Week Number Calculator
Instant results as you type.
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
- Pick a date.
- Read its ISO week number and, importantly, its ISO year.
- 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.