Week number guides
Everything about weeks, in plain language: how week numbers work, why systems disagree, how to use them in spreadsheets, calendars, payroll, and planning.
ISO 8601 week guide
The complete rules of the ISO week date system — definitions, edge cases, and code.
How many weeks in a year
52 or 53? The exact arithmetic, plus every 53-week year through 2055.
Week number formats
YYWW date codes, CW/KW, WW, ISO notation — every week code, decoded.
Week start conventions
Sunday or Monday? Why calendars disagree, country by country — and how it shifts week numbers.
Week numbers in calendar apps
Turn on week numbers in Google, Outlook, Apple, and Samsung calendars — with ISO-correct settings.
Week numbers in Excel
WEEKNUM vs ISOWEEKNUM, the week-year formula, YYWW codes — spreadsheet week numbers done right.
Fiscal weeks & 4-4-5 calendars
4-4-5, 4-5-4, 5-4-4, the NRF calendar — fiscal weeks decoded and mapped to ISO.
53-week payroll year
The years with an extra payday — why they happen and how payroll teams handle them.
Sprint cadence by week numbers
Name sprints by ISO weeks (2026-W29-W30), keep cadence over year ends, survive week 53.
Week of the month
"Week 3 of July" — counted three different ways. Which one people mean, and today's answer.
Start here
New to week numbers? Read the ISO 8601 week guide first — every other guide builds on it. Then check week start conventions to understand why your phone's calendar might disagree with your colleague's spreadsheet, and keep the calculator handy for everything else.