ISO 8601 Week Numbers Explained

Last reviewed on May 4, 2026

Every monthly calendar on this site has a thin column on the left labelled W1, W2, and so on. Those are ISO 8601 week numbers. They look obvious until they don't: week 1 is sometimes in the previous calendar year, December can land in week 1 of next year, and some years have 53 weeks instead of 52. This page explains the rule, why it works the way it does, and how to read week numbers without surprises.

The rule in one sentence

Under ISO 8601, week 1 of a year is the week that contains the first Thursday of January. Weeks start on Monday, so week 1 is also the first week with at least four days in the new year.

That single sentence drives every other oddity on this page.

Why Thursday?

A week straddles two calendar years whenever January 1 falls on Tuesday, Wednesday, Thursday, Friday, Saturday, or Sunday. ISO 8601 needed a tiebreaker for which year that mixed week belongs to. The standard picks Thursday because it's the day at the centre of the working week. If the Thursday of the week is in January, the whole week — including any leftover December days — counts as week 1 of the new year. If the Thursday is still in December, the whole week counts as the last week (52 or 53) of the old year.

Worked example: late December and early January

Take the boundary between 2025 and 2026. December 29, 2025 is a Monday; the Thursday of that week is January 1, 2026. So that whole week — Mon Dec 29 through Sun Jan 4 — is ISO week 1 of 2026, even though the first four days are still in 2025. On the January 2026 page, the W1 row contains both December 29 and January 1.

Compare that with the boundary between 2026 and 2027. January 1, 2027 is a Friday; the Thursday of that week is December 31, 2026. So the week of Mon Dec 28, 2026 through Sun Jan 3, 2027 is ISO week 53 of 2026, not week 1 of 2027. That's right — 2026 is a 53-week year.

52-week years and 53-week years

Most years have 52 ISO weeks. About one year in five or six has 53. The pattern depends on what day January 1 falls on and whether the year is a leap year.

53-week year if January 1 is a Thursday, or if it's a Wednesday in a leap year.
52-week year in every other case.

The next 53-week years from 2025 onward are 2026, 2032, 2037, and 2043. Anyone who runs a payroll, a fiscal quarter, or a content calendar by ISO weeks needs to know which years carry an extra week — it's the difference between 52 and 53 pay periods, or 52 and 53 weekly issues.

Why the W column on this site sometimes repeats

Each row of a monthly calendar represents one ISO week. The W number is computed from the Thursday of that row, which is why the same W can appear at the start of January and at the end of December for the same calendar year. It's also why the first row of a month sometimes shares a W number with the last row of the previous month — they're the same ISO week, split by the month boundary.

ISO weeks vs US "week-of-year" numbering

North American business calendars sometimes use a different rule: week 1 starts on January 1 regardless of the day of the week. Under that scheme, January 1 is always in week 1. Under ISO 8601, January 1 is in week 1 only if it's Monday, Tuesday, Wednesday, or Thursday. If you exchange spreadsheets with collaborators in different countries, check which scheme they use before relying on a week number — Microsoft Excel, for example, defaults to a US-style week numbering and needs an explicit option to use ISO 8601.

How to use ISO weeks day-to-day

Project planning. "Sprint W23" or "release W42" is shorter and less ambiguous than a full date range, especially across time zones.
Manufacturing and supply chain. Production schedules and shipment labels routinely use ISO week numbers.
Payroll. Bi-weekly pay periods are easier to align when both periods are described as W21–W22, W23–W24, etc.
Editorial calendars. Weekly publications use the W number to refer to issues without committing to a specific date.

A common pitfall: dates near the year boundary

December 31, 2025 is Wednesday — Thursday of that week is January 1, 2026. If you sort tasks by "year then week," a task dated December 31, 2025 with ISO week 1 would come before a task dated December 30, 2025 with ISO week 53 of 2025. Most spreadsheet sort orders won't catch that. The fix is to use the ISO 8601 "year-week" form together: 2026-W01, not just W1.

Quick checklist

Week 1 is the week containing the first Thursday of January.
Weeks run Monday through Sunday.
A year has 52 or 53 ISO weeks; 2026 has 53.
January 1 can be in week 52 or 53 of the previous year.
Always pair the week number with its ISO year (2026-W01) when sorting.

See the W column in action

Every monthly calendar on this site shows ISO weeks in the leftmost column. Pick any month from the 2026 overview to see the rule applied. The boundary months — January 2026 and December 2026 — show the most interesting cases.

Leap years — the rule and the next leap year.
Working days vs calendar days — counting business days around holidays.
Fiscal quarters and calendar quarters — where Q1, Q2, Q3, Q4 line up.