How to use this calculator
The week number page shows the current ISO 8601 week number and the Monday-to-Sunday dates that belong to it. Below that is a table of every week in the selected year with its start and end dates, and a lookup that turns any date into its ISO week number and ISO week-year. Choose a different year to see its table, or enter a date to find the week it belongs to.
Week numbers are not used the same way everywhere. Most of Europe and much of the rest of the world follow ISO 8601, where weeks start on Monday. In the United States, Canada and some other places, weeks are often counted from Sunday, with week 1 being the week that contains January 1. The page shows both numbers side by side so you can check which one a calendar, timetable or spreadsheet is using.
Formula and calculation method
ISO week = the Monday-to-Sunday week counted from week 1, the week that contains the year’s first Thursday (equivalently, January 4).
How ISO weeks are counted
ISO 8601 numbers weeks from Monday to Sunday. Week 1 of a year is the week that contains that year’s first Thursday, which is the same as the week that contains January 4. Every later week follows in sequence. Because a year has 365 or 366 days, 52 weeks plus one or two days, most years have 52 ISO weeks and some have 53. A year has 53 weeks when it begins on a Thursday, or when it is a leap year that begins on a Wednesday. The year a week is counted in is its ISO week-year, which can differ from the calendar year of a date near New Year.
How the US convention differs
The US convention used here starts weeks on Sunday and makes week 1 the week that contains January 1, however many days of that week fall in the new year. The first week can therefore be as short as one day, and the year can reach week 53, or week 54 in a leap year that begins on a Saturday. The two systems often give the same number for a date, but they can differ by one, especially on Sundays, which close an ISO week and open a US week.
Worked examples
October 29, 2026
October 29, 2026 is a Thursday. Its ISO week runs from Monday, October 26 to Sunday, November 1, and it is week 44 of ISO week-year 2026, written 2026-W44. In the US convention the same date is also week 44, but that week runs from Sunday, October 25 to Saturday, October 31. On Sunday, November 1 the numbers diverge: it is still ISO week 44, but it is US week 45.
January 1, 2027 belongs to 2026
January 1, 2027 is a Friday. The week containing it runs from Monday, December 28, 2026 to Sunday, January 3, 2027, and the year’s first Thursday is January 7, so this week is not week 1 of 2027. It is week 53 of ISO week-year 2026. ISO week 1 of 2027 starts on Monday, January 4, 2027. In the US convention January 1, 2027 is week 1 of 2027.
Why 2026 has 53 weeks
January 1, 2026 is a Thursday, so the week that contains it is week 1 of 2026. That week starts on Monday, December 29, 2025, which therefore belongs to ISO week-year 2026. Counting forward, week 53 starts on Monday, December 28, 2026 and ends on Sunday, January 3, 2027. Recent and upcoming 53-week years include 2015, 2020, 2026 and 2032.
Edge cases and practical checks
Dates near New Year
Up to three days at the start of January can belong to the last week of the previous ISO week-year, and up to three days at the end of December can belong to week 1 of the next. When you record or sort by week number, store the ISO week-year together with the week, as in 2026-W53, rather than the calendar year of the date. Otherwise the last week of December and the first week of January can collide or appear in the wrong order.
Spreadsheets and other software
Check which rule a program uses before comparing its numbers with this page. In Excel and Google Sheets, ISOWEEKNUM follows ISO 8601, while WEEKNUM with its default setting starts weeks on Sunday and counts the week containing January 1 as week 1, like the US column here. Some organisations use their own fiscal weeks starting on another date; those are not shown on this page.
Frequently asked questions
What week is it now?
The top of the page shows the current ISO week number and its Monday-to-Sunday dates, based on today’s date on your device. On October 29, 2026, for example, it is ISO week 44, which runs from October 26 to November 1. The US-convention number for the same date is shown next to it.
How many weeks are in a year?
An ISO week-year has 52 or 53 weeks. It has 53 when the year starts on a Thursday, or when it is a leap year starting on a Wednesday. In the US convention the week containing December 31 is usually week 53, because counting starts with a partial week; a leap year that starts on a Saturday reaches week 54.
Which date is week 1?
In ISO 8601, week 1 is the week that contains the year’s first Thursday, so it always contains January 4 and can start as early as December 29 of the previous year. In the US convention, week 1 is the week that contains January 1, starting on the Sunday on or before that date.
What does a notation like 2026-W44-4 mean?
It is the ISO 8601 week date format: ISO week-year 2026, week 44, day 4. Days are numbered from 1 for Monday to 7 for Sunday, so day 4 is Thursday. 2026-W44-4 is October 29, 2026. The shorter form 2026-W44 refers to the whole week.
Why does my calendar app show a different week number?
It may be set to start weeks on Sunday or to follow a regional convention. Some apps let you choose the first day of the week and the rule for week 1 separately. Compare its numbers with both columns on this page to see which convention it follows.
Is a week number the same as weeks elapsed?
No. A week number is a label for a particular calendar week. The number of weeks between two dates is a duration, which you can find by dividing the day difference by seven. Week 10 does not mean that ten complete weeks have passed since January 1.