What Week Is It? Week Number

How to use the What Week Is It? Week Number

1. The current week is shown straight away
2. Pick any date to see its week number and date range
3. Switch between ISO and US numbering or open the full week list

About this tool

See the current week number instantly, or find the week number of any date. Supports ISO 8601 (Monday start) and US (Sunday start) weeks, with a full list of the year's weeks. What Week Is It? Week Number is part of ToolNest's collection of free date & time. It works instantly in your browser with no account and no installation.

How week numbers work

There are two common systems, and they can give different answers for the same date:

• ISO 8601 — used in Europe, most of Asia and international business. Weeks start on Monday, and week 1 is the week that contains the year's first Thursday (so it always contains 4 January). A year has 52 or 53 weeks, and the first days of January can belong to the last week of the previous year. Example: Sunday 3 January 2021 is in week 53 of 2020.
• US system — weeks start on Sunday and week 1 is simply the week that contains 1 January, even if it is only one day long. Example: Thursday 1 January 2026 is in week 1, and Sunday 4 January starts week 2.

Week numbers in Excel and Google Sheets

=ISOWEEKNUM(A1) returns the ISO week. =WEEKNUM(A1) returns the US-style week (Sunday start); =WEEKNUM(A1, 2) starts weeks on Monday but still counts 1 January as week 1, so it is not the same as ISO.

Why it matters

Factories, schools, payroll and project plans often schedule by week number ("delivery in week 43"). When working with people in other countries, confirm which system they use — the ISO and US numbers differ for many dates.

Frequently asked questions