Use the week number calculator to find the ISO week number for any date, convert an ISO week number back into dates, see the Monday and Sunday for that week.
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Week number calculator for ISO week numbers and boundaries Find the ISO week number for any date, convert an ISO week code back to its Monday and Sunday, and see why some dates near New Year belong to the previous or next ISO year.
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Enter a date Provide a year, month, and day to find the ISO week number and week range.
Week number calculator guide: ISO week numbers, week 53
Use this week number calculator to find the ISO week number for any date, check the Monday and Sunday that define that week, or convert an ISO week code back into its full Monday-to-Sunday date range.
What is a week number?
A week number labels a seven-day block inside a year. In ISO 8601, weeks start on Monday and are numbered from 1 to 52 or 53. That makes week numbers useful for project schedules, reporting periods, manufacturing plans, and cross-border planning where month names alone are not precise enough.
This matters because the ISO system does not simply say that January 1 starts week 1 every time. Instead, it uses a rule that keeps full Monday-to-Sunday weeks aligned, which is why some dates at the start or end of a calendar year can belong to a different ISO year.
How ISO week numbers are assigned
ISO week 1 is the week that contains the first Thursday of the year. Another way to say the same thing is that week 1 is the first week with at least four days in the new year. Because weeks start on Monday, this rule keeps the numbering consistent across countries and across years.
That is why a date like 2025-01-01 can still be week 1, while a date like 2016-01-01 belongs to week 53 of the previous ISO year. The week is defined by the Monday-to-Sunday block, not by the calendar month boundary.
The reverse lookup follows the same rule in the other direction. If you already know the ISO year and week number, the calculator finds the Monday that starts ISO week 1 for that year and then steps forward the required number of full weeks.
ISO week 1 = the week containing the first Thursday of the year
This is the core ISO 8601 rule that determines when week numbering starts.
ISO week runs Monday to Sunday
The week start and week end shown by the calculator come from this fixed seven-day structure.
Week start for ISO week n = Monday of ISO week 1 + (n - 1) x 7 days
This is the practical reverse-lookup rule for converting an ISO week number back into a date range.
Why January 1 or December 31 can belong to another ISO year
The most common source of confusion is the new-year boundary. If January 1 falls on Friday, Saturday, or Sunday, that date may still belong to week 52 or 53 of the previous ISO year. If December 29, 30, or 31 falls in the week that contains the first Thursday of the next year, those dates can already belong to ISO week 1 of the next ISO year.
This is why current week number tools often show an ISO year that differs from the calendar year on edge dates. It is not an error. It is how ISO 8601 preserves complete Monday-to-Sunday weeks.
Why some years have week 53
Most ISO years have 52 weeks, but some have 53. A year gets week 53 when the calendar alignment leaves enough days at the end of the year to form another ISO week under the Thursday rule.
In practice, years that start on a Thursday usually have 53 ISO weeks. Leap years that start on a Wednesday also produce 53 ISO weeks. That is why week 53 appears in some years and not others.
Worked examples: 2025-01-01 and 2016-01-01
The date 2025-01-01 falls on a Wednesday. Under ISO rules, it belongs to week 1 of ISO year 2025, and that week runs from Monday 2024-12-30 through Sunday 2025-01-05. This is a good example of why an ISO week can begin in December even when the ISO year is already the new year.
The date 2016-01-01 falls on a Friday. It belongs to week 53 of ISO year 2015, and that week runs from Monday 2015-12-28 through Sunday 2016-01-03. This is the edge case many people mean when they ask why January 1 can belong to the previous ISO year.
When to use a week number calculator instead of other date tools
Use a week number calculator when you need an ISO week label, the Monday start date for a week, the Sunday end date for a week, or a reverse lookup from an ISO week code to real dates. This is common in logistics, operations, payroll planning, agile sprint planning, and planning documents that refer to week 14, week 35, or current week number instead of a full calendar date.
If you need the number of days between dates, use a date difference calculator. If you need to count business days, use a business days calculator. If you need a printable calendar with week numbers, that is a different tool from an ISO week lookup page like this one.
Week number to date lookup for schedules and sprint plans
A week number to date workflow is useful when a spreadsheet, ERP export, sprint board, or supplier plan gives you an ISO code such as 2026-W14 instead of a normal date range. In that case, the practical question is not what week a date belongs to. It is which Monday starts that ISO week and which Sunday ends it.
That reverse lookup matters because a week code is short but abstract. Turning it into seven dated rows makes handoffs, payroll cutoffs, release planning, and logistics windows easier to verify. This page now supports both directions: date to ISO week number, and ISO week to Monday-to-Sunday date range.
Frequently asked questions
What week number is it?
The answer depends on the date you are checking. This calculator returns the ISO week number for the exact year, month, and day you enter.
What is an ISO week number?
An ISO week number is the international week label defined by ISO 8601. ISO weeks start on Monday and week 1 is the week containing the first Thursday of the year.
Why can January 1 belong to the previous ISO year?
Because ISO weeks are full Monday-to-Sunday blocks. If January 1 falls late in a week that mostly belongs to the previous year, the ISO year can still be the previous one.
Why do some years have week 53?
A year has week 53 when the calendar alignment satisfies the ISO Thursday rule for an extra numbered week. Years that start on Thursday, and leap years that start on Wednesday, are common examples.
Do ISO weeks start on Sunday or Monday?
ISO weeks start on Monday and end on Sunday. The page uses this rule as a quick reference, but the surrounding assumptions and units still matter when you interpret the result.
Can December 31 belong to week 1 of the next year?
Yes. Dates at the end of December can belong to ISO week 1 of the next ISO year when they fall in the week containing the next year's first Thursday.
How many ISO weeks are in a year?
Most ISO years have 52 weeks, but some have 53. The page uses this rule as a quick reference, but the surrounding assumptions and units still matter when you interpret the result.
Why can the ISO year differ from the calendar year?
The ISO year follows week boundaries instead of month boundaries. Near New Year, a date can belong to a week that the ISO standard assigns to the previous or next ISO year.
How do I find the Monday and Sunday for an ISO week?
This calculator shows the week start and week end for your selected date, so you can see the Monday and Sunday that define that ISO week.
Can I convert a week number back into dates?
Yes. If you already know the ISO year and week number, use the reverse lookup mode to get the Monday start date, Sunday end date, and all seven dates in that ISO week.
Is this the same as a printable calendar with week numbers?
No. This tool is for looking up the ISO week number and range for a date. Printable calendar layouts with week numbers are a related but different format.
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