601 ordinal week designator, padded to two digits. 2026-W01 reads as the first ISO week of the 2026 ISO week-numbering year, running Monday 2025-12-29 through Sunday 2026-01-04.
Glossary
- ISO week
- A Monday-to-Sunday seven-day span numbered according to ISO 8601. Week 1 is the week containing the first Thursday of the calendar year.
- ISO week-numbering year
- The four-digit year associated with an ISO week. It can differ from the calendar year by a few days near January 1.
- Thursday rule
- The ISO 8601 rule that assigns each week to the year containing its Thursday. This is equivalent to "the week that contains January 4 is Week 1".
- Long year
- An ISO year that contains 53 weeks instead of 52. Occurs when January 1 is a Thursday, or a Wednesday in a leap year.
- YYYY-W## format
- The ISO 8601 ordinal-week notation. 2026-W01 means the first ISO week of 2026, regardless of where it falls in the calendar year.