You cannot convert it. Measurement scales are not equal. For example:

**Ratio scale** (zero has meaning): You can add, subtract, multiply and divide

**Interval scale** (zero has no meaning): You can add and subtract

**Ordinal scale** (e.g., Likert scale): You can rank

Ordinal scales such as a Likert scale only tell you that a rating of 3 is larger than a rating of 2. An interval scale would tell you that a 3 is 1 unit larger than 2. This is more information than provided by the Likert scale. A ratio scale would tell you that 3 is 1 unit larger than 2, AND that 3 is 1.5 times larger than 2. In addition, Likert scales are integer scales while interval and ratio scales may be subdivided to the limit of the measurement device.

In other words, a Likert scale simply does not provide the same information content as the other two scales. It's not a matter of converting.

When you use Likert ratings in regression, the results assume that the differences between a 1 and 2 are the same as the differences between any other adjacent ratings. In most cases this is simply not true. Therefore, used carefully, regression may provide insight into relationships, directionality and contribution. However, there will be higher than normal risks associated with your conclusions, and the regression model should not be used for predictions.