Week number calculator
This tool tells you which week of the year any date falls in, using both the international and US conventions. Week numbers are widely used in payroll, project planning, manufacturing schedules, and shared calendars, where teams refer to “week 23” rather than a specific date.
How it works
For the ISO 8601 number (weeks start Monday; week 1 is the week containing the year’s first Thursday), the calculator shifts your date to its nearest Thursday, then counts the whole weeks between that Thursday and the Thursday of week 1 (the week containing 4 January), adding one. Because the Thursday always lands in the “majority” year, this also yields the correct ISO year, which can differ from the calendar year at the start or end of December and January.
It also shows the US week number (weeks start Sunday; the week containing 1 January is week 1), the day of year (1 January = 1), and the weekday.
| Field | Convention |
|---|---|
| ISO 8601 week | Monday start, week 1 holds first Thursday |
| US week | Sunday start, week 1 holds 1 January |
| Day of year | Ordinal day, 1–365/366 |
Example
Take 1 January 2027, which is a Friday. Its nearest Thursday falls in late December 2026, so the ISO result is Week 53 of 2026, while the US convention counts it as Week 1 of 2027. The day of year is 1. This new-year overlap is exactly why showing both numbers matters.
When week numbers differ by convention
The two systems diverge most noticeably in the first and last days of the year:
- Late December dates — for example, 30 December 2024 is ISO week 1 of 2025, but US week 53 of 2024. A project plan using the wrong convention could misplace a deadline by an entire year.
- Early January dates — 2 January 2026 (a Friday) falls in ISO week 53 of 2025, not week 1 of 2026.
- Mid-year — the two systems are always in sync from roughly February to October, so the difference is a genuine edge case at the calendar boundaries.
Practical uses for week numbers
Payroll and HR — biweekly payroll in the US typically labels pay periods by week number. Knowing whether your system uses ISO or US numbering prevents schedule errors around the new year.
Manufacturing and logistics — European supply chains almost universally use ISO week numbers on delivery schedules, purchase orders, and production plans. Quoting “week 6” without knowing the convention can shift a delivery by one week.
Project management — agile sprints, gantt charts, and reporting windows often reference week numbers. Tools like Jira and Microsoft Project let you choose ISO or US. Confirm which your team uses before sharing a timeline.
Day of year — the ordinal day count (1–365 or 366) is useful for date arithmetic in programming, agriculture scheduling, and scientific data. Day 100 is always 10 April in a non-leap year.
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