What Is an ISO Week Number?
October 23, 2026
What an ISO week number is, how week 1 is defined, why a year has 52 or 53 weeks and how to read a number such as week 43 of 2026.
Blog
Practical guides to week numbers, calendar weeks and the rules behind them.
How to Find the Current Week Number
October 23, 2026
Several quick ways to find the current week number: the calculator, your calendar app, a spreadsheet formula, the command line and a mental shortcut.
How to Find the Dates of a Week Number
October 23, 2026
How to turn an ISO week number into real dates: find the Monday of week 1, add whole weeks and read off Monday to Sunday, with worked examples.
How Many Weeks Are in a Year?
October 23, 2026
A year has 52 weeks and one or two extra days, so ISO years have 52 or 53 weeks. Clear numbers, examples and what they mean for pay and planning.
Years With 53 Weeks Explained
October 23, 2026
Why some years have 53 ISO weeks, which years they are, how to spot one in advance and what a long year means for reports and payroll.
Why Week 1 Can Start in December
October 23, 2026
Why the first ISO week of a year sometimes begins on 29, 30 or 31 December, how to read such a date and what to do about it in planning and software.
ISO vs US Week Numbers
October 23, 2026
ISO week numbers start on Monday and need four days in the new year, while US numbers start on Sunday and count the week of 1 January. Compare them.
Which Day Does the Week Start?
October 23, 2026
Does the week start on Monday or Sunday? ISO 8601 says Monday, the US and several other countries use Sunday, and some use Saturday. Here is the full picture.
WEEKNUM and ISOWEEKNUM in Excel and Sheets
October 23, 2026
How WEEKNUM and ISOWEEKNUM differ in Excel and Google Sheets, which return types matter, and formulas for the week-year and the dates of a week.
Week Numbers in Project Planning
October 23, 2026
How teams use ISO week numbers in project plans, sprints and release schedules, and how to avoid the mistakes around New Year and week 53.
Week Numbers in Shipping and Logistics
October 23, 2026
How suppliers, carriers and manufacturers use week numbers for delivery windows, lead times and date codes, and how to read them correctly.
Week Numbers at School and University
October 23, 2026
How schools and universities use week numbers for terms, timetables, exams and even and odd weeks, and why week 53 breaks alternating schedules.
Week Numbers in Shift Planning and Payroll
October 23, 2026
How week numbers organise shift rosters, timesheets, weekly overtime and pay periods, and why the first day of the week and week 53 need care.
The ISO Week Date Format
October 23, 2026
How the ISO 8601 week date format works: the year, the letter W, the week number and the weekday digit, with examples and rules for using it.
Fiscal Weeks and the 4-4-5 Calendar
October 23, 2026
How companies build 52-53 week fiscal years and 4-4-5 or 4-5-4 calendars, how they differ from ISO weeks and why the 53rd week appears.
Pregnancy Weeks and Calendar Weeks
October 23, 2026
How pregnancy weeks are counted from the last period, how they differ from ISO calendar weeks and how to find the due date week. General information only.
How to Calculate a Week Number by Hand
October 23, 2026
A step-by-step method for the ISO week number of any date using the day of the year and the weekday, with the year-end corrections and worked examples.
Week Numbers Around the World
October 23, 2026
Where week numbers are part of daily life, which day each region starts the week on, and why the same date can have different week numbers.
Leap Years and Week Numbers
October 23, 2026
How leap years change weekdays and week numbers, which leap years have 53 weeks, and why the calendar repeats exactly every 400 years.
How to Write Week Numbers: W43, CW 43, KW 43
October 23, 2026
The common ways to write a week number, from W43 and CW 43 to KW 43 and the ISO form 2026-W43, with advice on which to use in plans and documents.