Week Number Calculator
Find the ISO week number and week-numbering year for any date.
This tool runs entirely in your browser. Your files are never uploaded to a server.
Week 1
ISO week-numbering year 2026
What this tool does
Finds the ISO 8601 week number for any date, along with that week's "week-numbering year" — which, right around January 1st, can differ from the date's calendar year.
How to use it
Pick a date, and the ISO week number and week-numbering year update immediately. Reset returns to the default example of 2026-01-01.
Worked example
2026-01-01 falls in ISO Week 1 of 2026. But 2027-01-01 falls in ISO Week 53 of 2026, not week 1 of 2027 — because 2026 starts on a Thursday, it's one of the years with 53 ISO weeks, and the first days of January 2027 still belong to that week.
Method
ISO 8601 weeks start on Monday, and a year's week 1 is defined as the week containing that year's first Thursday. The tool finds the Thursday of the same week as your chosen date, then counts how many full weeks separate that Thursday from January 1st of its own year — the year shown as the "week-numbering year" is the Thursday's year, not necessarily the input date's year.
Why the week-numbering year can differ
Because week boundaries don't align with January 1st, the first few days of January can belong to the last week of the previous year (as in the 2027 example above), and the last few days of December can belong to week 1 of the next year. This is standard ISO 8601 behavior, not a bug.
Limitations
This follows the ISO 8601 standard specifically — Monday-start weeks, first-Thursday rule — which differs from the US convention some spreadsheet WEEKNUM functions use by default (Sunday-start weeks, week 1 always containing January 1st). Compare against your target system's documentation if the numbers need to match exactly.
Privacy and processing
The calculation runs entirely in your browser using JavaScript. The date you enter is never uploaded or stored.
Frequently Asked Questions
- What does ISO 8601 week numbering mean?
- Weeks start on Monday, and week 1 of a year is the week containing that year's first Thursday — equivalently, the week containing January 4th.
- Why can a January date belong to the previous year's week 53?
- Because ISO weeks don't reset exactly at January 1st. If January 1st falls on a Friday, Saturday, or Sunday, it's still part of the last ISO week of the previous year, which can be numbered 53.
- Does this match the week numbers in Excel or Google Sheets?
- Only if you're using the ISO week mode. Many spreadsheet WEEKNUM functions default to a US-style system with Sunday-start weeks, which numbers weeks differently — check your spreadsheet's ISO-specific option for a match.
- Can I also find a date's ordinal day within the year?
- Yes, use the Day of Year Calculator for that instead.