ISO Week Number Calculator

ISO 8601 defines a week calendar that runs alongside the ordinary Gregorian calendar. Every week starts on Monday and ends on Sunday, and days are numbered 1 (Monday) to 7 (Sunday). A full ISO week date is written as year, week and day, for example 2026-W53-5 for Friday of week 53 in ISO year 2026, or in the shorter form 2026-W53 when the day does not matter.

The key rule is how week 1 is chosen. Week 1 is the week that contains the first Thursday of January. Two equivalent ways to say the same thing are that week 1 is the week containing 4 January, or the first week with at least four of its days in the new year. Because a week is assigned to whichever year owns its Thursday, every week belongs entirely to one ISO year and no week is ever split.

The calculator applies that rule directly. For a date, it finds the Thursday of the same Monday-to-Sunday week. The calendar year of that Thursday is the ISO week-numbering year, and the number of whole weeks between that Thursday and the Monday of week 1, plus one, is the week number. Converting the other way, it finds the Monday of week 1 for the chosen year, adds (week − 1) × 7 days, then adds the weekday offset.

• Weeks start: Monday (day 1) and end Sunday (day 7).
• Week 1: the week containing the first Thursday, which is always the week containing 4 January.
• Week range: 1 to 52, or 1 to 53 in long years.
• Result panel: the ISO format (for example 2026-W01-4), the readable calendar date, and the full Monday-to-Sunday range of that week.

ISO weeks are the default in most of Europe and in Nordic, German and Dutch wall calendars. They are also common in manufacturing and retail planning, sprint and release schedules, shipping and warehouse cut-offs, school timetables, and statistical reporting, because every week has exactly seven days and weeks never straddle a year.

• Planning from a week reference: choose ISO Week → Date, enter the ISO year and week, and read the Full Week Range to get the Monday and Sunday dates for a delivery window or sprint.
• Labelling a date: choose Date → ISO Week and copy the ISO format with the copy button so the week-numbering year is carried along with the week.
• What week is it now: the Current Week button fills in today's ISO week, and the Quick Reference table lists the current week, the first and last week of the ISO year, and year-boundary examples you can load with one click.
• Checking a year's length: the last row of the Quick Reference table flags a 53-week year.

The converter accepts ISO years from 1900 to 2100 and calculates in UTC, so the result does not shift with your device's time zone or daylight saving changes. If a week number from another system does not match, the difference is almost always the start day (Sunday vs Monday) or the rule used to pick week 1.