What is the difference between ISO 8601 and US week numbering?

ISO 8601 weeks start on Monday and week 1 is the week containing the first Thursday of the year (equivalently, the week with January 4th). US numbering starts weeks on Sunday and week 1 is simply the week containing January 1st. The two can differ by one, especially near year boundaries.

Why can a date be in week 52 or 53 of the previous year under ISO 8601?

ISO weeks belong to whichever year holds their Thursday. The first days of January can fall in the final ISO week of the prior year, so January 1st might be ISO week 52 or 53 of the previous year even though the calendar year has changed.

Which countries use ISO 8601 weeks?

Most of Europe, plus much of Asia and many ISO-aligned standards bodies, use Monday-start ISO 8601 weeks. The United States, Canada, and several other countries commonly use Sunday-start numbering, especially in business, payroll, and broadcast scheduling.

When does a year have 53 ISO weeks?

An ISO year has 53 weeks when January 1st is a Thursday, or when it is a Wednesday in a leap year. Otherwise the year has 52 weeks. About 71 of every 400 years are 53-week years.

Is week 0 ever valid?

ISO 8601 never uses week 0; the lowest week is 1. Some Sunday-start or simple-count systems do label the partial first week as week 0, but that is non-standard. This tool reports ISO weeks 1 through 53 only.

Week Number System Reference

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Two incompatible week-numbering systems are in everyday use, and they routinely disagree by one week near the start of the year. This reference explains the ISO 8601 (Monday-start) rule used across most of Europe and the US/broadcast (Sunday-start) rule, and computes both numbers for any date you pick.

How it works

ISO 8601 defines week 1 as the week containing the year’s first Thursday, which is the same as the week containing January 4th. Weeks run Monday to Sunday, and each week belongs to the year that owns its Thursday. To find the ISO week of a date, take the Thursday of that date’s week, then count weeks from the first Thursday of that Thursday’s year.

The US/broadcast system is simpler: weeks start on Sunday, and week 1 is just the week containing January 1st. Because the first partial week counts as week 1 and the start day differs, the US number is often one higher than the ISO number in January.

Example

For January 1st, 2026 (a Thursday): ISO 8601 places it in week 1 of 2026, because that week contains the first Thursday. The US system also calls it week

But January 1st, 2023 (a Sunday) is ISO week 52 of 2022, while the US system calls it week 1 of 2023 — a full-week disagreement.

Notes

When exchanging week numbers across systems, always state the standard and the year explicitly, for example 2026-W01 for ISO. Payroll, broadcast, and retail-calendar (4-4-5) systems each add their own variations, so never assume a bare “week 5” means the same thing in two organisations.