Can anyone tell me how to use the result of a select case statement for multiplication?

(SELECT CASE 
    WHEN unit = 'Sqft' THEN squareft
    WHEN unit = 'Lin.Inch' THEN value
 END) AS AREA, 
('AREA'* pieces) AS 'Total Area'

When I used my above query I get an error:

Error converting data type varchar to numeric.

Can anyone help?