# Week Number Calculator Calculate the week number of a date within a year. Supports different week number calculation standards (ISO 8601, US standard, etc.) for project management, time planning, and data analysis. > Canonical page: https://elysiatools.com/en/tools/week-number-calculator - **Category:** Utilities - **Keywords:** week, number, calendar, date, year, iso, calculate, time ## Overview The Week Number Calculator is a precise utility designed to determine the week number of any given date within a year. By supporting multiple international standards like ISO 8601 and US-based calculations, it helps professionals, project managers, and analysts align their schedules and data reporting with specific calendar requirements. ## Inputs - **Date Input** (textarea): Enter date(s) - one per line for multiple dates (e.g., "2024-01-01", "Jan 15, 2024", "today") - **Week Standard** (select) - **Multiple Dates** (checkbox): Process multiple dates (one per line) - **Output Format** (select) - **Include Detailed Date Information** (checkbox): Include additional date details like quarter, day of year, leap year info - **Start of Week (Advanced)** (select) - **First Week Contains (Advanced)** (select) ## When to use - Aligning project milestones with specific calendar weeks for reporting. - Converting historical dates into standardized week numbers for data analysis. - Planning recurring tasks or events based on specific week-start conventions. ## How it works - Enter your target date or a list of dates into the input field. - Select your preferred week standard (e.g., ISO 8601, US, or European) to define how weeks are counted. - Optionally adjust advanced settings like the start day of the week or first-week criteria. - Generate the results in your chosen format, such as a simple list, table, or JSON. ## Use cases - Project management scheduling and sprint planning. - Financial reporting and quarterly performance tracking. - Standardizing date-based data for database imports. ## Frequently asked questions ### What is the difference between ISO 8601 and US standards? ISO 8601 starts weeks on Monday and defines the first week of the year as the one containing the first Thursday. The US standard typically starts on Sunday and considers the week containing January 1st as the first week. ### Can I calculate week numbers for multiple dates at once? Yes, simply enable the 'Multiple Dates' option and enter one date per line in the input box. ### Does this tool support leap years? Yes, the calculator automatically accounts for leap years when determining the week number and day of the year. ### What output formats are available? You can export your results as a simple list, a formatted table, a JSON array, or CSV format. ### Can I customize the start day of the week? Yes, you can override the standard settings by selecting a specific start day like Monday, Sunday, or Saturday in the advanced settings. ## Related tools - [Date Difference Calculator](https://elysiatools.com/en/tools/date-difference): Calculate the difference between two dates in various time units - [Day of Year Converter](https://elysiatools.com/en/tools/day-of-year-converter): Convert any date to its day-of-year number (1-366) and back, with ISO week number, leap-year check, and days remaining in the year. - [ISO Week Number and Fiscal Quarter Anchor Finder](https://elysiatools.com/en/tools/iso-week-number-and-fiscal-quarter-anchor): Convert any date to ISO 8601 week number, day-of-year, Julian Day, and fiscal-quarter boundaries (4-4-5, 4-5-4, 13-week, or standard). - [Working Days Calculator](https://elysiatools.com/en/tools/working-days-calculator): Calculate working days between two dates, excluding weekends and holidays - [Zodiac Date Calculator](https://elysiatools.com/en/tools/zodiac-date-calculator): Calculate zodiac sign based on birth date and show date ranges for all zodiac signs - [Age Calculator](https://elysiatools.com/en/tools/age-calculator): Calculate age from birth date in years, months, days, and total time units - [Nepali Bikram Sambat & Thai Solar Calendar Bridge](https://elysiatools.com/en/tools/nepali-bikram-sambat-and-thai-solar-calendar-bridge): Convert any date between Gregorian (AD), the Nepali Bikram Sambat (BS) calendar, and the Thai solar (Buddhist Era, BE) calendar. 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