Week Number Calculator
Find the ISO week number, day of the week, and day of the year for any date.
What this calculator does
Enter any date and this calculator returns its ISO 8601 week number (a standard numbering used in payroll, project planning and manufacturing schedules), along with the day of the week and how many days into the year that date falls.
How ISO week numbering works
The ISO 8601 standard defines week 1 of a year as the week containing that year's first Thursday (equivalently, the week containing January 4th), with weeks running Monday to Sunday. This differs from some calendar apps that start week 1 on January 1st regardless of weekday — which is why the same date can show a different "week number" depending on which convention a system uses.
Worked example
September 4, 2026 falls in ISO week 36 of 2026, on a Friday, and is the 247th day of the year. Businesses that plan by week number (common in manufacturing, logistics and some payroll systems) use this figure to label a specific 7-day period unambiguously, regardless of which month it happens to straddle.
Why week numbers matter
Week-based planning is common in supply chain and manufacturing ("ship by week 42"), academic and fiscal calendars, and some payroll cycles that pay biweekly by ISO week rather than by calendar month. Because months have different lengths and don't divide evenly into weeks, referring to "week 36" is often less ambiguous across systems than referring to a date range in prose.
A note on year boundaries
Near the start or end of a year, the ISO week can belong to a different year than the calendar date suggests — for example, December 31st can fall in "week 1" of the following year, or January 1st can fall in the final week of the previous year, if that's where the Monday-to-Sunday week actually lands. This is expected ISO behavior, not an error.
Frequently asked questions
The ISO 8601 standard defines week 1 as the week containing the year's first Thursday, so that every week has a majority of its days in the same calendar year. This means January 1st sometimes falls in the last week of the previous year instead.
Not always — some apps and regions use a Sunday-start week or start week 1 on January 1st regardless of weekday, which produces different week numbers than the ISO standard used here. Check which convention a specific system uses if the numbers need to match exactly.
It's a running count of how many days have elapsed since January 1st of that year, sometimes called the ordinal date — useful in some scheduling and data systems that reference dates this way.
Yes, near year boundaries — a date in very late December can fall in week 1 of the following ISO year, and a date in very early January can fall in the last week of the previous ISO year.