ToolForage.
DATE & TIME CALCULATOR

ISO Week Calculator

Choose a calendar date to find its ISO week number, week-numbering year, and full Monday-to-Sunday week range.

Calculator

YOUR RESULTS
ISO week date2020-W53-5
ISO week number
53
ISO week-numbering year
2,020
Week starts Monday
2020-12-28
Week ends Sunday
2021-01-03

Page guide

ISO week date and interpretation

The full code uses ISO year, week number, and weekday number. Monday is weekday 1 and Sunday 7. Near New Year, the ISO year can differ from the date’s ordinary calendar year.

To identify the weekday of a calendar date, use the Day of the Week Calculator and follow its own input conventions.

The formula

ISO weeks begin Monday; week 1 contains January 4. The week-numbering year is the year containing the week’s Thursday.

Locate the Thursday in the date’s Monday-start week to determine the ISO year. Count weeks from that year’s beginning; its first week is the one containing January 4.

Verified worked example

January 1, 2021 is a Friday in ISO week 53 of 2020, written 2020-W53-5. That week begins December 28, 2020 and ends January 3, 2021.

How to use this calculator

  1. Choose a valid date from calendar year 1000 through 9999.
  2. Calculate its ISO week code and compare the ISO year with the calendar year.
  3. Read the Monday and Sunday endpoints; a supported boundary week may extend into an adjacent year.

ISO Week input conventions

ISO week numbering organizes complete Monday-to-Sunday weeks. Week one is the week containing January 4, equivalently the first Thursday of the year. Because complete weeks cross the New Year boundary, an early-January date can belong to the previous ISO year and a late-December date can belong to the next one.

Interpreting iso week date

The last digit in the full week date numbers Monday as one through Sunday as seven. Calendar year and ISO week year should not be mixed when building labels: combining a January calendar year with the previous year’s week 53 creates a misleading code. This tool treats the entered date as a calendar day in UTC for deterministic arithmetic; it does not convert an instant from a device time zone or apply a fiscal calendar.

Assumptions & limitations

What this calculation assumes

  • Weeks begin Monday and week one contains January 4 under the ISO convention.

What to keep in mind

  • ISO calendar weeks only; no fiscal weeks, local Sunday-start rules, or date-time zone conversion.

Common questions

Can an ISO year have week 53?

Yes. Some ISO week-numbering years have 53 weeks. The final week’s days can extend into the following Gregorian calendar year.

Why does my calendar app show another number?

It may use Sunday-start weeks or define week one as the week containing January 1. Those conventions differ from ISO Monday-start week numbering.

What is the final digit in an ISO week date?

It is the weekday number, with Monday as 1 through Sunday as 7. For example, the 5 in 2020-W53-5 means Friday.

Sources & further reading

Reviewed on