A grocery list, at its most basic, is just a list of random things you’ll need to stock up on: things like milk, eggs, cheese, bread, cereal, and so on. This list changes every so often and you usually have to make a new one each time. The reverse shopping list functions more like an inventory list, a master list of everything you have in your pantry or your fridge.

A good way to start a reverse grocery list would be that you dig out everything in your cabinet and see what you have. You can note down what you usually have in your cabinet (such as spices, boxed macaroni and cheese, cans of soup, etc.) on a Google Doc or your phone’s notes app. It’s like taking inventory at a store, seeing how much of a certain item you carry or have in stock at the moment. Then, if you run out of soup over the week, you simply bring up the master list on your phone and put a checkmark next to the soup, telling you that you will definitely need it next time. When you get the soup on your next grocery run, you remove that checkmark since the item is now back in stock in your pantry. This way, rather than running around the store filling your cart with items that you can’t remember if you need, you know exactly what you have to replace.