Why do recipes make you buy more food than you need?

A recipe calls for two tablespoons of buttermilk, a quarter cup of coconut milk, one tablespoon of tomato paste, a handful of coriander. The shop sells buttermilk by the litre, coconut milk by the tin, tomato paste in a jar meant to season a dozen dinners, coriander in a bunch that has clearly never met a recipe asking for so little of it. None of that is you shopping badly. It is two systems — a recipe and a supermarket shelf — that were never designed to line up in the first place, and the gap between them is exactly where the leftover buttermilk, the half-tin of coconut milk, and the wilting herbs come from.

A recipe is written for a reader with an empty kitchen

A recipe has to work for anyone who opens it, which means it is written as though the reader is starting from nothing — because most of them, for most ingredients, are. The person writing it has no way of knowing whether you already have half a jar of tomato paste in the fridge door, so the instruction is simply "one tablespoon," a precise number chosen to make the dish taste right, with no reference at all to how that tablespoon is actually sold. That precision is a feature for the dish and a liability for the shopping trip: the recipe optimises for one plate tasting correct, not for the container it came out of ending up empty.

Packaging is solving a different problem entirely

The tin, the jar, the bunch is not sized around any recipe either — it is sized around manufacturing, shelf life, shipping pallets, and what a shopper is willing to pay for at that size. Herbs get bundled because that is how they are harvested and displayed, not because any dish needs that much. Buttermilk comes in a carton sized for baking projects and drinking, not for the tablespoon a marinade happens to want. Two entirely reasonable systems — a recipe optimising for taste, a package optimising for retail — produce a mismatch as a simple side effect of neither one having been designed with the other in mind. Nobody along that chain made an error. The gap was there before you ever picked up a shopping list.

Shopping for a dish is a narrower kind of shopping than shopping for a household

There is also a second gap, less about units and more about direction. Shopping with a specific recipe in hand means the list is built entirely around what that one dish is missing, with no reference to what else the fridge could use or what is about to expire. It is easy to walk out with exactly what a recipe named and nothing that connects to anything else you already own — a jar bought to solve a single dish's requirement, with no natural next use once that dish is done. An onion or an egg is never really "for" one recipe; it slides into whatever gets cooked next without anyone deciding it should. A tablespoon-sized purchase bought for one specific step does not have that flexibility built in, because it was never chosen with a second use in mind.

The surplus does not feel like waste on the day you buy it

This is the part that makes the pattern easy to miss in the moment: buying a full carton for a two-tablespoon marinade does not feel wasteful at checkout. The carton is fresh, sealed, exactly what the recipe named — nothing about it looks like a mistake yet. The waste only becomes visible a week or two later, once the rest of the carton has sat in the door shelf through several more dinners that had no use for buttermilk, and by then it reads as forgetfulness rather than what it actually was: a purchase that was oversized relative to the dish from the moment it left the shelf.

What actually helps

• Before buying a recipe-specific item, ask what else it goes into. Buttermilk works in pancakes as well as marinades; coconut milk carries into a curry or a soup later in the week. If nothing else on the week's plan uses the rest of the container, that is worth knowing before checkout, not after.
• Look for a substitute already in the kitchen before buying the exact ingredient. A recipe naming buttermilk can often be answered with milk and a splash of lemon or vinegar; tomato paste can sometimes be swapped for a spoonful of an opened jar of pasta sauce. The recipe wants a flavour and a texture, not necessarily the specific product.
• Buy the smallest available size for a single-use ingredient, even if the larger size is better value on paper. A small jar that gets fully used beats a large one that gets a tenth used and then forgotten.
• Treat the leftover portion as a new ingredient the moment you get home, not as a problem to solve later. A note of what is now open and how much of it is left turns "there's some coconut milk somewhere" into something you can actually plan a second dish around.

The direction that avoids the problem entirely

None of the fixes above remove the mismatch — packaging is not going to start matching recipe quantities. What removes it is planning in the other direction: starting from what is already in the fridge and freezer and asking what it can become, rather than starting from a dish and asking what has to be bought to complete it. Cooked that way, the open carton of buttermilk from last week's pancakes is not a leftover looking for a use — it is simply part of what tonight's options already include. Munch keeps track of what is actually open and in the fridge, freezer and pantry, and when it suggests what to cook it groups recipes by what you can make in full from what is already there — so the ingredient that would otherwise sit half-used in the door has a real chance of being the thing that decides dinner, instead of the thing dinner forgot about.