Below is a short list of basic, commonly-used Excel formulas and their R counterparts. The R functions are part of Base R, in that they do no require third-party packages. This is not an exhaustive list by any stretch.
BAsic Arithmetic Functions
Excel formula R function SUM sum AVERAGE mean COUNTA length MAX max (sometimes pmax) MIN min (sometimes pmin) STDEV sd ABS abs CEILING, ROUNDUP ceiling FLOOR, ROUNDDOWN floor ROUND round EXP exp LOG, LN log SQRT sqrt Basic Text Functions
Excel formula R function CONCATENATE paste LEFT, MID, RIGHT substr LOWER tolower UPPER toupper LEN nchar FIND grepl, grep and others REPLACE gsub and others More Formulas
Excel formula R function ISNA is.na MATCH which REPT rep RANK rank VLOOKUP merge ( sort of) Excel Features
Here are some Excel features and some of their close counterparts in R:
Excel Feature R function(s) Format Cells as.Date, as.numeric, as.character (sort of) Navigating / inspecting table head, tail, str, summary, dim, nrow Text to Columns strsplit (sort of)