ISO 8601 Week Number Calculator
Pick any date and the tool returns its ISO 8601 week number (e.g. "2026-W20"), the ISO year, the weekday and the Monday-to-Sunday range of the containing week. ISO 8601 is the week numbering used in international business, ERP/supply-chain calendars, fiscal weeks, Agile sprints and tools like Jira/GitLab/Outlook. The quirks: weeks start on Monday, week 1 of a year is the week containing 4 January (equivalently, containing the first Thursday) — so early January can fall into the last week of the previous ISO year, and late December can roll into ISO week 1 of the next year.
Enter a valid Gregorian date (YYYY-MM-DD).
ISO 8601 year-week
2026-W20
Mon, 11 May 2026
11 May – 17 May
Day of the week (Mon → Sun)
ISO year
2026
Weeks in this ISO year
53
ISO date code
2026-W20-5
Formula
weekday = (Gregorian-weekday + 6) mod 7 + 1 (1 = Mon, 7 = Sun) ordinal = day-of-year for the input date week = floor((ordinal − weekday + 10) / 7) if week < 1 → roll back to the last ISO week of the previous year; if week > weeksInYear → roll forward to ISO week 1 of the next year.
- · Weeks start on Monday and end on Sunday (NOT Sunday-first as in the US calendar).
- · Week 1 is defined as the week containing 4 January — equivalently, the week of the year's first Thursday (ISO 8601:2019 §4.1.4).
- · "Long" ISO years have 53 weeks; they occur when 1 January falls on a Thursday, or when the year is a leap year and 1 January or 31 December is a Thursday. All other years have 52 weeks.
- · Examples: 2021-01-01 (Friday) belongs to 2020-W53; 2024-12-30 (Monday) belongs to 2025-W01.
- · The full ISO date format is YYYY-Www-D where D is the weekday (1 = Mon … 7 = Sun). E.g. 2026-W20-5 = ISO week 20 of 2026, on Friday.
- · ISO week dates are standard in international business, ERP / supply-chain planning, manufacturing schedules, banking reconciliation, ISO-audited processes, and dev tools like Jira / GitLab / Outlook.
- · All math is in UTC, so the result does not shift with the browser's timezone.
Frequently asked
How does the ISO week differ from the US-style week number?
Two differences: (1) the start of the week — ISO weeks start on Monday, US-style weeks usually start on Sunday (some systems use Saturday). (2) the definition of "week 1" — ISO week 1 contains 4 January / the year's first Thursday; US conventions vary, the most common being "the week containing 1 January", which makes 1 January always week 1. Retailers using a 4-4-5 fiscal calendar instead anchor on the Sunday closest to 1 January. The result is that ISO and US-style week numbers for the same date often disagree by 1–2 weeks, so cross-border teams need to agree on which standard they are using.
Why is January 1 sometimes shown as "W52" or "W53" of the previous year?
Because ISO week 1 is the week containing 4 January. If 1 January is a Friday, Saturday or Sunday, the Thursday of that week falls back in December — so the whole week still belongs to the previous ISO year. E.g. 2021-01-01 was a Friday → that week's Thursday was 2020-12-31 → the whole week is 2020-W53. Same with 2017-01-01 (Sunday → 2016-W52) and 2023-01-01 (Sunday → 2022-W52). The simplest mental model: a week belongs to the ISO year of its Thursday.
Why do some years have 53 ISO weeks instead of 52?
A common Gregorian year has 365 days (52 weeks + 1 day); a leap year has 366 (52 + 2). Because ISO requires week 1 to contain 4 January, every five or six years that extra day(s) push the calendar far enough for a 53rd ISO week to appear. Specifically, an ISO year has 53 weeks when 1 January is a Thursday (common year), or when the year is a leap year and 1 January or 31 December is a Thursday. Long ISO years over the next few decades: 2004, 2009, 2015, 2020, 2026, 2032, 2037, 2043, 2048, 2054. They occur in 71 of every 400 years — about 17.75 % of years.
How do I get the same ISO week number in Excel or Google Sheets?
Both Excel and Google Sheets have ISOWEEKNUM() — `=ISOWEEKNUM(A1)` returns the ISO week number (1–53) for the date in A1, matching this tool. To get the ISO year use `=YEAR(A1 - WEEKDAY(A1, 2) + 4)` (the calendar year of the week's Thursday, which is the ISO year by definition). Note that the older WEEKNUM() defaults to US-style week numbering — always reach for ISOWEEKNUM() when you need the ISO standard.
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