A teaspoon of kosher salt is not a teaspoon of kosher salt

The two brands of kosher salt sold in almost every American supermarket weigh very different amounts by volume. Diamond Crystal, the version most professional kitchens use, gives about 2.8 grams to a teaspoon. Morton, the version most home kitchens have on hand, gives 4.8 grams to the same teaspoon. That is not a rounding difference or a manufacturing tolerance. It is a factor of nearly two on the same nominal ingredient.

The number for regular table salt, for reference, is 6 grams per teaspoon. So Morton kosher sits close to table salt by weight; Diamond Crystal is less than half. A recipe developed with Diamond Crystal and cooked with Morton, using the same teaspoon count, ends up close to twice as salty as the writer intended. In the reverse direction, using Diamond Crystal for a recipe developed with Morton produces food that tastes flat.

Why the same salt weighs different amounts

Salt is salt at the crystal level. Sodium chloride has a chemical density of 2.16 grams per millilitre no matter which brand you buy. What changes between brands is the shape and size of the crystals, and how those crystals pack into a spoon.

Diamond Crystal uses the traditional Alberger evaporation process. Brine is boiled slowly in shallow open pans, and the salt forms as hollow, pyramid-shaped flakes on the surface. These flakes are thin and irregular; when you scoop them into a teaspoon, they bridge and stack loosely, trapping a great deal of air. The teaspoon that looks full is mostly air with a scatter of salt.

Morton uses a rolling process that produces small, dense, roughly cubic flakes. These pack together more like table salt does — tightly, with much less air between the crystals. The teaspoon that looks full is largely salt.

Both are pure sodium chloride. Both are labelled "kosher salt". Both cost about the same per pound. But the same volume of one weighs nearly twice the other because of how the crystals sit in the spoon.

What professionals do about it

Serious Eats, Bon Appétit, the New York Times cooking section, and most modern American cookbook authors specify Diamond Crystal when they publish a recipe with a salt measurement in teaspoons. It is the professional-kitchen default. Simply Recipes, older regional cookbooks, and much of Southern recipe writing assume Morton, because that is what stocked the pantries the recipes were tested in.

When a recipe does not name a brand, the safest guess is Diamond Crystal for anything written by a national US food publication in the last twenty years, and Morton for anything older or regional. When the wrong assumption costs you, the food is memorably off.

Bon Appétit took the unusual step, a few years ago, of publishing both measurements for salt-sensitive recipes — a zucchini scone recipe that called for either one tablespoon of Diamond Crystal or one and a half teaspoons of Morton. That is exactly the ratio the physics predicts.

The rule

If a recipe specifies kosher salt without naming a brand, and the recipe matters, do one of three things. Weigh the salt in grams instead of measuring it in teaspoons. Ask the writer, or check whose kitchen the recipe came from. Or default to Diamond Crystal for modern recipes and Morton for older ones, and adjust to taste.

If you cannot do any of those and you know only one thing about your salt, it is this: table salt weighs about 6 grams per teaspoon, Morton kosher is about 5, Diamond Crystal is about 3. Half a teaspoon of table salt is roughly one teaspoon of Diamond Crystal. Weigh, and none of this matters.

The label side of the same problem

The word "kosher salt" on a box does not tell you what a spoonful weighs, but at least you can weigh it. The word "peanuts" on a candy bar tells you they are somewhere in the ingredient list, but not where or how much or whether the flavour is real or synthetic. Labels are shorthand, and the shorthand hides more than it reveals.

That is why I also built SwapGenie — it reads the barcode, checks the ingredients against what you personally avoid, and suggests something better when the packet claims one thing and lists another.

Two ends of the same problem: knowing exactly what is in your hand, rather than what the label suggests.