Week Number Calculator

How to Use the Week Number Calculator

1. Pick any date in the date picker - defaults to today.
2. Read the stat panel: ISO week number (1-53), day of year (1-366), days remaining in the year, day of week, calendar quarter (Q1-Q4), and whether the year is a leap year.
3. Try different dates by re-selecting to see how year boundaries and leap days move the numbers.

Dates near year boundaries are where the ISO rule gets interesting: January 1, 2023 is ISO week 52 of 2022 (yes, really); December 31, 2018 is ISO week 1 of 2019. The tool handles those edge cases automatically.

The Algorithm Behind The Numbers

ISO 8601 week numbering is not "divide day-of-year by 7." The canonical algorithm is: take the Thursday of the week that contains your target date (Monday-based weeks), note what calendar year that Thursday falls in - that is the "week-numbering year." Then find January 4 of that same year (January 4 is always in ISO week 1 by construction), figure out which Monday starts that week, and count how many seven-day blocks separate your week from that anchor. The reason: defining week 1 as "the week containing the first Thursday" guarantees that weeks 1 through 52 or 53 always contain a majority of their days in the stated year, eliminating the awkwardness of a "half week" at either boundary. The component uses a compact implementation of exactly this rule, working from getDay() and a temporary Date clone so daylight saving transitions never shift the answer.

Who Actually Cares About Week Numbers

• European payroll and scheduling software - many German, Scandinavian, and Dutch companies identify pay periods, shifts, and project milestones by "week 14" or "KW 23" (Kalenderwoche).
• Agile/scrum teams that run one- or two-week sprints and label sprint numbers by the first ISO week they cover.
• Manufacturing lot codes. Date codes on electronics and consumables often read "YYWW" (e.g. 2415 = 2024 week 15) - you see this on microchips, canned food, and pharmaceuticals.
• Financial reporting where firms close books on a 4-4-5 or 4-5-4 week pattern aligned to ISO weeks rather than calendar months.
• Pregnancy tracking - "38 weeks" is the standard gestational milestone, always counted as weeks.
• Project retrospectives where "what did we ship in week 37?" is easier to answer than "what did we ship September 9 through 15?"

Gotchas That Surprise People

• ISO is not US numbering. US/Canadian calendars often use week 1 = the week containing January 1, Sunday-start. That system's "week 1" and ISO's "week 1" can disagree by a full week. When a US office emails "week 1" to a German office, they may mean different dates.
• 53-week years. Years starting on a Thursday (in a regular year) or a Wednesday (in a leap year) contain 53 ISO weeks. Recent examples: 2015, 2020, 2026. A payroll system that hard-codes 52 weeks will break.
• January 1 may be in the previous year's week 52 or 53. January 1-3 of some years belong to the prior year's final ISO week, because the Thursday rule pushes them there. Filing "annual reports for week 1" can miss those first days.
• The Date.prototype.getDay() method returns 0 for Sunday, not 1 for Monday - a common source of off-by-one bugs when implementing ISO week math by hand.
• Microsoft Excel's WEEKNUM function defaults to US numbering; you have to pass WEEKNUM(A1, 21) for ISO. ISOWEEKNUM is a separate function added in Excel 2013.

ISO 8601, US Weeks, And Why There Are Two Systems

ISO 8601 (ISO 8601-1:2019) is the international standard. Weeks run Monday through Sunday; week 1 contains the year's first Thursday, equivalently the week containing January 4; years have 52 or 53 weeks. The format 2024-W15 or 2024-W15-3 (where -3 is Wednesday) is an ISO-valid expression. The ISO week-numbering year may differ from the Gregorian calendar year near New Year's: January 1, 2023 was ISO 2022-W52. US/Canadian week numbering is informal: weeks start Sunday, week 1 contains January 1, and the year has 53 or 54 partial weeks. Canadian federal HR uses ISO; US commercial software overwhelmingly uses the US system. The week containing a given date is not the same span in each.

Week Numbers In Excel, Python, And The Browser

Python's datetime.isocalendar() returns a named tuple (year, week, weekday) and is the right tool in a notebook. Pandas exposes .dt.isocalendar() on Series for vectorised operation on a column of dates. Excel has ISOWEEKNUM(date) for ISO and WEEKNUM(date, [return_type]) for US and other systems - the return_type argument is where bugs hide. Moment.js's .isoWeek() and date-fns's getISOWeek() wrap the same algorithm. The native browser has no direct ISO week API - Intl.DateTimeFormat does not expose week numbers (a known gap that the upcoming Temporal proposal will close via Temporal.PlainDate.prototype.weekOfYear). Until Temporal ships, the compact implementation in this page, or a 10-line helper, is the way. This page is the right pick when you want the answer in three seconds without opening DevTools or Excel.