Having online access to recipes from all over the world from both amateurs and professionals alike is one of the great things about the internet. Unfortunately, professional recipes and international recipes usually measure ingredients in grams and milligrams, while American home bakers are used to spoons and cups. You can convert between them, though the conversions aren't always accurate.
Weight vs. Volume Measurements
It's difficult to come up with accurate conversions from milligrams to teaspoons for a lot of ingredients, because they don't really measure things the same way. Milligrams are a unit of weight, and teaspoons are a unit of volume.
That means a good conversion requires two things. One is an ingredient with a known weight per volume, and the other is your own attention to careful measuring. Sometimes those two things are easy to arrange, but other times, they can be problematic.
Milligrams to Teaspoons for Salt
Consider the case of salt, which is one of the small handful of ingredients in which this conversion is especially useful. The Dietary Guidelines for Americans suggest keeping daily intake to 2,300 mg of sodium or less for optimum health. As it happens, that's exactly the amount of sodium in 1 teaspoon of ordinary table salt.
The "ordinary table salt" part is important, and it explains why this is a difficult conversion. Table salt has a small and consistent crystal size, which makes it easy to know how much is in a teaspoon. Coarse kosher salt, pickling salt and flakes of sea salt all take up different amounts of space, so how many milligrams you can fit in a teaspoon will vary with the type of salt.
Even worse, the larger and more irregular the crystals, the more that the amount you fit into each spoonful will vary. That's partly what makes the conversion so difficult.
Grams to Teaspoons for Other Ingredients
It gets more complicated when you're measuring ingredients that can be dense and packed, or light and fluffy. This is almost everything you bake with, from brown sugar and confectioners' sugar to cocoa and, especially, flour.
There are a couple of ways to convert these between weight and volume measures. One is to look up ingredient conversion charts online, and do the math. Most charts give you a weight in grams per cup of the actual ingredient. A cup is 8 fluid ounces, and there are 2 tablespoons per ounce and 3 teaspoons per tablespoon. Armed with that information, you can divide that weight per cup into a weight per teaspoon.
This isn't quite ideal, because the way you actually wield your cup or spoon has an effect on the weight of ingredient you get into a given measure. This means that another person's carefully measured conversion chart won't necessarily correspond to the way you scoop your ingredients. A better option is to buy or borrow an accurate scale, scoop 10 or 20 teaspoons of the ingredient, then weigh it and average them. That compensates for any inconsistency between spoonfuls.
This is also the technique to follow if you're working in the other direction: converting volume measurements from teaspoons to milligrams.
The Best Conversion Is No Conversion
The only ingredient that has a 100 percent accurate conversion between weight and volume is water. By definition, 1 milliliter of water has a weight of 1 milligram; 1 liter weighs a kilogram, and so on.
For other ingredients, the best and simplest way to work is not to convert at all, but to make a habit of using weight measures whenever they're available. This is how professional bakers work, because the larger your batch, the greater the risk that inaccurate volume measurements will sabotage your efforts.
Digital kitchen scales are surprisingly inexpensive, and weighing your ingredients instead of scooping them removes one of the most common ways your baking can go wrong.