Everything about life changes inside of prison, from a person's daily schedule to the ways they're allowed to use the Internet. That new lifestyle also changes what prisoners use instead of money. Paper currency isn't typically allowed, which means inmates have to get creative with their finances.
So what do prisoners use instead of money? Food is a popular choice. Ramen noodles are worth their weight in gold, and spices and instant coffee can be useful bartering tools, too. But it's not just physical items used as currency in prison - sex is sometimes traded for goods on the inside as well. And that's not to mention the other unbelievable things smuggled into prisons and used like cash.
Things that serve as money in jail may seem worthless to those on the outside, but after spending time in the clink, they'd be bartering stamps and cigarettes like the rest of them.
According to a sociological study of prison inmates in 2016, ramen noodles are likely the most popular prison currency. Ramen is used to purchase a variety of services from other inmates, including bunk cleaning and laundry. The noodles can also be traded for black market items like fresh fruit and vegetables smuggled from the kitchens.
What makes ramen so valuable? Many prisons have been forced to make cutbacks on food costs, and have inadvertently created an underground economy of high-calorie noodles.
In the U.S., postage stamps are actually considered legal tender. So, they are the only form of prison currency that could actually be used legally in everyday life outside of prison.
Stamps hold a high value because snail mail is one of the few sanctioned forms of communication prisoners have with the outside world. And as a bonus, inmates are allowed to have stamps, making them ideal currency.
Since the mid-2000s, inmates have started using cans or pouches of mackerel for currency. Inmates started using the containers of oily fish, dubbed "macks," because they cost about a dollar at commissary and no one wanted to eat them. Commissary has limits on how many macks prisoners can get each week, so the market is relatively self-regulated.
Green dot cards, or "dots," are prepaid MasterCard or Visa cards that can be purchased at most convenience stores or gas stations. While paper money is bulky and difficult to conceal in prison, dots offer a relatively anonymous way to move large amounts of money around. The cards themselves are banned in prisons; instead, inmates use text messaging to access accounts linked to the dots.
In 2008, an investigation into Baltimore area prisons found that a prominent prison gang was using dots to fund their inmate protection service with the help of corrupt guards.