Today in the excellent Food Lab series, Kenji Lopez-Alt reverse engineers the In-N-Out burger.

According to the In-N-Out nutrition guideline, replacing the Spread with ketchup results in a decrease of 80 calories per sandwich. I know that ketchup has about 15 calories per tablespoon, so If we estimate that an average sandwich has about 2 tablespoons of sauce on it (that’s the amount that’s inside a single packet), then we can calculate that the Spread has got about 55 calories per tablespoon (110 calories in two tablespoons of Spread minus 30 calories in 2 tablespoons of ketchup = 80 calories difference in the sandwich). With me so far?

It just so happens that relish has about the same caloric density as ketchup (15 calories per tablespoon), and that mayonnaise has a caloric density of 80 calories per tablespoon. Using all of this information and a bit of 7th grade algebra, I was able to quickly calculate that the composition of the Spread is roughly 62 percent mayo, and 38 percent ketchup/relish blend.

Here’s the recipe to make your own at home. Pairs well with make-at-home McDonald’s french fries. See also make-at-home Shake Shack burger.