My federal criminal defense attorney said he needs to perform a "Guideline Calculation" using the United States Sentencing Guidelines. What does he mean?

A sentence of imprisonment in federal court is greatly influenced by the U.S. Sentencing Guidelines. The guidelines contain hundreds of rules that are designed to measure two things. First, they are designed to measure the severity of a defendant’s criminal conduct. Second, they measure the extent of a defendant’s criminal history. These two measurements combine to produce an advisory guideline range of imprisonment that will give the judge a starting place for sentencing a convicted defendant.

During the discovery and analytical phase of your case, your attorney will perform a guideline analysis based upon the guidelines and the evidence in your case. From that analysis, he or she will give you an estimate of the range in months of the prison term that you face if you plead guilty or are found guilty. You will consider this estimated guideline range in each decision you make from this point forward. It is very important that this analysis is performed correctly—or you will be making important decisions based upon unreliable information.