ISO Week Number Calculator

Direct answer

Enter a Gregorian date to find its ISO week number, ISO week-year and ISO weekday. ISO 8601 standardizes date and time representations; ISO week-years can differ from calendar years near New Year.

Variables and formula

• Year, month and day: a valid Gregorian date.

ISO week = week containing the date's Thursday, numbered from the first ISO week of its week-year

Reproducible example

January 5, 2026 is Monday in ISO week 2 of ISO week-year 2026. January 1, 2021 belongs to ISO week 53 of week-year 2020.

Interpretation

Use the week-year alongside the week number for reports, schedules and filenames. A calendar year alone can be misleading at the boundary.

Assumptions and limitations

• The date is Gregorian and valid.
• ISO weeks start Monday and ISO weekdays run 1 through 7.
• Local timestamp and locale-specific week rules are excluded.

Common mistakes

• Pairing week 1 with the wrong calendar year.
• Using a locale’s Sunday-start week rule.
• Passing an impossible date such as February 30.

Assumptions

• The entered date is a valid Gregorian calendar date.
• The calculation uses ISO week-date conventions.
• Time zones, timestamps and locale-specific week numbering are excluded.

Sources and methodology

CalcMotive publishes the formula and assumptions so you can decide whether the estimate fits your use case. See our methodology standards.

• ISO 8601 Date and Time Format; International Organization for Standardization; accessed Aug 10, 2026