Week Number Calculator

"We ship in W38" is unambiguous — provided everyone agrees which week that is. The tool above gives the ISO 8601 week number for any date, together with the ISO year, which is not always the calendar year.

How it works

Pick a date; it defaults to today. The result is the ISO week number and the ISO year, and the copy button produces the standard combined form, 2026-W38.

ISO 8601 defines the week precisely:

• Weeks run Monday to Sunday. Monday is day 1, Sunday is day 7.
• Week 1 is the week containing the year's first Thursday — equivalently, the week containing 4 January, and equivalently again, the first week with the majority of its days in the new year.
• A year has 52 or 53 ISO weeks. 53-week years occur roughly every five or six years, when 1 January falls on a Thursday, or on a Wednesday in a leap year.

The interesting consequence is at the year boundary. 31 December 2025 falls in ISO week 1 of ISO year 2026; 1 January 2027 falls in week 53 of ISO year 2026. That is why the tool reports the ISO year as a separate figure rather than assuming the calendar year of the date you typed — a week number without its ISO year is ambiguous for about six days a year, and those are exactly the days when year-end reporting happens.

Dates are parsed as local midnight and the week arithmetic is then done in UTC, so the answer never shifts by a day depending on your time zone.

ISO weeks are not the only convention

This matters if you are comparing numbers across systems. Two other conventions are in wide use:

• US / broadcast weeks run Sunday to Saturday, and week 1 is simply the week containing 1 January — so most years the US week number is one higher than the ISO number in early January, and the two can disagree for the first weeks of the year.
• Simple ordinal weeks just count blocks of seven days from 1 January, which produces a "week 53" stub of one or two days and no stable weekday alignment at all.

Spreadsheets hedge: Excel's WEEKNUM defaults to the US convention and needs ISOWEEKNUM (or WEEKNUM(date, 21)) for the ISO one. If a dashboard and a spreadsheet disagree by one, this is almost always why.

Working with weeks in code and in data

Do not compute week numbers with dayOfYear / 7. Use the platform's ISO support: WeekFields.ISO in java.time, %G-W%V in strftime, EXTRACT(ISOYEAR ...) and EXTRACT(WEEK ...) in PostgreSQL. Optional and the date-time API covers the temporal fields and adjusters that make this a one-liner rather than a source of year-boundary bugs.

Weekly is also the most common reporting grain, and it is where the 53-week year bites: a year-on-year comparison of "week 1 to week 52" silently drops a week of data, and a chart keyed on calendar year plus week number will double-count or lose the boundary week. Keying on the ISO year–week pair fixes both. Business intelligence covers choosing a time grain and building a date dimension that carries ISO week and ISO year as first-class columns, which is the durable fix.

Related tools

To count the days between two dates, use days between dates; to exclude weekends, business days; for a countdown to a fixed milestone, days until date.