Find the week number of any date, in the ISO and US systems.

ISO week 1 is the week that holds the first Thursday of the year, and weeks run Monday to Sunday.

ISO week 1 = the Monday to Sunday week that contains January 4

That rule is the whole system, set out in ISO 8601 and described in the ISO week date article. A week belongs to the year that holds most of its days, so the Thursday decides. In 2026, January 1 is a Thursday, so the week of Monday, December 29, 2025 is week 1 even though three of its days are in 2025.

ISO writes a day as a year, a week and a weekday number from 1 for Monday to 7 for Sunday. Monday, November 2, 2026 is 2026-W45-1. The calculator shows that form under the week number, which is handy for filenames and for systems that sort dates as text.

A 53-week year happens about every five or six years, and 2026 is one.

A normal year is 52 weeks and 1 day, so the calendar slides by a day every year, or by two in a leap year. When the slide pushes a Thursday into the last days of December, the year gets a 53rd week. The years with 53 weeks are those that start on a Thursday, plus leap years that start on a Wednesday. From 2020 to 2032 that is 2020, 2026 and 2032.

So the ISO year 2026 has 53 weeks. Its week 1 starts on Monday, December 29, 2025, and its week 53 ends on Sunday, January 3, 2027. The ISO year 2027 has 52 weeks and starts on Monday, January 4, 2027. Anyone who reports by week should expect a week 53 in 2026 and none in 2027, and check any weekly total that is compared with the same week a year earlier.

The US system starts the week on Sunday and counts the week with January 1 as week 1.

Excel's WEEKNUM function, with its default setting, works this way: the week begins on Sunday, and the week that contains January 1 is week 1, whatever day of the week that is. Microsoft's separate ISOWEEKNUM function returns the ISO number instead. The US numbering restarts on January 1, so the week that crosses New Year gets two numbers: December 27, 2026 to January 2, 2027 is week 53 for the days in 2026 and week 1 for January 1 and 2 in 2027. The US table in the tool lists that week as week 53 under 2026 and as week 1 under 2027.

In 2026 the two systems give the same number from Monday to Saturday, and a Sunday is one higher in the US system, because the US week starts a day before the ISO one. Near New Year they part ways: January 1, 2027 is in week 53 for ISO and week 1 for the US system. Pick the system your report, your payroll or your supplier uses, and keep using it all year. A table that mixes the two is a week out on some days.

Use the tables to find the dates of any week number before you plan around it.

The week table under the tool lists every week of two years, with the first and last day of each. It opens on 2026 and 2027, jumps to the years of a date you pick, and the Earlier years and Later years buttons move it by two. Weekly production plans, sprint calendars, delivery windows and holiday rotas are all written as “week 44”, and the table turns that into a start date and an end date without counting on a calendar.

• For the days between two dates, use the days between dates calculator.
• To step a date forward by whole weeks, use the date calculator.
• To count only working days in a stretch of weeks, use the business days calculator.
• For overtime inside a workweek, the time card calculator totals the hours.

A week number tells you when the work is due, not who will still be there to do it.

Plans written in weeks assume the same people turn up in week 12 as in week 1. TeamPredict reads public LinkedIn signals to flag resignation risk early, so a gap in a plan shows up as a warning while there is still time to talk.