Week Number Calculator: Find the ISO Week of Any Date

Plenty of businesses plan in weeks rather than dates: production runs, deliveries, payroll periods, school timetables and sprint cycles. The problem is that "week 14" means different things depending on the calendar system. The Week Number Calculator turns a date into its ISO week number, or a week number back into dates.

#What the ISO week standard says

ISO 8601 defines the week rules used across most of Europe and in international business:

• Weeks start on Monday.
• Week 1 is the week that contains the first Thursday of the year.
• A year has 52 or 53 weeks.

Those rules exist so that every week belongs to exactly one year and every year starts on the same weekday.

#Why week 1 can start in December

Because week 1 is defined by the first Thursday, the last days of December can fall into week 1 of the following year. Likewise, the first days of January can belong to the final week of the previous year. That is why an ISO week-year can differ from the calendar year printed on the same date.

#How to use the calculator

1. Open the Week Number Calculator.
2. Pick a date to see its ISO week number, or enter a year and week number.
3. Read the Monday start date, the Sunday end date and the ISO week-year.
4. Copy the result in the form 2026-W17 when you need to reference it.

#ISO weeks versus the US system

The common US calendar starts weeks on Sunday and puts 1 January in week 1. For most of the year the two systems agree closely, but around New Year they can differ by a week. If you are coordinating across countries, state which system you mean rather than writing "week 1" on its own.

#Practical uses

• Planning: agree delivery dates as "week 32" so they survive date changes.
• Invoicing: many payroll and invoicing cycles are weekly.
• Logistics: shipping windows are frequently quoted in weeks.
• Scheduling: school terms and project sprints are often week-based.

#Frequently asked questions

What is an ISO week number? The week of the year under ISO 8601, starting Monday, with week 1 containing the first Thursday.

How many weeks are in a year? 52 or 53. Years with 53 weeks occur roughly every five to six years.

Why is 1 January sometimes week 52 or 53? If it falls on Friday, Saturday or Sunday, ISO rules place it in the last week of the previous year.

Why does my calendar disagree? It may use the US Sunday-start system. Check its week-number setting.

Can I find the current week number? Yes. Enter today's date to see the current ISO week.