To calculate the total cost of a menu you'll first need to calculate the theoretical food cost of the menu items. Multiply the quantity of the menu item sold by the direct food cost of that menu item. For example, if you sell 10 salads that cost you $3 each, your theoretical food cost is $30 ($3 x 10 = $30).
You can then do this for each menu item sold and calculate the total theoretical menu cost. For example, let's say all the menu items sold total $100 in cost.
Finally, you'll want to express this as a percentage. You will take the total theoretical menu cost and divide it by the total sales of your menu to achieve your theoretical food cost %. In this example, let's say you had sales today of $400. This means your total theoretical food cost is 25% ($100 / $400 = 0.25).