Changing properties in attribute field

[StewartWhitney]

I have not been able to change the properties for attribute fields that I manually add.  For example I add in a field named Acres so that I can calculate the size of a certain polygon.  These polygons are normally small so the acres are tenths so I go into field properties > Numeric > number format and set decimal places to 3 and I can't get the number format setting to take effect in the attribute table. 

Is this because I add the field myself?

SW

[TrishRice]

These polygons are normally small so the acres are tenths so I go into field properties > Numeric > number format and set decimal places to 3 and I can't get the number format setting to take effect in the attribute table.

How do you define the new field as you create it?  In my (relatively short) experience, I've only been able to define the decimal places while creating a new field.  I'm not sure if there's a way to change that once the field is already established.

Here's what I'd try, but maybe this is what you're already doing.
Add Field-->select "Float" type-->precision of 3, scale of 3.

[JohnBrand]

Is the field holding your acreages a 'float' or 'double' data type?  If it is an 'integer' type, you can't use decimals.