Week Number Calculator

Businesses plan in week numbers, and two calendars can disagree about which week a date belongs to — especially at the turn of the year. This calculator uses ISO 8601, the standard that payroll, manufacturing and project systems follow: weeks start on Monday, and week 1 is the one containing the first Thursday of the year.

How to use it

1. Pick a date.
2. Read its ISO week number and, importantly, its ISO year.
3. Note that near 1 January the ISO year can differ from the calendar year.

Examples

• 4 August 2026 falls in ISO week 32 of 2026.
• 1 January 2021 falls in ISO week 53 of 2020 — not week 1 of 2021.

Frequently asked questions

What is an ISO week number?

Under ISO 8601, weeks run Monday to Sunday and week 1 is the week containing the first Thursday of the year — equivalently, the week containing 4 January. It is the definition used across European business, payroll and manufacturing systems, which is why a shared plan needs everyone using the same one.

Why do two calendars disagree about the week in January?

Because many simple calculators count weeks as days-since-1-January divided by seven, which is not the ISO rule. Under ISO, the first days of January often belong to the last week of the previous year — 1 January 2021 sits in week 53 of 2020. The calculator reports the ISO year alongside the week so the mismatch is visible rather than confusing.

Can a year have 53 weeks?

Yes. A year has 53 ISO weeks when it starts on a Thursday, or when it is a leap year starting on a Wednesday. That happens roughly every five to six years, and it is a recurring source of trouble in annual planning spreadsheets that assume 52.

Do weeks start on Sunday or Monday?

It depends on the convention. ISO 8601, and most of Europe and Asia, start on Monday. The United States commonly starts on Sunday, so a US calendar can show a different week number for the same date. This calculator uses ISO, and says so, because that is what business systems expect.