Sum and Divide a Bigint, Results with decimal precision?

I am storing a bigint value (for file sizes) in a table. I need to group on one column and for the filesizes (which are in bytes). I would like to have a column showing them by GB. This would mean Sum(FileSize/1024/1024/1024) which is not showing any decimal places. My research seems to indicate this may be due to truncation rather than rounding.

I have tried many options of cast and convert, but cannot seem to find any information about how to sum and divide a bigint, and maintain the decimals. If I take the sum and divide it in Excel, I get the decimals, which tells me there has to be 开发者_如何学JAVAa way to do this in SQL.

Any help is appreciated.

Sum(CAST(FileSize AS FLOAT)/1024/1024/1024)