Week Number Calculator

Q: "What is the ISO week number?"

"ISO 8601 defines weeks: Week 1 is the week containing the first Thursday of the year. Monday is day 1."

Q: "How do I get the week number for a date?"

"Enter the date. The calculator returns the ISO week number (1–53) and the week-year (which can differ from calendar year near Jan 1)."

Q: "Why does week-year sometimes differ from the year?"

"E.g. Jan 1, 2023 is a Sunday, so it belongs to week 52 of 2022. The week-year is 2022."

Q: "What date format can I use?"

"YYYY-MM-DD or your locale format (e.g. MM/DD/YYYY, DD/MM/YYYY)."

Q: "How many weeks in a year?"

"Most years have 52 weeks. Some have 53 (when the year has 53 Thursdays)."

Get ISO week number and year for any date. Example: Jan 1, 2025 → Week 1, 2025. Copy result.

Week number (ISO 8601) is the week of the year (1–53) for a date. Week 1 is the week with the year’s first Thursday. Monday is the first day of the week. Enter a date. The calculator returns the ISO week number and week-year (which can differ from the calendar year near Jan 1). Copy the result or Copy with labels.

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Examples

Jan 1, 2025 → Week 1, 2025
Dec 31, 2024 → Week 1, 2025 (ISO)
Jan 1, 2023 → Week 52, 2022

FAQ

What is the ISO week number?

ISO 8601 defines weeks: Week 1 is the week containing the first Thursday of the year. Monday is day 1.

How do I get the week number for a date?

Enter the date. The calculator returns the ISO week number (1–53) and the week-year (which can differ from calendar year near Jan 1).

Why does week-year sometimes differ from the year?

E.g. Jan 1, 2023 is a Sunday, so it belongs to week 52 of 2022. The week-year is 2022.

What date format can I use?

YYYY-MM-DD or your locale format (e.g. MM/DD/YYYY, DD/MM/YYYY).

How many weeks in a year?

Most years have 52 weeks. Some have 53 (when the year has 53 Thursdays).