ISO Week Number Lookup

What ISO week numbers are

ISO 8601 defines week numbering so calendars, payroll systems, and supply-chain reports across the world can agree on what 'week 23' means. Week 1 of an ISO year is the week containing the year's first Thursday — equivalently, the week containing January 4. Weeks run Monday through Sunday. A consequence is that the last few days of December may belong to week 1 of the next ISO year, and the first few days of January may belong to the last week of the previous ISO year. That's why this tool prints both the calendar year and the ISO year.

Current week number — quick lookup

Open the page and the date input is pre-filled to today, so the panel below it already shows the current ISO week number, ISO year, weekday, day-of-year, and quarter. The 'Set to today' button resets it after you've poked around. Common pairings: 'what week is it?' — read the headline; 'what week was March 17?' — type 03/17/current-year; 'what week is Christmas in 2026?' — type 2026-12-25.

ISO year vs calendar year — the boundary cases

The two years agree for most dates but diverge at the year edges. For example, 2026-01-01 is a Thursday, so it lives in ISO week 1 of 2026 — calendar and ISO years match. But 2027-01-01 is a Friday, which means it sits in ISO week 53 of 2026 (because that week's Thursday, 2026-12-31, is in 2026). When you write filenames or report headers like 2027-W01, double-check which year the W-number is keyed to — the tool always shows the ISO year explicitly.

• 2025-12-29 (Monday) → ISO 2026-W01-1 (calendar year 2025, ISO year 2026)
• 2026-01-04 (Sunday) → ISO 2026-W01-7
• 2026-12-31 (Thursday) → ISO 2026-W53-4 (the 53rd week exists because Jan 1 was Thursday)
• 2027-01-03 (Sunday) → ISO 2026-W53-7

ISO week compact notation

ISO 8601 lets you write a calendar date as 'YYYY-Www-D' where ww is the week number (zero-padded) and D is the weekday (1 = Monday through 7 = Sunday). The tool emits this string ready to copy — useful when a database column expects ISO-week format or a payroll cycle reports period boundaries that way. Without the day suffix ('YYYY-Www') the value identifies a whole week.

Total weeks in 2026 and other years

Most ISO years have 52 weeks; some have 53. The 53-week years are the ones where January 1 falls on a Thursday, or where a leap year has January 1 on a Wednesday. The 'Total weeks this ISO year' row tells you which kind 2026 is — and the 'Weeks remaining' row counts down how many ISO weeks are left in the displayed ISO year, useful for project planning that ends 'by end of year'.

• 53-week years near 2026: 2020, 2026, 2032, 2037, 2043, 2048
• Year 2026 itself: 53 ISO weeks (Jan 1 2026 is a Thursday)
• Adjacent: 2025 has 52 weeks, 2027 has 52 weeks

Day-of-year and quarter

Two more useful counters share the panel. Day-of-year (1–365 or 1–366 in leap years) lets you write succinct fixture identifiers like 'sample-2026-241' for August 29. Quarter (Q1–Q4) maps a date onto the standard fiscal-quarter split: Q1 = Jan–Mar, Q2 = Apr–Jun, Q3 = Jul–Sep, Q4 = Oct–Dec. Many companies start their fiscal year on a different month; if yours does, shift the date forward by the same number of months before reading the quarter.

Computing ISO week numbers in code

Every major standard library has this built in, and it's usually one method call. The tool's algorithm matches what those libraries do, so values are reproducible across languages.

• JavaScript (Intl): new Intl.DateTimeFormat('en-US', { weekYear: 'numeric', week: 'numeric' }).formatToParts(date)
• Python: date.isocalendar() returns (iso_year, iso_week, iso_weekday)
• Go: t.ISOWeek() returns (year, week)
• SQL (PostgreSQL): EXTRACT(WEEK FROM ts) and EXTRACT(ISOYEAR FROM ts)
• Java: LocalDate.of(...).get(WeekFields.ISO.weekOfWeekBasedYear())