OTHER · WEEK NUMBER
Week Number Calculator
Find the ISO 8601 week number for any date. Handles year-boundary traps where December dates fall in week 1 of the next year, or January dates belong to the previous year's week 53.
About This Calculator
Look up the ISO week number for any date — the standard used by payroll systems, project management tools, and international date notation (2026-W01). Enter a date and instantly see which week it falls in, along with the ISO year and the day of the week.
How It Works
ISO 8601 defines week 1 as the week containing the year's first Thursday; weeks run Monday–Sunday. This means some January dates belong to the previous year's final week, and some late-December dates fall in week 1 of the next year. The calculator implements the ISO week algorithm explicitly using Julian Day Numbers — no library shortcuts — and is tested against the known year-boundary traps.
The Formula
1. Find Thursday of the input date's week (shift ±days so the weekday = Thursday). 2. The ISO year is the calendar year of that Thursday. 3. Week 1 Monday = the Monday of the week containing January 4 of the ISO year (January 4 always falls in ISO week 1 by definition). 4. Week number = floor((input JDN − week-1-Monday JDN) ÷ 7) + 1.
Frequently Asked Questions
- Why can a December date be in week 1 of the next year?
- If the last days of December fall in a week whose Thursday is in January of the next year, that whole week belongs to the next year by ISO 8601 rules. For example, 2024-12-30 is in week 1 of 2025 because that week's Thursday (2025-01-02) is in 2025.
- Why can a January date be in week 53 of the previous year?
- The reverse applies. If Jan 1 falls on a Friday or later, the week's Thursday is in December of the prior year — so Jan 1 belongs to that prior year's last week. For example, 2021-01-01 (Friday) is in week 53 of 2020.
- What does the ISO year mean?
- The ISO year is the year that "owns" the week — it's almost always the same as the calendar year, except at the very start and end of the year at year boundaries. The notation "2026-W01" means ISO year 2026, week 1.
- Do weeks always start on Monday?
- Yes, in ISO 8601. Some other systems (like the US) count weeks starting on Sunday, which produces different week numbers. This calculator uses the international ISO standard.