Convert dates to week numbers and vice versa.
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Choose a date to see its calendar week, week of month, weekday, and the week’s start and end dates.
You can also select a week number, a week of the month, or a day of the week to see the updated date.
You may also select a month to see the week containing its first day, and change a year to see the current date and week information for that year.
What is a week number?
A week number identifies a calendar week within a year. Instead of saying “the second week of March”, a team can refer to “week 11”. That compact, unambiguous reference is especially useful when people work across departments, locations, or languages.
Week numbers are widely used in Europe and in international business calendars. They appear in project plans, production schedules, delivery windows, staff rotas, school timetables, payroll planning, travel arrangements, and retail campaigns. A due date written as “week 38” is often easier to scan than a list of individual dates.
This converter uses the ISO 8601 week-date standard. Under ISO 8601, weeks start on Monday and week 1 is the first week that contains Thursday, or equivalently 4 January. Most ISO years have 52 weeks, while some have 53. Because ISO weeks can cross a calendar-year boundary, the first days of January can belong to the last week of the previous ISO year, and the final days of December can belong to week 1 of the next ISO year.
Use the date field to find the ISO week for a specific date, or choose a week number to see its dates. The month shortcuts are useful for planning: they open the week containing the first day of that month. The Sunday/Monday display preference changes the date range shown in the card and the weekday order, while the ISO week number remains Monday-based.
ISO 8601 weeks run from Monday to Sunday. Week 1 is the week containing 4 January, which is also the first week with at least four days in the new year. This rule keeps week numbering consistent across countries and makes it possible to identify a week with a year and a number, such as 2026-W01.
ISO 8601 weeks run from Monday to Sunday. Week 53 occurs in years where 1 January is a Thursday or in leap years where 1 January is a Wednesday. This ensures that week numbering remains consistent and that each ISO year has either 52 or 53 weeks.
ISO weeks are complete Monday-to-Sunday periods, not fragments cut at midnight on 1 January. For example, if New Year’s Day falls on a Friday, Saturday, or Sunday, it can be part of the final ISO week of the preceding year. The same principle can place late-December dates in week 1 of the following ISO year.
Week numbers make recurring work easier to coordinate. They are commonly used for project milestones, manufacturing and logistics schedules, stock planning, employee shifts, construction programmes, school terms, event planning, and weekly reports. They are particularly helpful when a team needs to discuss a period without repeating several dates.
No. ISO 8601 always starts the numbered week on Monday. The setting in this converter is a display preference: choose Sunday or Monday to change the range shown in the Week Dates card and the order of weekday buttons. The selected ISO week number does not change.
Select the ISO year, then choose a week from the Week of Year menu. The converter keeps the currently selected weekday where possible and updates the date, week start, week end, month, and week-of-month controls. You can then select another weekday to inspect a different date in the same week.