I have a spreadsheet that I have cells that I want to restrict entry to be either a whole number, or a half number.
Allow say 5
or 5.5
or 9.5
.
NOT allow say 3.2
or 5.7
.
I'd like to do this with a formula, not a macro.
I have a spreadsheet that I have cells that I want to restrict entry to be either a whole number, or a half number. Allow say NOT allow say I'd like to do this with a formula, not a macro. 


You can do it via data validation.
This basically means that cell A1 will accept values where the remainder when A1 is divided by 0.5 is 0 (i.e. it is a multiple of 0.5). If a user tries to put an invalid value, a warning will pop up and warn that that value is not accepted. You can customize the message. 

