ISO Week Number Calculator

Formula

ISO week number = week of the ISO week-year containing the date

Worked example

With Calendar date: 2026-10-21, iso week number is 43.

Calculation method

The rule is ISO week number = week of the ISO week-year containing the date.

Reading iSO week number

Day of year counts January 1 as day 1; an ISO week follows the ISO week-year; month length depends on the month and leap-year status. In the worked values on this page, calendar date 2026-10-21 produce iso week number 43.

Practical use

Use the result for schedules, reporting periods, and date labels. The specific task here is to find iso week number from calendar date.

Limits and common mistakes

An ISO week-year can differ from the calendar year near New Year. Month length and days remaining are calendar facts, not fixed 30-day approximations. Verify calendar date before interpreting iSO week number; a value measured on another basis would answer a different question.

Read the example and units

The worked values on this page use calendar date 2026-10-21. From that selected calendar date, the calculator reports iso week number 43. Read that number using this page’s calendar convention; one date can have an ordinal position, a number of remaining days, or a week label, and this page reports only the named result. The output follows this page’s specific calendar or elapsed-time convention; another convention can produce a different answer from the same dates or clock readings. Calendar counts depend on the exact dates and the rule for including endpoints, complete months, weekdays, or week numbering. A date is not a fixed number of seconds in a local time zone. Read the named output carefully before comparing it with a different date-counting convention.

Compare one changed input

For a second scenario, change only calendar date 2026-10-21 to calendar date 2026-10-28 and leave the other example entries alone. The result for iso week number changes from 43 to 44. Comparing one changed input at a time shows exactly which assumption drove the difference in this calculation. A calendar answer can change in discrete steps at a day, month, year, or ISO week boundary. Try a value near the boundary relevant to your question and check whether the start or end date is included.

How to interpret this kind of result

ISO weeks begin on Monday, and week 1 is the week containing the year’s first Thursday. Near New Year, the ISO week-year can differ from the ordinary calendar year printed in a date. This calculator reports the week number only, so verify the corresponding ISO week-year separately when labeling a report or schedule. For this iso week number calculation, calendar date determine iso week number; the displayed iso week number 43 belongs to that exact combination rather than to a different rate, term, or measurement basis.

Scope and practical checks

This is a calendar convention, not a count of seven-day periods since January 1. Different workplaces can use Sunday-start or custom fiscal weeks. Confirm the reporting system before comparing a computed ISO week with a week label from another source. Check the calendar date entries against the quantities in your own situation before using iso week number; this page cannot infer a field or condition that you did not enter.

Common questions

What does this iso week number result include?

The iso week number comes from calendar date using the displayed rule ISO week number = week of the ISO week-year containing the date. It does not automatically include a condition absent from the listed inputs. Check the stated rule for calendar boundaries, weekends, time zones, or elapsed time before applying a deadline.