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Week Number Calculator (ISO 8601)

Enter a date to get its ISO week number, weekday, week start and end, day of year and quarter; switch to reverse mode to turn a year, week number and weekday back into a date.

Week numbers turn up in sprint planning, weekly reports and European order forms — and there are two conventions: ISO 8601, used internationally and standard in Europe, and the North American habit of starting the week on Sunday with the week containing 1 January as week one. The same date can fall a week apart between them. This tool works in ISO 8601 and gives the week number, the weekday, the week's start and end, the day of the year and the quarter.

ISO details worth knowing: Monday begins the week, and week 1 is the week containing the first Thursday — so a late-December date can belong to week 1 of the next year, and an early-January date to the last week of the previous one (1 January 2027 falls in week 53 of 2026). Confirm which convention a colleague is using before comparing week numbers, and use the reverse mode to go from year, week and weekday back to a date.

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How to use

  1. Enter a date to read its ISO week number, weekday, week start and end, day of the year and quarter.
  2. Use reverse mode to look a date up from a year, week number and weekday.
  3. Note the year boundaries — late December and early January can belong to the adjacent year's weeks.
  4. Confirm that the other party also uses ISO 8601 before comparing week numbers.

How it works

The ISO week-number definition

ISO 8601 defines a week as starting Monday, with the week containing the year's first Thursday (equivalent to containing January 4) as week 1. The point is to make "week N" consistent across countries and systems, avoiding the same day landing in different weeks. So 2026-09-13 (Sunday) falls in week 37 of 2026, running 2026-09-07 (Monday) to 09-13 (Sunday).

How year-boundary ownership arises

Because week 1 must contain Thursday, year-boundary ownership often shifts around New Year: 2026-01-01 (Thursday) is week 1 of 2026, and that week starts 2025-12-29; conversely 2024-12-30 (Monday), still in 2024, belongs to week 1 of 2025. Writing weekly reports or production plans without noticing can misassign those days to the wrong year.

Can a year have 53 weeks

A year has 52 weeks plus 1 day (common year) or 2 days (leap year); when those extra 1-2 days fall on Thursday or later, the year gets a week 53. ISO calls a year with 53 weeks a "long year," occurring roughly every 5-6 years. 2026 has week 53 (2026-12-28 to 2027-01-03), while 2025 doesn't.

Code example

JavaScript The ISO week number algorithm 

function isoWeek(d) {
  const t = new Date(Date.UTC(d.getFullYear(), d.getMonth(), d.getDate()));
  const day = t.getUTCDay() || 7;                 // Sunday counts as 7
  t.setUTCDate(t.getUTCDate() + 4 - day);         // move to this week's Thursday
  const yearStart = new Date(Date.UTC(t.getUTCFullYear(), 0, 1));
  return {
    year: t.getUTCFullYear(),
    week: Math.ceil(((t - yearStart) / 86400000 + 1) / 7)
  };
}

isoWeek(new Date(2027, 0, 1));   // { year: 2026, week: 53 }

Python isocalendar does it in one line 

from datetime import date

d = date(2027, 1, 1)
d.isocalendar()   # (2026, 53, 5) → Friday of week 53 of 2026

# The reverse: year + week number + weekday to a date
from datetime import timedelta
iso = date(2026, 1, 4)   # the Thursday of the week holding the year's first Thursday
date.fromisocalendar(2026, 53, 5)   # 2026-12-31

FAQ

How does the ISO week number differ from the calendar week?

In the start point and the definition of week 1. The ISO week always starts Monday; week 1 is the week containing the year's first Thursday (equivalent to containing January 4). So early-January days may count as the previous year's last week, differing from the intuition that January 1 is week 1.

Why can January 1 belong to the previous year?

If January 1 falls on Friday, Saturday or Sunday, its week doesn't contain a Thursday, so by ISO rules it joins the previous year's last week. For example 2027-01-01 is a Friday, belonging to week 53 of 2026, not week 1 of 2027.

Does a year always have 53 weeks?

No. A common year of 365 days is 52 weeks + 1 day; a leap year of 366 days is 52 weeks + 2 days. When those extra 1-2 days include a Thursday, the year produces week 53. 2026 has week 53; 2025 and 2027 don't.

How do I quickly find this year's week number for today?

Just enter today's date; the result also gives "day of the year." If your weekly-report system uses the non-ISO "January 1 is week 1" convention, the two differ by 1 in the boundary weeks.

Will weekly reports using ISO week numbers mismatch?

Confirm which convention your team uses. Under ISO, early-year days may belong to the previous year's week 52/53 and late-year days to next year's week 1; if the team starts a new week on January 1, the two numbering schemes offset by a week at the boundary. This tool uses the ISO convention for cross-checking system-generated numbers.

What weekday should I pick when reverse-lookup up a date?

As needed: Monday for a report's start, Sunday for its deadline. The default is Monday. If a year has no week 53 and you enter 53, the tool rolls it into the next year per ISO rules and notes "no such week this year; rolled into next year," avoiding a silently wrong date.

Is week-number calculation timezone-related?

This tool computes from the date string you enter, with no timezone conversion. But in a cross-timezone distributed system computing weeks from "now," different server timezones can put the same instant on different dates and thus different weeks.