The three counting systems
Unlike week-of-year, week-of-month was never standardised. Three conventions circulate:
| System | Rule | Weeks per month | Where you meet it |
|---|---|---|---|
| Simple blocks | Days 1–7 = week 1, 8–14 = week 2, … | Always 5 (week 5 short) | Everyday speech, most software defaults |
| Calendar rows | Each printed calendar row is a week | 4–6 | Wall calendars, Outlook month view |
| First full week | Week 1 starts at the month's first Monday (or Sunday) | 4–5 | Payroll and shift schedules |
The same date can be "week 4" in one system and "week 5" in another — the calendar-row count even depends on whether the calendar starts weeks on Sunday or Monday (week start conventions). If a recurring event matters, don't say "week 3"; say which system, or better, use the nth-weekday form below.
The nth-weekday alternative
Recurring schedules avoid the ambiguity by naming the weekday instance instead: "the third Thursday of the month" is exact in every system, which is why standups, user groups, and option-expiry dates use it. The rule: day 1–7 holds the month's first of every weekday, 8–14 the second, 15–21 the third, 22–28 the fourth; day 29+ is a fifth instance that only some months have — recurring events set to "the fifth Friday" silently skip most months, so prefer "last Friday" when you mean the final one.
Formulas
' Simple-blocks week of month (Excel / Sheets)
=INT((DAY(A1)-1)/7)+1
' Calendar-row week of month, Monday-start rows
=ISOWEEKNUM(A1)-ISOWEEKNUM(EOMONTH(A1,-1)+1)+1
// JavaScript, simple blocks
const weekOfMonth = Math.ceil(d.getDate() / 7);For week-of-year — the number with an actual standard behind it — use the week number calculator.