Cernarus

Week Number Finder

This Week Number Finder computes the week number for a given Gregorian calendar date using multiple conventions: ISO 8601, a US-style Sunday-start rule, a simple Jan 1 start, or a fully custom week-start definition. Choose the method that matches your scheduling, reporting, or regulatory needs.

Results include the numeric week and the associated calendar year for that week. The tool includes clear notes on boundary cases (weeks that cross calendar years), leap-year handling, and the authoritative standards used to define week-counting rules.

Updated Nov 6, 2025

Calculates the ISO week number based on the ISO 8601 rule where weeks start on Monday and week 1 is the week containing January 4 (equivalently, the week with the year's first Thursday). This method returns the week number as an integer; in boundary cases the associated ISO week-year may differ from the calendar year.

Inputs

Results

Updates as you type

ISO week number

—

ISO week-year (may differ from calendar year)

2,025

OutputValueUnit
ISO week number——
ISO week-year (may differ from calendar year)2,025year
Primary result—

Visualization

Methodology

ISO 8601 is the international standard for week numbering: weeks start on Monday and week 1 is the week containing January 4 of the calendar year. This tool implements an established algorithm that converts the input date to a Julian Day Number (JDN), derives weekday and day-of-year, then applies the ISO formula to compute the week number.

The US-style method treats Sunday as the first day of the week and designates the week containing January 1 as week 1. The simple Jan 1 method divides the year into consecutive 7-day blocks beginning on January 1. The custom method shifts the start-of-week and first-week alignment per user selection.

All calculations use proleptic Gregorian rules (Gregorian leap-year rules applied for all years) and integer arithmetic to avoid rounding issues. Boundary cases (for example, ISO week 1 that begins in the previous calendar year) are handled according to the chosen method; see the FAQ for specifics.

Key takeaways

Select the method that matches your reporting or scheduling needs: ISO 8601 for international consistency, US-style for Sunday-based calendars, simple Jan 1 for the simplest partitioning, or Custom to match an organizational convention.

All computations use standard Gregorian calendar arithmetic and JDN conversions. Review boundary-case behavior before using results in compliance-sensitive contexts.

Expert Q&A

Which week-numbering standard should I use for legal or reporting purposes?

Use ISO 8601 for international reporting and many government or business standards. Some local or legacy systems use a US-style Sunday-start rule or simple Jan 1 counting. Confirm the requirement in your regulatory or contractual documentation before choosing a method.

How are leap years and Feb 29 handled?

This tool uses Gregorian leap-year rules (year divisible by 4, except centuries not divisible by 400). Day-of-year and JDN calculations account for Feb 29 automatically. The week-numbering method then assigns that date to the appropriate week.

What about dates that fall in the first or last week and belong to a different week-year?

ISO weeks occasionally assign early-January dates to the last ISO week of the previous ISO week-year, or late-December dates to week 1 of the next ISO week-year. The ISO method output includes the associated week-year. For simple methods, the week-year equals the calendar year.

How accurate are these calculations and what are the limits?

Calculations follow standard arithmetic and Gregorian calendar rules and are accurate for years in the range 1..9999. Edge cases depend on the chosen convention; users should validate critical schedules against authoritative sources. This tool is intended for civil calendaring only and not for astronomical or legal timekeeping that requires time-zone or leap-second accounting.

Can I automate batch processing with this algorithm?

Yes. The method is deterministic and can be implemented server-side in bulk. When automating, ensure you document which week-number convention is used and include a test set covering boundary dates (e.g., Jan 1–7 and Dec 25–31) to validate behavior.

Sources & citations