What Week
Is It Today?
ISO 8601 & US week numbers · Full year calendar · Instant lookup
| Week # | Start Date | End Date | Days in Year | Quarter |
|---|
Everything About Week Numbers
Professional week calendar tools in one place
ISO 8601 Standard
The international standard week — always starts Monday. Week 1 contains the first Thursday of the year.
US Week System
American-style weeks starting Sunday. Week 1 always contains January 1st regardless of what day it falls on.
Full Year View
See all 52 (or 53) weeks of the year in a beautiful month-by-month calendar with week numbers highlighted.
Date Lookup
Enter any past or future date and instantly see its week number, date range, day of year and more.
All Weeks List
Browse a complete table of every week in the selected year — with start/end dates and quarter info.
Live Auto-Update
The current week is always highlighted automatically. No manual setup needed — just open and use.
Week Number FAQ
? What is ISO 8601 week numbering?
ISO 8601 is the international standard. Weeks run Monday to Sunday. Week 1 is the week containing the year's first Thursday. This means January 1st may fall in week 52 or 53 of the previous year.
? How does US week numbering work?
In the US system, weeks start on Sunday. Week 1 always contains January 1st. This can result in 53 weeks in some years and weeks that span two years differently than ISO.
? How many weeks are in a year?
Most years have 52 ISO weeks. Roughly 71% of years have exactly 52 weeks, while about 29% (called "long years") have 53 weeks — this happens when January 1st or December 31st falls on a Thursday.
? What is the current week number?
Today is Sunday, April 26, 2026. It is ISO week 17 of 2026 and US week 18 of 2026.
? Can week 1 start in December?
Yes! Under ISO 8601, if December 29th, 30th, or 31st falls on a Monday, Tuesday, or Wednesday, those days belong to week 1 of the following year.
? What is a "week date"?
A week date is written as YYYY-Www-D (e.g. 2025-W07-3 = Wednesday of week 7, 2025). It is defined by ISO 8601 and used in business and government scheduling.