What HLOOKUP does
HLOOKUP is the horizontal counterpart to VLOOKUP. Instead of searching down the first column, it searches across the first row and returns a value from a lower row in the same column.
This is most useful when your source data is laid out horizontally, such as month names across columns or labels in the top row of a reporting sheet.
Practical examples
Find sales for a month stored across columns
=HLOOKUP("Mar",$B$1:$G$4,3,FALSE)
If "Mar" appears in the top row of the range, Excel returns the value from the third row of that same column.
Use cell references instead of typed labels
=HLOOKUP(A2,$B$1:$G$5,4,FALSE)
This is the better pattern for reusable worksheets because the lookup value can change without rewriting the formula.
Common mistakes and notes
Mixing up row numbers and worksheet row numbers
row_index_num is counted inside table_array. If the return value is in the fourth row of the selected table, use 4 even if that row happens to be worksheet row 27.
Forgetting about sorted data
Approximate matches need the first row sorted in ascending order. When in doubt, use FALSE.
Choosing HLOOKUP when the data is vertical
If your lookup key lives in a column rather than a row, VLOOKUP or INDEX plus MATCH is usually the better choice.