周数计算器

查找任意日期的 ISO 8601 周数，或查询特定周数和年份对应的日期。ISO 周从周一到周日，第 1 周定义为包含 1 月第一个周四的那一周。查看完整年度周历，当前周高亮显示。

Result

ISO Week NumberWeek 14
ISO Year2026
Day of WeekThursday
Week Starts (Mon)2026-03-30
Week Ends (Sun)2026-04-05

Frequently Asked Questions

周数计算器是如何工作的？

周数计算器根据 ISO 8601 标准或其他规则确定某个日期所在的周数。ISO 8601 规定：以周一为一周的第一天；包含 1 月 4 日的周为第 1 周（即年份的第一个周四所在的周）。一年可能有 52 或 53 周。

ISO 周数与美国周数有什么区别？

ISO 8601（国际标准）以周一为一周开始，第 1 周是年内包含首个周四的周；美国系统以周日为一周开始，第 1 周是包含 1 月 1 日的周。这导致年末/年初约有 1-2 周的数字差异。本计算器支持两种标准。

一年有多少周？

大多数年份有 52 周零 1 天（或在闰年有 52 周零 2 天）。在 ISO 8601 标准下，53 周年（Long Year）每 5-6 年出现一次，当 1 月 1 日是周四（普通年）或 1 月 1 日是周三/周四（闰年）时出现。

如何根据周数计算对应日期？

本计算器支持反向查询：输入年份和周数，可获得该周的开始日期（周一，ISO 标准）和结束日期（周日）。例如，2024 年第 15 周对应 2024 年 4 月 8 日（周一）至 4 月 14 日（周日）。

ISO 8601 Week Number Rules

The ISO 8601 standard defines a week numbering system used across Europe, most of Asia, and in many international business contexts. Under ISO 8601:

  • Week 1is the week that contains the year's first Thursday — equivalently, the week containing January 4.
  • Each ISO week runs from Monday to Sunday.
  • A year has either 52 or 53 ISO weeks. A 53rd week occurs when January 1 falls on Thursday, or (in a leap year) on Wednesday or Thursday.
  • The ISO week year may differ from the calendar year for days near January 1 and December 31. For example, December 31, 2019 belongs to ISO week 1 of 2020.

ISO Week Number Formula

  1. Find the Thursday of the week containing the date.
  2. The ISO year is the calendar year of that Thursday.
  3. Week number = ⌈ (days from Jan 1 of ISO year to that Thursday + 1) / 7 ⌉

Week Numbering Differences: US vs ISO

The United States uses a different week numbering convention from ISO 8601. Understanding both systems prevents confusion in international projects and reports.

PropertyISO 8601US (Common)
Week starts onMondaySunday
Week 1 definitionContains Jan 4 (first Thursday)Contains Jan 1
Total weeks per year52 or 5352 or 53 (different dates)
StandardISO 8601 (international)No formal standard
Used inEurope, Asia, international businessUS calendars, some software
Jan 1 always in Week 1?No (may be in prior year's week)Yes

This calculator uses the ISO 8601 standard (Monday start, Week 1 = first Thursday rule), which is what most programming languages and international reporting tools use.

How to Find the Week Number for a Date

To find the ISO week number for any date, use the Date to Week Number mode above:

  1. Select Date to Week Number at the top of the calculator.
  2. Enter the date using the date picker, or click the calendar icon to use today's date.
  3. The result instantly shows the ISO week number, ISO year, day of the week, and the full Monday–Sunday span of that week.

Example: January 1, 2026 is a Thursday — it falls in ISO Week 1 of 2026, which runs Mon Dec 29, 2025 through Sun Jan 4, 2026.

Full Year Week Table

The All Weeks in [Year] table at the bottom of the calculator lists every ISO week of the selected year, showing the Monday start date and Sunday end date for each week. The current week is highlighted for quick reference. This is useful for:

  • Planning sprint schedules and fiscal quarters
  • Verifying which week a specific date falls in
  • Identifying years with 53 ISO weeks (long years)
  • Cross-referencing week numbers between different systems

Years with 53 ISO weeks (recent examples): 2015, 2020, 2026, 2032. These occur when January 1 falls on Thursday, or when it falls on Wednesday in a leap year.

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