Most common case is you can make user defined functions by assigning them to a name in name manager. More on reddit.com

They do different things. Lambdas allow you to define a function that can simplify how you build a complex process. This is especially the case if you need to reference the same data multiple times. Perhaps your formula references column B:B 3 times, for whatever reason. If you move those values to column C, you have to make sure you update every reference in your function to C:C. If you miss one, you start getting incorrect outputs. In a Lambda, your input parameter makes the 3 references, so you just update the input argument at the end to redirect the function's reference. Easy. Also, You can save Lambdas in Named Ranges which allow you to give your custom functions a name that can be easily referenced. This is also offers a layer of protection when idiots colleagues accidentally delete the formulas. You don't need to rebuild the whole formula, you just call the function name you saved in Named Ranges again. Let is a different beast. Let allows you to make multiple calculations and save the intermediate results. In the olden days, I used to have to do a VLOOKUP, however if the lookup returned an empty value (which shows as 0), then the output should be empty, not 0. Without Let, this looks like this: =IF(VLOOKUP(A1,D:E,2,FALSE)=0,"",VLOOKUP(A1,D:E,2,FALSE)) You have to perform the lookup, check if it's 0, and then if it isn't, then look it up again to produce the output. Not very efficient.... With Let, we can save the 2nd lookup: =LET(vl,VLOOKUP(A1,D:E,2,FALSE),IF(vl=0,"",vl)) Now I can perform the lookup and save the value in vl. If vl is 0, then return an empty string, otherwise return vl. Now we only need to do the lookup once. EDIT: Fixed variable names in 2nd formula. Was supposed to say vl, not V1. More on reddit.com

Learn LET first and master it. LAMBDA will help you make custom formulas that you can name and call up like a regular function like =SUM(). But the real power of LAMBDA will be the helper functions BYROW, BYCOL, MAP, SCAN, REDUCE, and MAKE ARRAY. Save these YouTube tutorials and watch and rewatch. Soon you'll be able to make some of the most complicated calculations that you never could have imagined before lambda. The combination of Let and LAMBDA are the two most powerful functions in Excel. And the formulas are interchangeable with Google sheets. Once you start recall the knowledge without the aid of references, you could technically call yourself an excel programmer, instead of a knowledgeable user. Things that are normally locked behind VBA are so easy to create with LET and a LAMBDA. Let formula: https://youtu.be/blNIX1BjsYE LAMBDA: https://youtu.be/45v5NhPhopc More on reddit.com

I have a recursive one that builds all unique combinations of columns so that i can have smart data validations. Think a table of parts that has columns like vendor, model line, size, and color. So it builds a list of each model line, and for each model line, all the sizes, and for each size the colors. It's a fairly long formula, but it replaces about 1000 lines of vba. Edit: Since folks were asking, i put this up on github . it's basically just a markdown file right now, but i'll add the sample data as CSVs, and add the python i used to prototype this. This builds a spill range based on the columns to let you build multi level or inter-dependant data validations. The data validation formula parser is very very dumb. It will not allow functions that output an array. You must have real cells on a real range. More on reddit.com