Week Number Calculator

Date

Result

ISO week number31
ISO year2,026
Day of year213
Weekdayсуббота
Quarter3

The standard that decides which week is week 1

Find the ISO week number for a date, along with the day of the year, the quarter and the weekday.

Week numbers are everywhere in European business — production schedules, delivery dates, project plans — and the ISO 8601 rule behind them is not obvious.

Week 1 is the week containing the first Thursday of the year, and weeks start on Monday. That means 1 January can legitimately belong to week 52 or 53 of the previous year.

What this calculator shows

The ISO 8601 week number and the ISO year it belongs to
The day of the year and the calendar quarter
The weekday for that date

What to keep in mind

Uses ISO 8601: weeks run Monday to Sunday and week 1 contains the first Thursday.
The US convention differs — weeks start Sunday and week 1 simply contains 1 January.

FAQs

How is week 1 defined?

As the week containing the first Thursday of January — equivalently, the week holding 4 January. Any earlier days belong to the last week of the previous year.

Can a year have 53 weeks?

Yes. Years starting on a Thursday, and leap years starting on a Wednesday, contain 53 ISO weeks.

Why does the American week number differ?

The US convention starts weeks on Sunday and puts 1 January in week 1 regardless of weekday, so the two systems drift apart by a week.

Where are week numbers actually used?

Manufacturing, logistics and project planning across Europe. 'Delivery in week 34' is a normal way to write a date in those industries.

Worked example

A date near the year boundary

Input: 1 January 2027

Output: Week 53 of ISO year 2026

Note: This is exactly why the ISO year is shown separately: a January date can belong to the previous ISO year.

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Week Number Calculator

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