Week Number Calculator
Find the ISO week number (1–53) for any date.
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How it works
- Enter — type or paste your values or text.
- Get the result — it is worked out in your browser.
- Adjust — change the inputs to compare, or copy the result.
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About Week Number Calculator
Find the week number of any date, or the dates that belong to a given week. Week numbers are common in project plans, payroll, school timetables and shipping schedules, especially in Europe.
How weeks are counted (ISO 8601)
The international standard, ISO 8601, starts weeks on Monday. Week 1 is the week that contains the first Thursday of the year — which is the same as saying the week containing 4 January. Most years therefore have 52 weeks, and some have 53.
Why New Year can be confusing
Because week 1 is defined by that first Thursday, the first days of January can belong to the last week of the previous year. For example, 1 January 2027 falls in week 53 of 2026 under ISO rules. The reverse also happens: the last days of December can be counted in week 1 of the next year. When a date is close to New Year, always check both the week number and the year it belongs to.
Other systems
In the United States and some other places, weeks are often counted from Sunday, and week 1 is simply the week that contains 1 January. The same date can therefore have a different week number in a US calendar app than in an ISO one. If you are coordinating with another team or country, agree on which system you are using.
Examples
5 October 2026 is in ISO week 41 of 2026. Teams that plan in two-week sprints often name them by week numbers, such as "weeks 41–42".
To count the days between dates, use the Date Difference Calculator; for business days only, the Working Days Calculator.
How weeks are counted (ISO 8601)
The international standard, ISO 8601, starts weeks on Monday. Week 1 is the week that contains the first Thursday of the year — which is the same as saying the week containing 4 January. Most years therefore have 52 weeks, and some have 53.
Why New Year can be confusing
Because week 1 is defined by that first Thursday, the first days of January can belong to the last week of the previous year. For example, 1 January 2027 falls in week 53 of 2026 under ISO rules. The reverse also happens: the last days of December can be counted in week 1 of the next year. When a date is close to New Year, always check both the week number and the year it belongs to.
Other systems
In the United States and some other places, weeks are often counted from Sunday, and week 1 is simply the week that contains 1 January. The same date can therefore have a different week number in a US calendar app than in an ISO one. If you are coordinating with another team or country, agree on which system you are using.
Examples
5 October 2026 is in ISO week 41 of 2026. Teams that plan in two-week sprints often name them by week numbers, such as "weeks 41–42".
To count the days between dates, use the Date Difference Calculator; for business days only, the Working Days Calculator.
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