If you’re getting error msg 8115 that includes the message Arithmetic overflow error converting expression to data type…, it’s probably because you’re trying to convert a value to a data type that can’t handle that value. For example, trying to convert a number to a tinyint but the number’s too big to fit into a tinyint.

To fix this issue, make sure you convert the value to a data type that can handle the size of the value that you’re trying to convert.

Example of Error

Here’s an example of code that results in the error:

DECLARE @bank_balance smallmoney = -20;
SELECT CAST( @bank_balance AS tinyint );

Result:

Msg 8115, Level 16, State 2, Line 2
Arithmetic overflow error converting expression to data type tinyint.

This error occurred because I tried to convert a smallmoney value of -20 to a tinyint type. The tinyint type can only handle values in the range 0 to 255. The value I passed is outside of that range, and so we got the error.

We get the same error when doing an implicit conversion:

DECLARE @bank_balance smallmoney = -20;
DECLARE @wallet tinyint = @bank_balance;

Result:

Msg 8115, Level 16, State 2, Line 2
Arithmetic overflow error converting expression to data type tinyint.

Solution

To fix the issue, we should convert to a data type that can handle the number. In our case, we can convert to a smallint type:

DECLARE @bank_balance smallmoney = -20;
SELECT CAST( @bank_balance AS smallint );

Result:

-20

And for the implicit conversion:

DECLARE @bank_balance smallmoney = -20;
DECLARE @wallet smallint = @bank_balance;

Result:

Commands completed successfully.

We can verify the type with the sys.dm_exec_describe_first_result_set function:

SELECT system_type_name
FROM sys.dm_exec_describe_first_result_set(
    'DECLARE @bank_balance smallmoney = -20;
SELECT CAST( @bank_balance AS smallint )', 
    null, 
    0);

Result:

smallint