Forget the cliché about lemons and lemonade. Christian Wilkins can make lemonade out of thin air.

It’s true. At a restaurant, the former Clemson defensive lineman starts by ordering a glass of water. Then he asks for six lemon slices. He pours in a few sugar packets. Voilà! Free lemonade.

Wilkins has one of the biggest personalities in sports, but he boasts one of the smallest budgets. His lemonade trick is just one example of his world-class frugality, an approach he’ll take into the NFL after the Miami Dolphins selected him No. 13 overall in the draft Thursday.

Wilkins’s radical approach to conserving money has paid off. While many students go into debt during college, he’s saved up more than $15,000.

“It’s simple: My mind-set is just, save a whole lot more than you spend,” said Wilkins, who won two national titles with the Tigers. “And I’m not ever willing to spend much. I’m very low-maintenance when it comes to my needs.”

Christian Wilkins runs the 40-yard dash at the NFL combine. Photo: Michael Conroy/Associated Press

Wilkins started by renting a $300-a-month apartment (“It’s nice down South cause it’s cheaper living,” he said) then spent as little time there as possible. That kept electrical and water bills down. Wilkins spent nearly every moment he wasn’t in class at Clemson’s 140,000-square-foot football-operations facility, eating most meals and showering there.

“I even brush my teeth here,” he said in a recent phone interview.

Wilkins didn’t have a car. He got around Clemson on a bike or got rides from teammates (though he did help pay for gas, he said). He didn’t use credit cards. He was on a family phone plan until recently so he didn’t have a monthly bill. He didn’t pay for music or phone apps. He wore team-issued clothes.

“My teammates know I’m the cheapest guy in the world,” he said.

An NCAA rule change in Wilkins’s freshman year helped his savings plan. It allowed schools to help athletes cover the full cost of attendance—including things like transportation and personal expenses.

For decades, athletics-related aid was limited to room and board plus tuition, fees and books. That limit, along with many players’ families’ modest financial means, made it common for even full-scholarship athletes to graduate in debt.

In the academic year just ending, Clemson athletes on full scholarship living off-campus received $1,532 monthly for the cost of attendance, room and board. Cost-of-attendance amounts vary by school. Early in his college career, Wilkins also received federal Pell grants for students with financial need, he said.

In the academic year just ending, Clemson athletes on full scholarship, such as Christian Wilkins, living off-campus received $1,532 monthly for the cost of attendance, room and board. Photo: Richard Shiro/Associated Press

Wilkins’s approach to budgeting was to segment his money into four Bank of America accounts and follow a code for using each one.

He stocked the first account, for everyday spending like going out to eat, with about $150 a month. When that was gone, he stopped spending.

“My biggest weakness always is food,” said Wilkins, who is 6-foot-3 and weighs 315 pounds. “That’s like my only drug in life. But in my mind, I put that into a necessity.”

The second account was for rent and big purchases, like airline tickets. The third, in which he put roughly half of his monthly check, was for savings and investments. The fourth account was for emergencies. Having purpose-specific accounts helped keep him focused, he said.

“I feel like a lot of people just have one account, and they feel like ‘I want this, I want that,’” he said.

Christian Wilkins accepting the Campbell Trophy from NFF Chairman Emeritus Jon F. Hanson. Photo: Ben Solomon/National Football Foundation

Wilkins grew up the youngest of eight children in Springfield, Mass., and said his family didn’t have much, but had what it needed—food on the table and one another. Wilkins can’t pinpoint how he became a super saver but said that over time he realized how little he needed to be happy.

He earned an undergraduate degree in communication studies from Clemson in 2½ years, then finished a master’s degree in athletic leadership in December. That month Wilkins also won the National Football Foundation’s Campbell Trophy, also called the “academic Heisman,” given to the nation’s best football scholar-athlete.

During college Wilkins became certified and worked as a substitute teacher, mainly in kindergarten and elementary school, for about $80 a day. In the NFL, his pay will be slightly higher: He’s in line for a $10 million to $20 million contract or more, depending on where he’s drafted.

Christian Wilkins earned an undergraduate degree in communication studies from Clemson in 2 ½ years, then finished a master’s degree in athletic leadership in December. Photo: Chuck Burton/Associated Press

Pro athletes often find themselves bankrupt years after their windfall. Wilkins’s challenge, in contrast, might be spending enough money—he joked that he’s been “slightly hesitant” to invest in hiring a financial adviser.

He doesn’t have a budget for being in the NFL but said he admires former player Ryan Broyles, who said he lived on $5,000 monthly while in the league. Wilkins wants his future family to want for nothing, but said he’ll balance their desires with his penny-pinching instincts.

“I’ll be that guy still forever sneaking food into the movie theater,” Wilkins said. “But if I didn’t have time to run to the store before the movie, I’ll still buy that large popcorn.”

Share Your Thoughts What were your strategies for limiting spending and saving money in college? Join the discussion below.

Write to Rachel Bachman at rachel.bachman@wsj.com