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Week Number Calculator

Enter a date to find its ISO week number, ISO week-year, US week number, weekday number, and day of year.

Week number for 2026-05-06

ISO and US week numbers with day-of-year context.

ISO week: 19
ISO week year: 2026
ISO weekday: 3
US week number: 19
Day of year: 126

ISO-8601 vs. US Standard Week Numbering Systems

When two teams look up the "current week number" and get answers that are one week apart, they are almost always mixing the international ISO-8601 week date system with the traditional US / North American Sunday-start calendar system. Because a 365-day common year equals 52 weeks plus 1 extra day (and a 366-day leap year equals 52 weeks plus 2 extra days), calendar years never divide evenly into seven-day blocks. Each numbering standard resolves those leftover boundary days differently.

Under ISO-8601, every week is an unbroken seven-day Monday-through-Sunday period assigned wholly to whichever year holds at least four of its days (equivalently, whichever year holds that week's Thursday). Under standard US numbering, Week 1 starts strictly on January 1—creating a short partial week at the start of January and another partial week at the end of December. For a deeper walkthrough of edge cases, read our complete guide to ISO-8601 vs. US week numbers, or browse full annual tables for 2026 week numbers and 2027 week numbers.

Rule / Attribute	ISO-8601 Week System	US / North American System
First Day of Week	Monday (Day 1 = Mon, Day 7 = Sun)	Sunday (Day 1 = Sun, Day 7 = Sat)
Week 1 Definition	Week with the year's first Thursday (always includes Jan 4; 4+ days in new year)	Begins on January 1, regardless of weekday
Partial Boundary Weeks	Never — every ISO week is a full 7-day Monday–Sunday block	Yes — Week 1 and Week 53/54 are 1–6 days long unless bounded by Sun/Sat
Weeks Per Year	52 full weeks (364 days) or 53 full weeks (371 days in leap-week years)	53 week labels in almost all years; 54 labels if a leap year starts on Saturday
Year-End Crossovers	Dec 29–31 can belong to next year's W01; Jan 1–3 can belong to prior year's W52/W53	Week-year always matches the Gregorian calendar year of the date
Standard Notation & Users	YYYY-Www (e.g., 2026-W24) — ERP, freight, manufacturing, EU/global SaaS	Week 1–53 — US consumer planners, retail store calendars, domestic media

Spreadsheet, SQL & Code Formulas for Week Numbers

A common cause of reporting bugs in BI dashboards and ERP exports is pairing an ISO week number with a standard calendar year around New Year's Day. For example, formatting December 30, 2025 with calendar year YYYY plus ISO week IW produces 2025-W01—pointing to the wrong end of the year by 52 weeks! Whenever you extract ISO weeks in SQL or code, always extract the matching ISO week-numbering year alongside the week index.

Platform / Language	ISO-8601 Week Formula	US Sunday-Start Formula & Notes
Excel & Google Sheets	=ISOWEEKNUM(A1) or =WEEKNUM(A1, 21)	=WEEKNUM(A1, 1) Return type 1 = Sunday start, Jan 1 = Week 1
PostgreSQL	EXTRACT(WEEK FROM dt) TO_CHAR(dt, 'IYYY-"W"IW')	Always pair `IW` with ISO year `IYYY` (never `YYYY`) to prevent Dec/Jan rollover bugs
MySQL / MariaDB	WEEK(dt, 3) YEARWEEK(dt, 3)	WEEK(dt, 0) Mode 3 = ISO-8601 (1–53); Mode 0 = Sunday start (0–53)
Python (datetime)	dt.isocalendar() dt.strftime("%G-W%V")	dt.strftime("%U") `%G`/`%V` = ISO year & week; `%U` = Sunday-based week (00–53)
JavaScript (UTC)	Shift UTC date to nearest Thursday: `d.setUTCDate(d.getUTCDate() + 4 - (d.getUTCDay() \|\| 7))`, then divide days since Jan 1 by 7	Native `Date` has no built-in `getWeek()`; compute in UTC to avoid Spring/Fall DST off-by-one-hour shifts

How Supply Chains, Manufacturing & 4-4-5 Retail Accounting Use Week Numbers

Manufacturing Lead Times & Freight Windows (e.g., W24 Delivery)

Automotive, electronics, and industrial suppliers schedule factory capacity and container vessel sailings by ISO week—such as "Ex-Works W19 / Warehouse ETA W24." Using week codes solves two operational headaches at once: first, a cross-border purchase order stating 04/07/2026 cannot be misread as April 7 in Chicago and July 4 in Hamburg; second, ocean port congestion and customs clearance make week-level slotting far more accurate for dock and labor planning than single-day promises.

4-4-5 / 4-5-4 Retail Accounting & Sprint Cadences

Standard calendar months vary from 28 to 31 days and contain either four or five weekends, distorting month-over-month revenue comparisons for retailers, restaurants, and hospitality groups. Under a 4-4-5 or NRF 4-5-4 fiscal calendar, every quarter equals exactly 13 full weeks (91 days) subdivided into two 4-week periods and one 5-week period. Every fiscal period contains the exact same number of Saturdays and Sundays year after year, with a 53rd leap week added once every five to six years—such as in ISO leap-week year 2026—to realign the fiscal calendar with the seasons.

CSV

Complete 2027 week calendar comparing 52-week ISO boundaries against US week numbering.

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Frequently asked questions

What is the exact rule for ISO-8601 Week 1?

Under the international ISO-8601 standard, weeks always begin on Monday and end on Sunday. Week 1 (W01) of any year is defined in three mathematically equivalent ways: it is the week containing the year's first Thursday, the week containing January 4, and the first week that has at least four days (a majority of its seven days) inside the new calendar year.

Why can the ISO week-year differ from the Gregorian calendar year?

Because ISO weeks never split into partial weeks at New Year's, up to three days at the end of December or the start of January can cross into the adjacent ISO week-year. For example, Monday, December 29 through Wednesday, December 31, 2025 already belong to 2026-W01, while Friday, January 1 through Sunday, January 3, 2027 still belong to 2026-W53.

How does the US week number differ from the ISO week number?

Standard US calendar numbering starts weeks on Sunday rather than Monday, and Week 1 always begins on January 1 regardless of which day of the week January 1 falls on. Unless January 1 is a Sunday, US Week 1 and the final week of December are partial weeks of fewer than seven days, whereas every ISO week is a complete 7-day Monday-to-Sunday block.

Which Excel formula should I use for ISO vs. US week numbers?

In Microsoft Excel and Google Sheets, use =ISOWEEKNUM(A2) (or =WEEKNUM(A2, 21)) when you need Monday-start ISO-8601 week numbers from 1 to 53. Use =WEEKNUM(A2, 1) when you need traditional North American Sunday-start week numbers where January 1 is always Week 1.

Why do global supply chains and manufacturers quote delivery dates as W24 or W42?

Quoting production slots and ocean or air freight arrivals by ISO week (such as 'ETD 2026-W24') eliminates date-format confusion between US MM/DD/YYYY and European DD/MM/YYYY notation while giving factories and warehouses a realistic Monday-through-Sunday fulfillment window rather than a brittle single-day promise.