Preheat the oven to 350 degrees F (175 degrees C). Grease and flour a 9x13-inch pan. Sift flour, baking powder, baking soda, salt, and cinnamon together in a large bowl. Set aside.

Beat sugar and eggs together in a large bowl until thick and pale. Stir in oil. Gradually mix in the flour mixture until combined. Fold in carrots and walnuts. Spread batter evenly in the prepared pan.

Bake in the preheated oven until a toothpick inserted into the cake comes out clean, about 40 to 45 minutes.