Excel WEEKNUM Formula

What this does

WEEKNUM returns which week of the year a date falls in, and the answer depends entirely on the convention. Its default counts 1 January as being in week 1 and starts weeks on Sunday, which is the US system. European and most business reporting uses ISO 8601, where weeks start on Monday and week 1 is the one containing the first Thursday — which sometimes puts early January in week 52 of the previous year. Passing 21 as the return type gives ISO, and ISOWEEKNUM does the same with no argument to forget. Mixing the two across a report is a genuine and hard-to-spot reconciliation bug. The same idea underpins a lot of everyday Excel work, so the few minutes spent getting it right here pay back across every sheet you build afterwards. Treat it as a pattern, not a one-off, and it stops being something you look up and starts being something you reach for. Treat “excel weeknum formula” as a small repeatable workflow rather than a one-off click you hope to remember next time. Use a small test block before the live file, so any surprise in the affected formula shows up while it is still harmless. When a formula is involved, keep the inputs labelled beside it, reference cells instead of typing values, and apply number formatting only after the result checks out. That turns a calculation you can defend to a CFO or an auditor into a method you can reuse, explain, and defend when the workbook leaves your screen.

A worked example

ISO week number for European reporting: =ISOWEEKNUM(A2), or equivalently =WEEKNUM(A2, 21). The difference in action: for 1 January 2027 the US default returns week 1 while ISO returns week 53 of 2026. Grouping a sales table by ISO week: put =ISOWEEKNUM(A2) in a helper column and pivot on it. Week numbers are the classic silent reconciliation bug, because two systems can both be right and still disagree. If there is any chance you will reuse this, drop it into a small template tab right now: a labelled input area on the left and the formula beside it, checked once against the tool above. Next time the same question comes up, the answer is a single paste away instead of a rebuild from memory.

In Google Sheets

Google Sheets handles this almost identically to Excel. The formula syntax above is the same, and the menu lives under a slightly different label rather than a ribbon tab. Use the platform toggle at the top of the page to switch every keyboard shortcut between Windows and Mac, and expect at most cosmetic differences in naming. The aim was to get you unstuck fast and leave you a little more capable than a copy-paste would. The answer is at the top, the tool proves it, and the detail above shows why it holds — so the next time a colleague asks, you can answer without reaching for search. Treat “excel weeknum formula” as a small building block rather than a chore. Once the inputs sit in their own cells and the formula reads from them, the same setup answers a dozen related questions with a tweak, and Excel keeps every dependent figure current as the data changes. The tool above is there so you can rehearse and verify before committing anything to a real workbook; the steps and worked example are there so the logic sticks. Get it right once and it stops costing you time — it starts saving it, every time the question comes back around.

Common mistakes

• Using the default return type in a European report, which produces week numbers off by one against every other system.
• Assuming early January is always week 1; under ISO it can be week 52 or 53 of the previous year.
• Mixing conventions across sheets in the same workbook, which is nearly impossible to spot in a reconciliation.

Frequently asked questions