What is a week number?

A week number is simply a label that tells you which week of the year a date falls in. Instead of saying "the last week of September," planners, payroll departments, factories, and project managers say "week 40." It is compact, unambiguous across languages, and perfect for scheduling. The catch is that there is more than one way to number the weeks of a year. The two dominant systems — ISO 8601 and the US traditional system — can assign a different number to the very same day, especially near New Year's Eve and New Year's Day. A third convention, used widely across the Middle East and parts of Asia and Africa, starts the week on Saturday. Knowing which system your calendar, employer, or software uses saves you from off-by-one errors in plans, reports, and deadlines.

ISO 8601: the international standard

ISO 8601 is the international standard for dates and times, maintained by the International Organization for Standardization and first published in 1988. Its week-date system is the one used across most of Europe, much of Asia, and in international business. Three rules define it. First, every week begins on Monday and ends on Sunday. Second, week 1 of a year is the week that contains the year's first Thursday — which is the same as saying it is the week that contains January 4th. Third, the ISO year of a week is the Gregorian year that contains that Thursday. This elegant rule means every ISO week belongs entirely to one ISO year, even when some of its days fall in December or January of a neighboring calendar year. The standard notation writes an ISO week date like 2026-W40, meaning the 40th week of ISO year 2026. Most programming languages and databases — from Python's isocalendar to SQL Server's iso_week — implement exactly this definition, which is why ISO week numbers are the safest choice for software, data pipelines, and anything crossing borders.

US week numbering: Sunday start, January 1st anchored

The traditional US convention works differently. Weeks begin on Sunday, and week 1 is simply the week that contains January 1st — or more precisely, in the widely used strftime %U definition, week 01 begins with the year's first Sunday, and any days before that first Sunday belong to week 00. This keeps every US week inside a single calendar year: a US week never borrows days from the previous December or the next January. Microsoft Excel's WEEKNUM function with its default setting follows a close cousin of this logic (Sunday start, week 1 contains January 1st), which is why American office calendars, school planners, and wall calendars show week numbers that differ from ISO around the turn of the year. For example, January 1, 2026 is a Thursday: under ISO 8601 it sits in week 1 of 2026, but under the US %U convention it sits in week 0, because the first Sunday does not arrive until January 4th. Neither answer is wrong — they just answer different questions.

Saturday-start weeks: the Islamic convention

In many countries across the Middle East, North Africa, and parts of Asia, the week is conventionally considered to begin on Saturday. This reflects long-standing weekend patterns — historically Thursday–Friday or Friday–Saturday weekends in several Gulf states — and it persists in calendars, newspapers, and broadcast schedules even where the official workweek has shifted. Numbering weeks from Saturday works exactly like the US system with a different anchor: week 1 is the week containing the year's first Saturday, and earlier days are week 0. Comparing the same date across Monday-start, Sunday-start, and Saturday-start systems reveals how much the "week number" depends on cultural convention rather than astronomy. If you schedule meetings or shipments with partners in Saudi Arabia, the UAE, Egypt, or Bangladesh, checking the Saturday-start week number avoids the classic confusion of arriving a day early or late.

Why the same date can have three different week numbers

The differences between systems only bite near year boundaries, but when they bite, they bite hard. Take December 29, 2025, a Monday. Under ISO 8601, its week contains Thursday, January 1, 2026 — so it is week 1 of ISO year 2026, and its week runs December 29 through January 4. Under the US system, December 29, 2025 is simply in week 52 of calendar year 2025, running December 28 through January 3. Under the Saturday-start system it is week 52 of 2025 as well, but running December 27 through January 2. Three systems, three labels, three different seven-day windows. For the vast middle of the year the labels usually agree or differ by a constant offset, but the date ranges they describe always start on different weekdays. This is why the WhichWeek comparison cards above show not just the numbers but the full Monday-to-Sunday, Sunday-to-Saturday, or Saturday-to-Friday range for each system: the number alone does not tell you which days are included.

Can a year have 53 weeks?

Yes — and it happens more often than most people expect. A year has 53 ISO weeks whenever January 1st falls on a Thursday, or on a Wednesday in a leap year. Recent and upcoming 53-week ISO years include 2015, 2020, 2026, and 2032. The extra week, sometimes called "week 53" or the "leap week," creates real-world consequences: an extra week of payroll for salaried staff paid weekly, an extra week of rent or subscription billing, and a thirteenth four-week accounting period for retailers. Many large retailers actually run a formal 52-53-week fiscal year, defined by the National Retail Federation, precisely to keep every accounting period exactly seven days long. In 2026, ISO week 53 runs from Monday, December 28, 2026 to Sunday, January 3, 2027 — a week that belongs to ISO year 2026 even though most of its days sit in calendar year 2027. If your budgets or contracts are defined in weeks, always check whether the year in question has 52 or 53 of them.

