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Enter a week number and year
Click Find Dates to see the date range// find start and end dates for any ISO week
Convert any ISO week number and year to start/end dates instantly. Find Monday–Sunday date ranges for any week number. Free browser-based week calculator.
Enter a week number and year
Click Find Dates to see the date rangeType any ISO week number from 1 to 52 (or 53 for long years).
Enter the year. Supports 1900–2100. Defaults to current year.
Click Find Dates to see Monday–Sunday start/end dates in multiple formats.
This tool converts an ISO 8601 week number and year into exact calendar dates. ISO weeks always start on Monday and end on Sunday. Week 1 is defined as the week containing the first Thursday of the year. Some years have 53 weeks.
An ISO week number follows the ISO 8601 standard. Weeks run Monday through Sunday. Week 1 is the week containing the year's first Thursday, meaning it always has at least 4 days in the new year.
Most years have 52 ISO weeks. A year has 53 weeks when January 1st falls on Thursday (in common years) or Wednesday or Thursday (in leap years). About 71 out of every 400 years have 53 ISO weeks.
ISO Week 1 always starts on the Monday of the week containing the first Thursday of January. This means Week 1 can begin as early as December 28 of the previous year or as late as January 4.
The tool outputs three formats: ISO 8601 (YYYY-MM-DD), US format (MM/DD/YYYY), and European format (DD.MM.YYYY). You can copy each individually or copy the full week range string.
Yes. By the ISO 8601 standard, every week begins on Monday (day 1) and ends on Sunday (day 7). This differs from the US convention where weeks start on Sunday.
The converter supports years from 1900 to 2100, covering all practical business, historical, and planning use cases without any accuracy loss.
Yes. Since Week 1 is anchored to the first Thursday, its Monday may fall in the previous December. For example, if January 1 is a Friday, Week 1 starts on December 29 of the prior year.
This tool divides week numbers into quarters of 13 weeks each: Q1 = weeks 1–13, Q2 = weeks 14–26, Q3 = weeks 27–39, Q4 = weeks 40–52/53. Note this is an approximation — fiscal quarters may differ.
The Week Number to Date Converter is a free, browser-based tool that instantly calculates the exact start and end dates for any ISO 8601 week number. Whether you are managing project sprints, processing payroll, handling logistics data, or working with time-series APIs, knowing the precise Monday-to-Sunday date range for a given week number is essential.
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ISO 8601 is the international standard for representing dates and times. Under this standard, a week is defined as seven days running from Monday (day 1) to Sunday (day 7). Week 1 of any year is defined as the week containing the first Thursday of that year — this guarantees that Week 1 always has at least four days in the new year.
This definition means that the very first week of a year can begin as early as December 28 of the previous year, and as late as January 4 of the current year. Similarly, the last days of December might belong to Week 1 of the following year rather than Week 52 or 53 of the current one.
ISO week numbers are widely used in business, engineering, and scientific contexts because they provide a consistent, timezone-independent way to refer to periods of time. Some common use cases include:
Converting a week number to a calendar date follows a specific algorithm based on the ISO 8601 standard. The process involves finding the Thursday of the target week (since Week 1 is defined by its Thursday), then computing the Monday and Sunday around it. This tool handles the full calculation automatically — just enter your week number and year, and it returns the exact Monday and Sunday dates in ISO, US, and European formats.
Most years have exactly 52 ISO weeks. However, a year has 53 ISO weeks when January 1 falls on a Thursday in a common year, or on a Wednesday or Thursday in a leap year. Years with 53 weeks occur approximately 71 times every 400 years. Recent and upcoming examples include 2015, 2020, 2026, and 2032. This tool correctly identifies 53-week years and prevents you from entering an invalid week number for a given year.
The ISO week standard differs from the US convention in one key way: US calendars typically start the week on Sunday rather than Monday. This means a date that falls in "Week 14" by US counting might fall in "Week 13" or "Week 14" by ISO counting, depending on the day. When working with international systems or European partners, always confirm which week numbering convention is being used. This converter strictly follows ISO 8601.
For each week number and year you enter, the tool displays the full date range in three formats. The ISO 8601 format (YYYY-MM-DD) is the safest choice for databases and APIs. The US format (MM/DD/YYYY) is familiar to North American users. The European format (DD.MM.YYYY) is standard across much of continental Europe. Each date can be copied individually, or you can copy the complete range string (e.g., "2025-04-07 to 2025-04-13") for quick pasting into documents or emails.
Once you view a result, the Prev / Next buttons let you step through adjacent weeks without re-entering values. The tool automatically handles year boundaries — stepping back from Week 1 will take you to the last week of the previous year, and stepping forward from the last week takes you to Week 1 of the next year.