How much is “medium-sized” in a recipe?

“Medium-sized” is not a measure but a range; two onions or two potatoes called by the same name are not the same weight. If you want a recipe to come out the same every time, what is reliable is weight, not count. No gram equivalent is given here on purpose, because any figure given would again reduce a range to a single value.

Medium compared with what?

The medium-sized in a recipe is the vegetable the person writing the recipe was holding that day. Its equivalent in the reader's hand is whatever came into their own store that week; there is no reason for the two to match.

The difference in size does not come from a single cause. Variety, season, where it was grown and the seller's size sorting decide the result together; even the same store can sell noticeably different sizes in different weeks.

The same uncertainty exists with household containers. How much measures named after themselves, such as spoons and glasses, can vary is set out with its source in the article on spoons and glasses; expressions of count are the vegetable-side equivalent of the same problem.

In which recipes does this difference matter?

In stews the difference usually does not show. A slightly larger onion does not spoil the dish; a dish cooking in a pot carries that flexibility.

Where the ratio decides the result, the same difference becomes visible. In baking, in recipes such as cakes and buns, or in the consistency of a sauce, the ratio between ingredients builds the result directly.

The difference also accumulates. The deviation in one medium potato may stay unimportant; in a recipe calling for three medium potatoes the same deviation triples and changes the cooking time too.

The same uncertainty shows in the cooking time as well. A larger piece cooks more slowly; the time written in the recipe was worked out for the size in the hand of whoever wrote it, and may not have an exact equivalent in your own pot.

What goes in place of a count?

The most reliable equivalent is weight. If the recipe is your own, weighing once and writing the result in the margin makes that recipe permanently repeatable.

The second route is volume: “a glass of chopped onion” describes a narrower range than “a medium onion”. Measuring the chopped form gives a more consistent result than estimating the size of the whole.

With someone else's recipe, making the first attempt as written and noting the result is enough. What gets corrected next time is not the recipe but its equivalent in your kitchen.

In some recipes the count is left flexible on purpose. If the exact weight of the onion going into a dish does not matter, the recipe writes it as a count; telling which item is flexible and which is not is more useful than weighing everything.

What if you have no scale?

Consistency can be built without a scale too. Always buying the same size when you shop fixes your own kitchen's “medium”; any measure that is fixed becomes repeatable even if it is not accurate.

Chopping and measuring with a container does the same job. The article on fixing your measure to a single container explains this method in detail and it is the most practical answer for kitchens without a scale.

What can you do today?

You can weigh the onion or potato you use in a recipe you make often, once, and write it in the margin. That one-off job keeps that recipe from being left to guesswork again.

If you have no scale, you can try measuring with the same container every time after chopping; even without knowing the real volume of the container, the result comes out the same every time.

Do not try to convert all your recipes at once. Converting the count into a measure in the single recipe you make most often is already enough to see where the difference matters.