Week Number Calculator
Find the ISO week number and ISO week-year for a date.
Results are provided for general informational use. Verify important financial, health or legal figures before relying on them.
About this Week Number Calculator
The Week Number Calculator reports the ISO 8601 week number for a selected date. ISO weeks begin on Monday, and week 1 is the week containing the year’s first Thursday.
ISO week numbering, used widely in Europe and international project management, defines a week as starting on Monday and belonging to whichever year contains that week's Thursday, which occasionally places the first few days of January in the final week of the previous year.
For related tasks, try Date Difference Calculator, Day of Year Calculator or Add Days to Date.
How the calculation works
The calculation shifts the selected date to the Thursday of its ISO week and then measures the week position from the start of the ISO year.
How to use it
- Choose a calendar date.
- Select Calculate week number.
- Review the ISO week and ISO week-year.
Practical example
A date near the start or end of a year can belong to the adjacent ISO week-year depending on the weekday layout.
January 1, 2023 fell on a Sunday, and under ISO week rules, since fewer than 4 days of that week belonged to 2023, that date is actually counted as part of ISO week 52 of 2022, not week 1 of 2023.
Things to double-check
Because the ISO week-year can differ from the calendar year for a small number of days at the start of January and end of December, a date like December 31 might report as ISO week 1 of the following year rather than the final week of the current year, which is correct ISO behavior.
ISO week numbers
The ISO 8601 standard starts weeks on Monday and defines week 1 as the week that contains the first Thursday of the year, which is the same as the week containing 4 January. 7 October 2026, a Wednesday, falls in ISO week 41. Because 1 January 2026 is a Thursday, week 1 runs from 29 December 2025 to 4 January 2026, and 2026 has 53 ISO weeks. Dates in early January can belong to the last week of the previous year.
Week-year versus calendar year
Because of this rule, the ISO week-year can differ from the calendar year for a few days at each end. 30 December 2024 is in week 1 of 2025, and 1 January 2027 falls in week 53 of 2026. When you write a week number, write the week-year with it, such as 2026-W41, to avoid confusion in reports and schedules.
Where week numbers are used
Many European companies use week numbers for production schedules, delivery dates and project plans. Schools plan terms by week, and calendars in some countries display them. A supplier saying delivery in week 44 means the week beginning on Monday 26 October 2026. Check the numbering system used, since it is not universal.
Other week numbering systems
In the United States, weeks commonly start on Sunday and week 1 is the week containing 1 January, so week numbers can differ from the ISO ones by one. Some other systems start on Saturday. If you exchange week numbers with someone in a different country, confirm the system. The tool follows ISO 8601 and shows the matching week-year.
Years with 53 weeks
Most years have 52 ISO weeks, but some have 53. A year has 53 weeks when 1 January falls on a Thursday, or on a Wednesday in a leap year. Recent and upcoming examples are 2020, 2026 and 2032. This matters for annual reports, production calendars and payroll, because week 53 may be missing from a template that assumes 52. When you build a schedule, check the last week of the year and the first week of the next to make sure dates are not counted twice or skipped.
Worked examples
Wednesday 7 October 2026 falls in ISO week 41. Thursday 31 December 2026 is in week 53, because 2026 has 53 weeks. Friday 1 January 2027 also belongs to week 53 of 2026, since its Thursday, 31 December, is in 2026. Monday 4 January 2027 starts week 1 of 2027. Always write the week-year with the week number, such as 2026-W53, to avoid mistakes at the turn of the year. The calculator shows both numbers, so you can label reports and schedules clearly.
Production, payroll and school calendars
Week numbers are widely used by manufacturers, logistics firms and some employers because they make schedules short and easy to compare across years. A supplier who promises delivery in week 44 means the week beginning Monday 26 October 2026. Some businesses use a retail calendar with 4-4-5 week months, which differs from the ISO calendar. Schools and universities in some countries number their teaching weeks from the first week of term. Make sure you know which system a document uses before you plan around a week number.
ISO and US week systems
The United States commonly uses a system in which weeks start on Sunday and week 1 is the week that contains 1 January. Under that system, 1 January 2026, a Thursday, is in week 1 and the week starts on Sunday 28 December 2025. Under ISO rules, week 1 starts on Monday 29 December 2025. The two systems can differ by one week for much of the year. If you work with partners abroad, state the system you are using, and check how your software numbers weeks.
Weeks in spreadsheets and code
Spreadsheet programs have functions for week numbers, and the choice of option decides the system: in many programs, the ISO version is a separate function or a specific argument. Programming languages offer the ISO calendar as well, for example the isocalendar method in Python, which returns the ISO year, week and weekday. Use the ISO year and not the calendar year when you group data by week, otherwise the first and last days of the year may end up in the wrong group.
Key terms explained
A week number counts the weeks of a year. The ISO 8601 week starts on Monday, and week 1 is the one with the first Thursday of the year. An ISO week-year is the year to which the week belongs and can differ from the calendar year at the start and end. Some countries number weeks from Sunday, and week 1 may be the week with 1 January. A fiscal calendar divides a financial year into periods such as 4-4-5 weeks. A weekday is a day from Monday to Friday.
Rules of thumb and quick checks
An ISO year has 52 or 53 weeks. Week 1 always contains 4 January, and the last week contains 28 December. Add 7 days to move to the next week number. Dates in early January can belong to week 52 or 53 of the previous year, and dates at the end of December can belong to week 1 of the next. Quote the week together with the week-year, such as 2026-W41.
Frequently asked questions
Does ISO week 1 always start on January 1?
No. ISO week boundaries depend on Mondays and the first Thursday rule.
Can December dates belong to next year’s week 1?
Yes. Some late-December dates do.
Can early January belong to the prior ISO year?
Yes.
Why does January 1st sometimes belong to the previous year's last week?
ISO week numbering assigns a week to the year that contains its Thursday; if January 1st falls late in the week, that partial week's Thursday actually falls in the prior year, so the week is numbered as part of that prior year.
Does every year have exactly 52 weeks under the ISO standard?
No, most years have 52 ISO weeks, but years where the extra days work out correctly (roughly once every 5-6 years) have 53 ISO weeks instead.
Keep exploring
View category →More Date & Time
Date Difference CalculatorFind calendar years, months, days and total days between two dates.Day of Year CalculatorFind a date’s ordinal day number within its year.Add Days to DateAdd or subtract a number of calendar days from a chosen date.Business Days CalculatorCount weekdays between two dates while excluding Saturdays and Sundays.Time Duration CalculatorFind elapsed hours and minutes between two clock times.Hours & Minutes to DecimalConvert hours and minutes into decimal hours.