What is it?
Week Number shows the ISO week number of today or any Date, the US-style week number, and the dates of that week.
Who is it for?
Factory, logistics and project teams that plan by week number, students and office staff using planners, and Excel users checking week formulas.
How to use it
- Pick a Date. Today is shown at first.
- Read the ISO week number, the US-style week number and the dates of that week.
ISO or US week?
- ISO 8601 (Europe, India in most business tools, Excel’s
ISOWEEKNUM): weeks start on Monday, and week 1 contains the year’s first Thursday. Some years have 53 weeks. - US: weeks start on Sunday, and week 1 contains 1 January.
- Near New Year the two can differ, and 1 January can be in week 52 or 53 of the previous ISO year.
Uses
- Factory production plans, shipping schedules and sprints that use “week 32”.
- Pregnancy and school timetables count weeks differently; use their own calculators.
In Excel and Sheets
=ISOWEEKNUM(A1) gives the ISO week. =WEEKNUM(A1) gives the US-style week by default.
Worked examples
- 1 January 2027 is a Friday. In ISO weeks it belongs to week 53 of 2026, because the week containing it has its Thursday in 2026. In US numbering it’s week 1 of 2027.
- 4 January is always in ISO week 1. That’s an easy way to remember the rule.
Writing ISO weeks
The standard format is 2026-W45, and with a day 2026-W45-3 (Wednesday of week 45). Planners, factories and software use this format to avoid confusion.
Why years have 52 or 53 weeks
A year has 52 weeks and one or two days extra. Every five or six years, those days add up to a 53rd ISO week, which happens when the year starts on a Thursday (or a Wednesday in a leap year).
Common mistakes
- Mixing ISO and US numbering in one team: agree on one, usually ISO.
- Assuming week 1 starts on 1 January: in ISO it can start in late December.
Related tools
What Day Was This Date · Working Days Calculator · Date Converter
Errors and how to fix them
- My planner shows a different week number: it probably uses the US rule (weeks start on Sunday) while the ISO number follows the European rule. The tool shows both; agree with your team which one to use.
- Nothing is shown: the date box is empty. Pick a date.
Open the tool: Week Number
Still stuck?
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Questions, or an error not listed here?
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