Week Number Calculator
Find the ISO 8601 week number and week-numbering year for a date, the week’s date range and a US-style week number.
Your inputs
Results
ISO 8601 week
Week 53 of 2026
- ISO week number
- 53
- ISO week-numbering year
- 2,026
- That ISO week runs
- 2026-12-28 to 2027-01-03
- US-style week (Sunday start, week of Jan 1 = 1)
- 1
- ISO weeks in that week-year
- 53
✓Exact calculation — rounded only for display
What this calculator does
ISO weeks start on Monday, and week 1 is the week containing the year’s first Thursday. That means early January can belong to the previous year’s last week.
Formula and method
ISO week = number of the Thursday-anchored week; week-year = year of that week’s Thursday
Worked example
Calculated by this tool with its default inputs:
Inputs
- Date: 2027-01-01
Results
- ISO 8601 week: Week 53 of 2026
- ISO week number: 53
- ISO week-numbering year: 2,026
- That ISO week runs: 2026-12-28 to 2027-01-03
- US-style week (Sunday start, week of Jan 1 = 1): 1
- ISO weeks in that week-year: 53
How to read the results
Years have 52 or 53 ISO weeks.
Assumptions and limitations
- US-style numbering starts weeks on Sunday with 1 January in week 1; other conventions exist.
Questions
Why is 1 January 2027 in week 53?
It is a Friday, so its week’s Thursday falls in 2026.
Methodology and verification
- Result type
- Exact mathematical calculation (rounded only for display)
- Formula version
- 1.0
- Automated verification
- 4 reference and validation test cases, checked against independently computed values. The site cannot be built if any of them fail.
- Editorial review
- Awaiting human editorial review. The explanation was drafted with AI assistance; the formula and examples are verified by the automated tests above.
Privacy
Everything you enter is calculated in this browser and is not sent to Numerith or anyone else. Inputs are added to a share link only if you choose to include them. Privacy policy