float and real (TransactSQL)
Approximatenumber data types for use with floating point numeric data. Floating point data is approximate; therefore, not all values in the data type range can be represented exactly. The ISO synonym for real is float(24).
TransactSQL Syntax Conventions
float [ (n) ]
Where n is the number of bits that are used to store the mantissa of the float number in scientific notation and, therefore, dictates the precision and storage size. If n is specified, it must be a value between 1 and 53. The default value of n is 53.
n value  Precision  Storage size 

124  7 digits  4 bytes 
2553  15 digits  8 bytes 



The SQL Server float[(n)] data type complies with the ISO standard for all values of n from 1 through 53. The synonym for double precision is float(53).
Data type  Range  Storage 

float   1.79E+308 to 2.23E308, 0 and 2.23E308 to 1.79E+308  Depends on the value of n 
real   3.40E + 38 to 1.18E  38, 0 and 1.18E  38 to 3.40E + 38  4 Bytes 
Values of float are truncated when they are converted to any integer type.
When you want to convert from float or real to character data, using the STR string function is usually more useful than CAST( ). This is because STR enables more control over formatting. For more information, see STR (TransactSQL) and Builtin Functions (TransactSQL).
Conversion of float values that use scientific notation to decimal or numeric is restricted to values of precision 17 digits only. Any value with precision higher than 17 rounds to zero.
ALTER TABLE (TransactSQL)
CAST and CONVERT (TransactSQL)
CREATE TABLE (TransactSQL)
Data Type Conversion (Database Engine)
Data Types (TransactSQL)
DECLARE @local_variable (TransactSQL)
SET @local_variable (TransactSQL)