About

In global supply chains, software development sprints, and international fiscal planning, the definition of a "week" is rarely universal. A misalignment between ISO-8601 (European standard) and US Standard (Sunday start) systems can lead to missed shipping deadlines or contractual breaches. This tool eliminates ambiguity by providing a strict, algorithmic determination of week numbers across multiple systems.

Unlike basic calendar widgets, this engine handles edge cases such as 53-week years, overlapping fiscal boundaries, and the specific mathematical criteria defining Week 1. It is designed for Project Managers requiring precision in scheduling, ensuring that W₁₄ in Berlin matches the timeline in New York.

Formulas

The calculation of the ISO week number relies on finding the Thursday of the current week. The mathematical definition states that Week 1 is the first week with a majority (4 or more) of its days in January.

W iso = floor DOY - DOW + 10 7

Where DOY is the ordinal Day of the Year (1-366) and DOW is the Day of the Week (1 for Monday through 7 for Sunday). If the result is 0, the date belongs to the last week of the previous year.

For Fiscal Calculations involving an offset t₀:

W fiscal = ceil (date - start) days 7

Reference Data

Year	ISO Week 1 Starts	Total Weeks	Leap Year	Dominical Letter
2023	Jan 02 (Mon)	52	FALSE	A
2024	Jan 01 (Mon)	52	TRUE	GF
2025	Dec 30 (2024)	52	FALSE	E
2026	Dec 29 (2025)	53	FALSE	D
2027	Jan 04 (Mon)	52	FALSE	C
2028	Jan 03 (Mon)	52	TRUE	BA
2029	Dec 31 (2028)	52	FALSE	G
2030	Dec 30 (2029)	52	FALSE	F
2031	Dec 29 (2030)	53	FALSE	E
2032	Jan 05 (Mon)	52	TRUE	DC

Frequently Asked Questions

Microsoft Excel (specifically `WEEKNUM`) and Outlook often default to the US system where Sunday is the first day of the week and the week containing January 1st is always Week 1. This tool defaults to ISO-8601 (European/International Standard), where Monday is the start, and Week 1 must contain at least 4 days of the new year. You can toggle the "System" setting to "US Standard" to match Outlook.

In the ISO system, a year has 53 weeks if it starts on a Thursday (or is a leap year starting on Wednesday). This occurs approximately every 5.6 years. This tool automatically detects these anomalies. For example, 2020 and 2026 contain 53 ISO weeks.

Select the "Fiscal" tab in the settings. You can input your organization's specific "Start Date" (e.g., April 1st or October 1st). The calculator will then compute the week number relative to that start date, rather than January 1st.

Yes. The JavaScript `Date` object and our custom logic handle dates effectively back to year 100 AD, accounting for the Gregorian calendar cycle, although ISO-8601 was only formally standardized in 1988.