Barack Obama will become the first sitting US president to visit a federal prison on Thursday, in a push to reform one of the most expensive and crowded prison systems in the world.

The statistics against US prisons are many: 2.2 million prisoners are housed in the United States, which is more men and women behind bars than the top 35 European countries combined.

During his visit to El Reno prison in Oklahoma, Obama will advocate for fairer sentencing and better professional integration for former inmates, among other measures.

"Our incarceration rate is four times higher than China's" Obama said Tuesday, adding that prisons were four times less crowded in 1980 and two times less crowded just 20 years ago.

Nearly a quarter of the world's prison population is concentrated in American jails, while the United States accounts for less than five percent of the world's population.

One of Obama's first orders of business will be to change the duration of prison time for inmates.

Laws adopted since 1980 that are "tough on crimes" have filled federal and state prisons with non-violent offenders, according to Human Rights Watch.

"Sentences are much much longer by order of magnitude than in other countries," said Michele Deitch, a law professor at the University of Texas.

Obama has taken a stance against disproportionate sentencing, particularly for small-time drug traffickers who are locked away for years.

"In far too many cases, the punishment simply does not fit the crime," Obama said Tuesday as he called for a vast overhaul of the US justice system that would give convicts voting rights, curb the use of solitary confinement and end mandatory sentences.

"If you're a low-level drug dealer, or you violate your parole, you owe some debt to society. You have to be held accountable and make amends. But you don't owe 20 years," Obama said.

Deitch questioned other types of convictions as well, such as that of a Florida resident who was locked up for two and half years for having sex in the middle of a beach. The prosecutors had tried to put the individual away for 15 years.

She also said many youths are tried as adults and receive sentences that are "hugely troubling and very destructive."

Around 71,000 minors are incarcerated in the United States.

- One African-American in 35 -

Obama has also focused on the fact that African-Americans are more likely than whites to be arrested for the same offenses and then given harsher sentences.

Again, the numbers are clear: black and Latino Americans represent 60 percent of the prison population while only around 30 percent of prisoners are white.

One African-American in 35 and one Latino in 88 are in prison, for white people the ratio is one person imprisoned for every 214 individuals.

Meanwhile, one in nine black children has a parent in jail, Obama said.

As a result of mass incarceration, the US prison system has seen an explosion in costs. At $80 billion, the budget for jails represents a third of the Department of Justice's annual spending.

But all that money doesn't mean prisons are in good condition.

A report by the Columbia Journalism Review on Illinois state prisons revealed vermin, flooded basements, a general lack of sanitation and crowding, with "some serving time for nothing more than driving on a revoked license."

Obama has also tried to raise awareness on the perverse effects of solitary confinement, which houses inmates "in tiny cells for 23 hours a day, sometimes for months or even years at a time."

"If those individuals are ultimately released, how are they ever going to adapt?" he said. "It's not smart."