Weeks, quarters, and the business calendar

Businesses rarely think in months alone; they think in quarters, and quarters are built from weeks. A calendar quarter is three months, but thirteen weeks make a cleaner planning unit — which is why many companies use a 4-4-5 week quarter pattern (four weeks, four weeks, five weeks) for internal reporting. Knowing the current week number tells you instantly how far through the quarter you are and how many weeks remain to hit a target. The quarter panel above shows each quarter's date range alongside its ISO week span, so you can translate "Q3" into "weeks 27 through 39" without mental arithmetic. The "weeks remaining" figure is equally practical: with 14 weeks left in a 53-week year, a sales team knows exactly how many weekly run-rates stand between today and the annual target. Week-based thinking turns vague end-of-year pressure into a concrete countdown.

How to convert a week number back into dates

Reverse lookup — turning "week 40 of 2026" into actual calendar dates — is the feature most week-number sites forget, yet it is the one planners need most. Someone emails "deliver in week 40," and you need to know whether that means late September or early October, and on which weekday the week starts. The conversion depends on the system: ISO week 40 of 2026 runs Monday, September 28 to Sunday, October 4; US week 40 of 2026 runs Sunday, October 4 to Saturday, October 10; Saturday-start week 40 of 2026 runs Saturday, October 3 to Friday, October 9. The rule of thumb for ISO: find the Monday of week 1 (the Monday of the week containing January 4th), then add seven days per week. For US and Saturday-start systems: find the first Sunday or Saturday of the year, then add seven days per week, remembering that days before that first anchor day are week 0. The reverse-lookup tool above does this arithmetic for you, validates impossible inputs like "week 54," and shows every day of the resulting week.

Week numbers in software: Excel, Google Sheets, SQL, and code

If you work with data, you will meet week numbers in your tools whether you look for them or not. In Microsoft Excel, WEEKNUM defaults to the US-style Sunday-start system, while ISOWEEKNUM (available since Excel 2013) returns the ISO 8601 week — mixing the two in one spreadsheet is a classic source of silent errors. Google Sheets offers both ISOWEEKNUM and WEEKNUM with a type argument. In SQL Server, DATEPART(iso_week, date) gives the ISO week while DATEPART(week, date) depends on the session's DATEFIRST setting. Python's datetime.isocalendar() returns the ISO year, week, and weekday in one call. JavaScript has no built-in ISO week function, which is why every JS date library ships its own implementation — and why the math on this page is computed from first principles with the Thursday rule rather than trusting a shortcut. Whenever you store weeks in a database, store the underlying date as the source of truth and derive the week label in a documented view; implicit week functions differ across engines and versions.

Everyday uses: payroll, projects, logistics, and health

Week numbers quietly run large parts of modern life. Payroll departments process weekly and biweekly pay cycles by week number. Construction and manufacturing schedules are drawn as week-by-week Gantt bars; factory products are often stamped with a year-week code (you will find "2440" style stamps on tires, electronics, and food packaging). Logistics companies quote transit times in weeks and plan capacity around numbered weeks. Epidemiologists track disease surveillance by week — the US CDC's MMWR week system, which starts weeks on Sunday, is a cousin of the conventions compared on this page. Agile software teams run one- or two-week sprints anchored to week numbers. Even fitness and habit programs ("week 6 of the training plan") lean on the same idea. A shared, unambiguous week label is what lets a factory in Vietnam, a warehouse in Rotterdam, and a head office in Chicago agree on what "week 40" means.

Fun facts about week numbers

A few curiosities to impress your colleagues: the ISO week-numbering year can start as early as December 29 or as late as January 4 of the calendar year. Years that begin on a Thursday have 53 Thursdays — and 53 ISO weeks. The earliest possible date for ISO week 1 is December 29, which happens when January 1 is a Monday. Because the Gregorian calendar repeats every 400 years, the complete pattern of 52- and 53-week years repeats on a 400-year cycle. The "week date" format (2026-W40-2 for Tuesday of week 40) is fully specified in ISO 8601 and sorts alphabetically in chronological order — a neat property calendar dates lack. And the longest possible gap between the same week number in the US and ISO systems is a full week plus the year-boundary shift, which is why December dates deserve a double-check every single year.

How to use WhichWeek

WhichWeek answers "what week number is it?" the moment the page loads — the giant number in the hero is today's ISO 8601 week, with the US and Saturday-start equivalents beside it. Use the date picker to check any past or future date and compare all three systems side by side, including the exact seven-day range each system assigns. Use reverse lookup when someone gives you a week number and you need the dates: pick the system, enter the year and week, and get the full range with every weekday listed. The quarter panel shows where you are in the year and how many weeks remain, and the year-table builder generates all 52 or 53 weeks with their date ranges — hit Print / PDF to export a clean, ad-free sheet for your wall or your reports. Everything runs entirely in your browser: no account, no tracking of your dates, no limits. Bookmark it, and never wonder which week it is again.