Week Number Calculator (ISO 8601 & US)
Find the ISO 8601 week number and day of year for any date. Critical for European business, project planning, and SQL date queries.
What this calculates
Week numbers (W1-W53) are heavily used in European business, supply chain, manufacturing, and project planning. The ISO 8601 standard differs from the US convention — ISO weeks start on Monday and W1 is the week containing the first Thursday of the year. This calculator gives both ISO and US (Sunday-start) week numbers plus the day-of-year.
Formula & how it works
ISO week 1 = week containing the first Thursday of January (or equivalently, week containing Jan 4). Weeks start Monday. Some years have 53 weeks. Day of year (ordinal): 1-366. US convention: weeks start Sunday, week 1 contains Jan 1 — simpler but less standardized.
Worked example
Date: 2026-05-15 (Friday). ISO week: W20. US week: W21. Day of year: 135 (out of 365). New Year's Day 2026 was a Thursday — making the first Thursday Jan 1, so W1 starts Dec 29, 2025 in ISO. Confusingly, late-December dates often belong to W1 of the following year.
Frequently asked questions
Why use ISO weeks?
Standardized globally. Software (Postgres, Java, Python) defaults to ISO weeks. European businesses report sales/production by ISO week. Spreadsheets handle it via WEEKNUM(date, 21) in Excel/Sheets.
Can a year have 53 weeks?
Yes. ISO years that have 53 weeks happen when Jan 1 is a Thursday OR when it's a leap year and Jan 1 is a Wednesday. Examples: 2020 (53 weeks), 2026 (53 weeks).
Difference from US WEEKNUM?
US WEEKNUM (Excel default) makes Jan 1 always week 1 regardless of day. So Jan 1 Saturday makes a 1-day W1. ISO never has < 4-day weeks. Use Excel's `WEEKNUM(date, 21)` for ISO.
Day of year use cases?
Scientific data, agriculture, military scheduling, gross seasonal averaging. Aviation pilots reference 'Julian dates' (day of year) for fuel calculations and route planning.
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