A missing half-inch really adds up.

A card-carrying Subway devotee is fighting back against sub-par sandwiches, filing a lawsuit alleging the fast-food giant ripped off customers for more than $142 million with its short “Footlongs.”

Jason Leslie, 32, of Marlboro, NJ, says he was spurred on to sue the sandwich maker yesterday after he saw an article in The Post last week that revealed the “$5 Footlong” subs often don’t measure up.

Leslie, who carries a Subway Rewards card and estimates having eaten about 50 of the sandwiches a year since he was 18, bought a Footlong meatball sub four days later for $8 that ended up being short of 12 inches. “They advertise in all these commercials, ‘Footlong, Footlong, Footlong,’ and now I feel like an idiot,” the used-car dealer told The Post. “I can’t believe I fell for that trick.

“The sandwiches are anywhere between a half-inch to an inch shorter . . . I feel cheated.”

In the federal class-action suit, filed in Trenton, Leslie’s attorneys estimate that 25 percent of the company’s revenues come from selling Footlong subs, totaling $2.85 billion a year.

Roughly 5 percent of that — or $142.5 million — represents “unfair and deceptive revenue,” the suit alleges, based on an assumption that each Footlong is at least a half-inch short of 12 inches.

“Plain and simple, the advertising is deceptive,” said Jerry Marks, founding partner of Red Bank, NJ, law firm Marks & Klein, which is representing Leslie.

It’s the second lawsuit filed this week against Subway. A state suit filed Tuesday in Burlington County, NJ, is also targeting the allegedly deceptive marketing.

“A foot is 12 inches. They call it the Footlong, making people believe they’re getting a foot-long sandwich,” said attorney Stephen DeNittis. “If they were calling it the ‘Big Sandwich,’ or the ‘Big Kahuna,’ this case wouldn’t have been filed.” DeNittis, who represents Charles Pendrak and John Farley, said he planned to file a similar state suit in Philadelphia today.

The lawyer hired an investigator to measure sandwiches at 17 Subways near his office, and found that all sold Footlong subs that were less than 12 inches, he said.

He is seeking compensation for the missing inches, and to “either get [Subway] to sell sandwiches that are foot long, or get them to put up a disclaimer, similar to what McDonald’s does with its Quarter Pounder,” he said.

A Subway spokesman said in a statement, “We have redoubled our efforts to ensure consistency and correct length in every sandwich we serve. Our commitment remains steadfast to ensure that every Subway Footlong sandwich is 12 inches.”