Week numbers with ISOWEEKNUM
In B2, find the ISO week number of the delivery date in A2, the week number used on calendars in Europe, with ISOWEEKNUM.
| A | B | C | D | E | F | |
|---|---|---|---|---|---|---|
| 1 | Delivery date | Week | ||||
| 2 | Mar 17, 2027 | |||||
| 3 | Jan 4, 2027 | |||||
| 4 | Jan 1, 2027 | |||||
| 5 | ||||||
| 6 | ||||||
| 7 | ||||||
| 8 |
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Why this works
ISO weeks start on Monday, and week 1 is the week with the year's first Thursday in it. Mar 17, 2027 is in week 11. Fill the formula down and you'll see why the rule matters: Jan 4, 2027 is in week 1, but Jan 1, 2027 is a Friday in week 53 of 2026.
Common mistake: Using WEEKNUM for European week numbers. In years that start late in the week it is one week ahead of the calendar.
The answer
- B2
=ISOWEEKNUM(A2)
WEEKNUM starts weeks on Sunday by default and calls the week of Jan 1 week 1. ISOWEEKNUM follows ISO 8601, where weeks start on Monday and week 1 holds the first Thursday. =WEEKNUM(A2,21) gives the ISO week too.
+10 points