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I know that when SQL Server handles an implicit conversion between types it converts the lowest priority one to the highest one.

But what is the result datatype when I perform an operation between decimals with different precisions?

The type of the result is determined by the rules set out in Precision, scale, and Length (Transact-SQL):

The following table defines how the precision and scale of the result are calculated when the result of an operation is of type decimal. The result is decimal when either:

• Both expressions are decimal.


• One expression is decimal and the other is a data type with a lower precedence than decimal.

The operand expressions are denoted as expression e1, with precision p1 and scale s1, and expression e2, with precision p2 and scale s2. The precision and scale for any expression that is not decimal is the precision and scale defined for the data type of the expression. The function max(a,b) means the following: take the greater value of "a" or "b". Similarly, min(a,b) indicates to take the smaller value of "a" or "b".

* The result precision and scale have an absolute maximum of 38. When a result precision is greater than 38, it's reduced to 38, and the corresponding scale is reduced to try to prevent truncating the integral part of a result. In some cases such as multiplication or division, scale factor won't be reduced, to maintain decimal precision, although the overflow error can be raised.

In addition and subtraction operations, we need max(p1 - s1, p2 - s2) places to store integral part of the decimal number. If there isn't enough space to store them that is, max(p1 - s1, p2 - s2) < min(38, precision) - scale, the scale is reduced to provide enough space for integral part. Resulting scale is MIN(precision, 38) - max(p1 - s1, p2 - s2), so the fractional part might be rounded to fit into the resulting scale.

A convenient quick way to see the resulting type is to use SQL_VARIANT_PROPERTY:

DECLARE @V1 decimal (9, 6) = 123.456789;
DECLARE @V2 decimal (7, 2) = 12345.67;

SELECT 
 [BaseType] = SQL_VARIANT_PROPERTY(@V1 + @V2, 'BaseType'),
 [Precision] = SQL_VARIANT_PROPERTY(@V1 + @V2, 'Precision'),
 [Scale] = SQL_VARIANT_PROPERTY(@V1 + @V2, 'Scale');

Output:

╔══════════╦═══════════╦═══════╗
║ BaseType ║ Precision ║ Scale ║
╠══════════╬═══════════╬═══════╣
║ decimal ║ 12 ║ 6 ║
╚══════════╩═══════════╩═══════╝

db<>fiddle demo

The question is a bit vague but I believe the below is what you're after

It works the same way it does in mathematics:

DECLARE @a DECIMAL (5,2) = 100.02
DECLARE @b DECIMAL (8,3) = 10125.020

SELECT @a
SELECT @b


SELECT @a + @b 

will add the number

one hundred point zero two

to

ten thousand, one hundred and twenty five point zero two zero

which will result in

ten thousand, two hundred and twenty five point zero four zero

When running the code above, SQL Server outputs 10225.040