Preheat oven to 350 degrees F (175 degrees C). Grease a 9x13-inch baking dish.
Whisk flour, sugar, cocoa powder, and baking soda together in a bowl; add water, canola oil, and vinegar. Mix until batter is smooth. Pour batter into prepared baking dish.
Bake in the preheated oven until a toothpick inserted in the center of the cake comes out clean, about 30 minutes. Cool in the pan before slicing.
