Custpom Lookup formula not working.

[countryfan_nt]

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Hello friends, Hope all is well! Please help me fix the formula below: when I place Nov in A13; and end up getting 8! when I enter Aug, I get: #N/A =LOOKUP( I13, {"Jan","Feb","Mar","Apr","May","Jun","Jul","Aug","Sep","Oct","Nov","Dec"}, {"1","2","3","4","5","6","7","8","9","10","11","12"}) Please help, thank you very much in advance!

 

[KRice]

LOOKUP requires the lookup array to be in ascending order.
You might try a different approach:
=MONTH(DATEVALUE(A13&1))

 

[countryfan_nt]

hmmmm, can you please help me fix the formula?

 

[KRice]

The formula I suggested is a more compact way to return the number of the month.
=MONTH(DATEVALUE(A13&1))

If you really want to use a lookup function, then consider something like this:
=VLOOKUP( A13, {"Jan",1;"Feb",2;"Mar",3;"Apr",4;"May",5;"Jun",6;"Jul",7;"Aug",8;"Sep",9;"Oct",10;"Nov",11;"Dec",12},2,FALSE)
VLOOKUP used with the FALSE option (4th argument) does an exact match and it does not require the lookup values to be in ascending order.

And if you really want to use the LOOKUP function, which requires a sorted (ascending) lookup array, then the arrays need to be manually sorted:
=LOOKUP( A13, {"Apr","Aug","Dec","Feb","Jan","Jul","Jun","Mar","May","Nov","Oct","Sep"}, {4,8,12,2,1,7,6,3,5,11,10,9})