Week Number Calculator

Find the ISO week number for any date instantly. See the current week number and plan your schedule effectively.

ISO 8601

Standard

Planning

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What is an ISO Week Number?

Most of the world, specifically in business and government contexts, uses the ISO 8601 date standard. According to this standard, weeks start on Monday, and the first week of the year is the week that contains the first Thursday of the year.

This means that Week 01 of any year is the first week with at least 4 days in that year. Consequently, a year can have 52 or 53 weeks.

How Week Numbers are Calculated

Calculating the week number manually can be tricky because of the "First Thursday" rule. Here is the logic simplified:

Week Start: Monday
Week 1: The week containing January 4th, or equivalently, the week with the year's first Thursday.
Years with 53 Weeks: If January 1st is a Thursday (or Wednesday in leap years), that year will have 53 weeks.

Common Uses

Business Planning: Quarterly reports and project sprints are often scheduled by week numbers.
Supply Chain: Manufacturing and delivery schedules frequently reference week numbers (e.g., "delivery on W42").

❓ Frequently Asked Questions

Why does the year sometimes change?

Sometimes, the last days of December belong to Week 1 of the next year, or the first days of January belong to the last week of the previous year. For example, Dec 29, 2025 might be in Week 1 of 2026.

Which week is Week 1?

Week 1 is the first week with a Thursday in the new year. It typically includes January 4th.

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