A float is 4 bytes wide, whereas a double is 8 bytes wide. You should probably prefer to use double in most cases. You may check IEEE floating point standard.
Also to note that floats can be doubles, but doubles can never be floats because doubles have the larger range.
Range of float is 1.40129846432481707e-45 to 3.40282346638528860e+38
Range of double is 4.94065645841246544e-324d to 1.79769313486231570e+308d