Two and a half million.

That’s roughly the number of Americans in jail right now. But that number becomes even grimmer if you include the entire correctional system. Just the title of a March 2009 study by Pew Center on the States, “One in 31: The Long Reach of American Corrections,” easily puts things into perspective: The number of incarcerated adults in the U.S. combined with those under criminal justice supervision amounts to one out of every 31 adults, or about 3.2 percent of the population.

Dissecting who’s actually in prison reveals drastically higher rates for men (one in 18), African-Americans (one in 11) and those from inner-city neighborhoods.

State expenditures for prisoners now come in second only to Medicare costs.

Even with a litany of statistics to illustrate just how many Americans are in the correctional system, Piper Kerman makes an unlikely ambassador for life on the inside.

Kerman, a drug felon and erstwhile prisoner, is the author of “Orange Is the New Black: My Year in a Women’s Prison,” a first-person account of her yearlong incarceration in the federal correctional institute in Danbury, Conn.

Kerman’s background leaves little to suggest her middle-age, 2004 incarceration: Kerman was a product of suburban middle-class New England and reached a zenith in her upbringing with her 1992 graduation from Smith College.

An adventurous streak settled in. “I had chafed within the safe confines of Smith, graduating by a narrow margin, and longed to experience, experiment, investigate,” Kerman writes. “It was time to live my own life.”

Post-undergrad and waiting tables at a microbrewery in Northampton, Mass., Kerman crossed paths with “Nora” — virtually all the book’s characters are given pseudonyms — a “raspy-voiced midwesterner,” lesbian companion and drug smuggler.

Months of companionship and international travels gradually drew Kerman into a one-time misadventure, currying drug cash from Chicago to Brussels. That crime caught up with Kerman years later, through the bust of Nora’s drug ring, leading to Kerman’s 1998 indictment for drug smuggling and money laundering. A torturous six-year gap would come before Kerman actually went to prison.

When Kerman moves to actually serving the sentence — which she does rather expeditiously — she gives readers a vivid sense of all the discomforts of a first-time prisoner. All the quirks of prison life in Danbury are on display, from the “count” — a five times daily ritual of counting all the prisoners — to the ritual of older prisoners only being allowed to make up a room to pass inspection: “I was getting used to feeling completely idiotic,” Kerman writes. “It was as if I’d been home-schooled my whole life and then dropped into a large, crowded high school. Lunch Money? What’s that?”

Other discomforts of prison life are on full display — including full body searches after personal visits to an incident in which one of Kerman’s fellow prisoners peed in front of another cell out of spite — but the book is also chock full of moving personal dynamics. From Kerman’s roommates to her fellow grounds workers, you get a sense of the real stories of many women whose lives placed them on a path to prison.

With all of the detail that Kerman presents — a visceral, intimate amount — one wonders why she would share her account with the world. In an interview, Kerman said one motivation came down to others’ interest: “I found when I came home from prison that everyone I knew sort of across the board without exception was really interested in hearing about the experience,” she said. “Folks were very sort of voraciously interested in hearing details about the experience, and I think that the experience that I had was really very different from what they expected. It was certainly different from what I expected.”

But her strongest incentive was more high-minded: “It relates to so many other millions of Americans’ experiences,” she said. “We have the largest prison population in the world. There are almost 2.5 million Americans in prison right now, and there are more than 7 million Americans on probation and parole. But there are also families and there are children and all the communities around them. In addition to my personal experience, it’s an issue that affects a lot of people.”

“Orange is the New Black” — Kerman’s title came from a friend’s mailing of a Bill Cunningham’s “On the Street” fashion column in the New York Times that showed all manner of orange women’s apparel — gives a more complete and complex understanding of who’s actually in prison and what happens to them while they’re there.

Kerman’s experience reveals an incarceration system that brings scant resources to rehabilitation both on the inside and out. “There are few resources going toward making sure that when people come home from prison that they can re-enter society and get their life on track in a way that is possible and feasible for them,” Kerman said.

Now on the outside for several years, Kerman has an even more blunt aim than awareness: “I hope that people read this and say, ‘Perhaps we shouldn’t have so many Americans in prison. Maybe prison isn’t really a great remedy for some of these problems,'” she said. “I think we can all agree, I hope, that it would be far better to be spending government dollars, taxpayer dollars on things that build our communities and make our communities stronger, instead of prison cells.”

To that end, Kerman now sits on the board of the Women’s Prison Association, an organization in New York that focuses on advocacy and direct services for women with a criminal justice history.

As for actually lowering the number of prisoners, the author was able to rattle off some things that Congress could do, including equalizing the sentences for crack and powder cocaine; fully funding the Second Chance Act, which takes some criminal records off the books; and applying some of the innovations seen at the state level. Michigan, for example, she said, “has done a really good job of reducing their prison population without seeing an increase in crime and creating savings for the taxpayers.”