SQL Server 2008 :: DOUBLE Precision For Calculations / Convert To Double?

I am performing a series of calculations where accuracy is very important, so have a quick question about single vs double precision variables in SQL 2008.

I'm assuming that there is an easy way to cast a variable that is currently stored as a FLOAT as a DOUBLE prior to these calculations for reduced rounding errors, but I can't seem to find it.

I've tried CAST and CONVERT, but get errors when I try to convert to DOUBLE.

For example...

SELECT CAST(1.0/7.0 AS FLOAT)
SELECT CONVERT(FLOAT, 1.0/7.0)

both give the same 6 decimal place approximation, and the 6 decimals make me think this is single precision.

But I get errors if I try to change the word FLOAT to DOUBLE in either one of those commands...

SELECT CAST(1.0/7.0 AS DOUBLE)

gives "Incorrect syntax near )"

SELECT CONVERT(DOUBLE, 1.0/7.0)

gives "Incorrect syntax near ,"