Excel formulas, found by function name
Excel formulas aren't actually that many. SUM, IF, COUNTIF, SUMIF, VLOOKUP, XLOOKUP, TEXT — about twenty functions cover nearly everything you'll need at a desk job. What's missing isn't more functions. It's knowing which one applies to the situation in front of you.
You know SUMIF, then the moment a second condition shows up you're not sure what changes. You use VLOOKUP every week and still don't know why it sometimes just won't return anything.
The list itself is in Korean — search by function name instead
The situations and explanations in this box are written in Korean for now, so typing an English phrase like "duplicate" or "weekday" into the search box won't find anything yet. What does work in any language is the function name itself: search VLOOKUP, SUMIF, TRIM, COUNTIF, or any other function, and it matches because the formula text is written the normal Excel way, in English. If you already know roughly which function you need, that's the way in until the situation text gets an English pass.
Formulas here are grouped by what they're for: sum and count, look up, text, dates, conditions, duplicates and rank. There's also a separate "when it fails" group for reading error codes.
Fill in your cell names once
Each formula has placeholders like {range} or {lookup value}. Type your actual sheet's range in once, and it stays filled in on the next formula you open too. Range names on a given sheet rarely change, so you shouldn't have to retype them.
Arguments are separated by commas here — the US and UK default. If your regional settings use a comma as the decimal separator instead, as much of continental Europe does, Excel expects semicolons between arguments there. Swap every comma for a semicolon if a formula won't parse.
Three places people get it wrong
SUMIF and SUMIFS take arguments in a different order. SUMIF goes range, criteria, sum range. SUMIFS puts the sum range first, then pairs of criteria range and criteria. The order flips the moment you go from one condition to several, and that's exactly where it trips people up.
VLOOKUP without FALSE as the last argument returns an approximate match instead of an exact one, and on an unsorted table that comes back completely wrong. Get in the habit of always adding it.
Weekday from a date is one TEXT call: =TEXT(cell,"dddd") for the full name, "ddd" for the abbreviation. If you've ever seen "aaaa" in a template instead, that's the Korean weekday code — dddd/ddd is the English equivalent.
Why VLOOKUP throws #N/A when the value is right there
The most common cause is a type mismatch, not a missing value. If one side stores 1001 as a number and the other stores "1001" as text, Excel treats those as different values — the small green triangle in the corner of a cell is the tell. Wrap the lookup value in VALUE() if it's stored as text and the table holds numbers, or append &"" if it's the other way around. If you can't tell which, try both.
If the type looks right and it's still failing, an invisible trailing space is the usual culprit. Wrap the lookup value in TRIM and try again.
The other errors
#VALUE! means a cell you're adding contains text, or looks empty but actually holds a space — click into it to check. #REF! shows up when a formula was pointing at a row or column you deleted; undo first, fix the formula second. #DIV/0! means you divided by an empty cell or a zero. ##### isn't an error at all — the column is too narrow, and double-clicking the column border fixes it. If a formula shows as literal text instead of calculating, the cell is formatted as Text: switch it to General, then press F2 and Enter in that cell to force it to recalculate.
Nothing here came from an AI
Every formula was run in Excel by hand before it went on this list. Ask an AI for a formula and the simple ones come back fine — the more conditions you stack in, the more often a wrong one comes back looking entirely plausible. The problem is you have no way to check it except by running it, and if you don't already know what's wrong, a wrong answer just sits there in your sheet looking correct.
This tool doesn't do the calculating either. That's Excel's job.