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.

What is the Week Number System Reference?

Reference for ISO 8601 (Monday-start) versus US/Canada (Sunday-start) week numbering, including the week-1 rule for each system and a live calculator that shows both week numbers for any date. It runs free in your browser on Gera Tools, with nothing uploaded.

Week Number System Reference

Name: Week Number System Reference
Creator: Gera Tools
License: https://creativecommons.org/licenses/by/4.0/

Get one useful tool a week

Like this tool? Enter your email and we'll send you one genuinely useful Gera tool a week — plus a link to come back to this one. No spam, one-click unsubscribe any time.

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.

The two systems compared

Property	ISO 8601	US / Broadcast
Week start day	Monday	Sunday
Week 1 rule	Week containing the first Thursday (= Jan 4)	Week containing January 1st
Week belongs to	The year owning its Thursday	The calendar year of Sunday
Max weeks per year	52 or 53	52 or 53
Used widely in	Europe, ISO standards, most of Asia	USA, Canada, broadcast TV scheduling

How ISO 8601 week numbers are calculated

ISO 8601 defines week 1 as the week containing the year’s first Thursday, which is equivalent to 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 any date:

Find the Thursday of that date’s week (Monday = day 1, Sunday = day 7).
Identify which calendar year that Thursday falls in.
Count how many Mondays have passed since the first Monday on or before January 4th of that year.

This means the first days of January can legally belong to the final ISO week of the previous year, and late December can fall in ISO week 1 of the next year.

How the US system works

The US/broadcast system is simpler: weeks start on Sunday, and week 1 is just the week containing January 1st. A partial week at the start of January is week 1 regardless of how many days it contains. 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 early January.

Worked examples of disagreement

For January 1st, 2026 (a Thursday): ISO 8601 places it in week 1 of 2026 because that week’s Thursday is January 1st itself, which falls in 2026. The US system also calls it week 1 of 2026, so the two systems agree.

But January 1st, 2023 (a Sunday) is ISO week 52 of 2022: that Sunday’s Monday–Sunday week has its Thursday on December 29, 2022, so ISO assigns it to 2022 week 52. The US system calls January 1st, 2023 week 1 of 2023 — a full-year and full-week disagreement despite referring to the exact same day.

When 53-week years occur

An ISO year has 53 weeks when January 1st falls on a Thursday, or when it falls on a Wednesday in a leap year. This happens roughly 71 times in every 400-year cycle. A US/Sunday-start year also occasionally has 53 weeks when the year spans 53 Sundays.

Practical guidance

Always write ISO week numbers with the year: 2026-W01, not just W01 or week 1.
When exporting dates across systems (payroll, ERP, retail 4-4-5 calendar, broadcast TV), confirm which standard the receiving system expects.
Database functions like PostgreSQL EXTRACT(WEEK FROM date) typically use Monday-start ISO weeks, but SQL Server’s DATEPART(wk, date) uses Sunday-start US weeks — check your dialect’s documentation.
Never assume a bare “week 5” means the same thing in two organisations.