US vs ISO week numbers
The same day can be “week 44” in one country and “week 44” in another — yet cover different dates. Here is where the two counts come from, and the dates where they pull apart.
The difference is two choices: which day starts the week and which week counts as week 1. ISO 8601 starts weeks on Monday and anchors week 1 to the year’s first Thursday. The common US calendar convention starts weeks on Sunday and calls the week containing January 1 “week 1”, even if that week starts in December.
The definitions side by side
| ISO 8601 | US convention | |
|---|---|---|
| First day of the week | Monday | Sunday |
| Week 1 | Contains the first Thursday (or January 4) | Contains January 1 |
| Boundary behaviour | A week belongs to the year of its Thursday; up to 3 days can spill into a neighbouring ISO year | The week containing January 1 always opens the new year’s count, starting as early as December 28 |
| Status | International standard (ISO 8601) | Customary calendar practice — no single official US standard |
Dates where the systems disagree
| Date | ISO 8601 | US convention |
|---|---|---|
| Jan 1, 2023 | Week 52 of 2022 Mon Dec 26, 2022 – Sun Jan 1, 2023 | Week 1 of 2023 Sun Jan 1, 2023 – Sat Jan 7, 2023 |
| Jan 1, 2026 | Week 1 of 2026 Mon Dec 29, 2025 – Sun Jan 4, 2026 | Week 1 of 2026 Sun Dec 28, 2025 – Sat Jan 3, 2026 |
| Oct 27, 2026 | Week 44 of 2026 Mon Oct 26, 2026 – Sun Nov 1, 2026 | Week 44 of 2026 Sun Oct 25, 2026 – Sat Oct 31, 2026 |
| Jan 1, 2027 | Week 53 of 2026 Mon Dec 28, 2026 – Sun Jan 3, 2027 | Week 1 of 2027 Sun Dec 27, 2026 – Sat Jan 2, 2027 |
| Dec 30, 2024 | Week 1 of 2025 Mon Dec 30, 2024 – Sun Jan 5, 2025 | Week 1 of 2025 Sun Dec 29, 2024 – Sat Jan 4, 2025 |
Two patterns explain nearly every mismatch. Around New Year, ISO pushes early-January days back into the old year whenever January 1 lands late in the week — January 1, 2023 is ISO week 52 of 2022, but plainly week 1 of 2023 in US counting. The rest of the year, the one-day offset between Monday-start and Sunday-start weeks means the counts tick over on different days, so a Sunday belongs to the next US week but still closes the current ISO week.
Which one will you meet?
European employers, airlines, rail and freight schedules, and most international project plans use ISO weeks. US wall calendars, planners and much American software use the Sunday-start count. When a bare “week 12” crosses the Atlantic in an email, it is worth one clarifying question — or sidestep the whole issue by naming the Monday (or Sunday) date instead. The calculator shows both counts for any date, so you can translate on the spot.