Four years after he walked out of City Hall for the last time, Ray Nagin is about to learn what the rest of his life has in store for him.

The news may be grim. A presentence investigation prepared by federal probation officers calls for a sentence of 20 years or more based on the former mayor’s conviction in February on 20 counts of bribery, wire fraud, money laundering and filing false tax returns.

The courtroom will almost certainly be packed when U.S. District Judge Ginger Berrigan metes out Nagin’s punishment at a hearing that begins at 10 a.m. She has some discretion to veer from the federal sentencing guidelines that underlie the recommendation by probation officers, but not that much.

By now, Berrigan has performed her own calculation of the guidelines, using the same manual that the probation office uses. The manual allows for limited wiggle room. Among the potentially aggravating factors that Berrigan will have to weigh: Was Nagin the leader of a conspiracy involving five or more people? Did he obstruct justice by lying on the witness stand and to investigators? And what was the financial loss caused by his misdeeds?

Nagin, 58, will have a chance to address the court before Berrigan imposes her sentence, but it’s unclear whether he plans to do so, or what he might say if he does. He has maintained his innocence throughout the investigation and trial.

Letters sent to the court from his wife and his sons during the past week continue to assert his innocence, and urge Berrigan to set aside the verdict on the basis of prosecutorial misconduct. They claim Nagin was railroaded by a crooked businessmen with self-preservation in mind, an argument that Nagin’s lawyer, Robert Jenkins, made unsuccessfully at trial.

Once Berrigan imposes the sentence, she has the option of remanding Nagin to the custody of the federal Bureau of Prisons or allowing him to remain free while his appeal is pending. But legal observers say neither of those outcomes is likely; rather, she will probably order Nagin to surrender to the Bureau of Prisons on a specified date a month or two from now. That is a courtesy judges typically afford white-collar defendants who are not thought to pose a danger.

It will be up to the Bureau of Prisons to determine where Nagin will serve his sentence, though the judge can make a recommendation. Often, judges ask that defendants be placed in a facility near their family to make visiting easier.

But those requests are not always honored. And in Nagin’s case, he’s likely to start off in a low-security prison rather than a minimum-security “camp,” under Bureau of Prison guidelines for inmates facing more than a decade in prison. Whatever sentence Berrigan metes out, Nagin will have to serve 85 percent of it under federal rules.

Follow Gordon Russell on Twitter @gordonrussell1.