Week Number Calculator
Week numbers run payroll, sprint planning and financial reporting, and the standard behind them — ISO-8601 — hides one quirk: a week belongs to the year containing its Thursday. That is why 31 December 2025 files under week 1 of 2026, and why this page prints the week-year beside the calendar year rather than mixing the two. Enter a date and it returns the week, the weekday and the days left before New Year.
Enter values
Results
- ISO week
- Week 43 of 2026
- Weekday
- Saturday
- Days remaining in the year
- 68
How is the Week Number calculated?
Enter day, month and year as three numbers, the same shape the cluster's other date pages use. The calculator locates the Thursday of the date's week and reads the year it lands in — that year names the ISO week. It counts weeks from the year's first Thursday for the number, names the weekday, and subtracts the day's position from the year's length for the days-remaining row. Impossible dates such as 31 February are refused in plain words rather than crashing the page.
Formula
ISO week 1 is the week containing 4 January — the year's first Thursday — and weeks start on Monday. Week number = weeks between the date's Thursday and that first Thursday, plus one. The week-year is the calendar year of that Thursday, which differs from the date's year near New Year. Days remaining = days in the calendar year − day of year.
Example
24 October 2026 is a Saturday, inside the week running Monday 19 to Sunday 25 October; its Thursday, 22 October, sits in 2026, so the page reports week 43 of 2026 with 68 days left in the year. The year boundary shows the quirk: 31 December 2025 carries its Thursday into 1 January 2026, giving week 1 of 2026 despite the calendar year 2025 — and 1 January 2026, itself a Thursday, agrees at week 1.
Things to remember
Once a team adopts ISO weeks — Monday as day one, no week ever empty — report figures stop arguing across the year boundary. Remember the week-year whenever a 'week 1' number shows in late December: it may already belong to the next budget year. Counting working days inside that week is the working-days page's job.
Frequently Asked Questions
What is the ISO week number standard?
ISO-8601 defines a week as a Monday-to-Sunday unit, with week 1 being the week that contains the first Thursday of the year. The rule guarantees every year holds either 52 or 53 complete weeks, which keeps weekly reporting aligned wherever the calendar flips over.
Why does 31 December show week 1 of next year?
Because each week belongs to whichever year contains its Thursday. When 29 to 31 December fall on Monday to Wednesday, their Thursday is already in January, so ISO-8601 files those days under week 1 of the following year — the calculator prints that year explicitly as the week-year.
Why can 1 January belong to the previous year?
If 1 January falls on a Friday, Saturday or Sunday, its week does not contain the new year's first Thursday, so ISO-8601 assigns the date to week 52 or 53 of the year before. The page shows week and week-year together, keeping the pair readable in both directions.
How many ISO weeks are in a year?
Either 52 or 53. A year gains a 53rd ISO week when it begins on a Thursday, or on a Wednesday in a leap year — 2020 qualified, and its final days carried week 53 all the way through to 31 December.
Related Calculators
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- Day of Week CalculatorDate & AgeFind the weekday, day of the year and ISO week number for any date, and see which ISO week early-January and late-December dates sit in.
- Working Days CalculatorDate & AgeCount working days (Monday–Friday) between two dates, excluding weekends and optional public holidays.
- Date Difference CalculatorDate & AgeFind the exact number of days, weeks, months and years between any two dates, with inclusive and exclusive counts.