Mustard powder is also known as ground mustard or dry mustard. It is the ground seeds of the mustard plant and you can use it to make a host of savory dishes. Mustard powder provides food with a pungent, tangy burst of acidity and a mild heat that can cut through the fat in meat dishes or add a distinctive sharpness to macaroni and cheese. If there is no mustard powder in your spice cabinet, there is still hope. Consider some of the alternatives.
Your best bet: Mustard seeds
The closest spice to dry mustard is the seed used to make it. If you have a spice grinder or a coffee grinder, you can grind your own mustard powder from whole mustard seeds. If you have neither of those appliances, consider crushing them on a clean surface with a heavy pot or large spoon. Note that crushed mustard seeds can be used as a part of a dry rub or in a stew, but will be too coarse to provide the same emulsification benefits that you get from mustard powder. When you are using whole seeds, use half the amount that your recipe requires for mustard powder.
A decent second choice: Regular prepared mustard
The main ingredients in prepared mustard are mustard powder and vinegar, which means that it can make an excellent mustard powder substitute. Note that the yellow mustard with which most Americans are familiar is somewhat milder than mustard powder so you will have to adjust the quantity when using this substitute. Use a tablespoon of prepared mustard for each teaspoon of mustard powder that your recipe requires. Dijon mustard is often a better choice than yellow mustard as its flavor is closer to that of mustard powder. Something else to note is that prepared mustard can affect the consistency of sauces and dressings. To prevent this, reduce the liquid in your recipe by a teaspoon to compensate.
In a pinch: Horseradish powder
Horseradish comes from a relative of the mustard plant, but is made from the root rather than the seeds. The similarity between the two makes it an excellent mustard powder alternative. Horseradish is hotter than mustard, which makes it perfect for adventurous eaters who want to give their dish a spicy kick. You should keep that additional spiciness in mind when using it as a substitute; however, this is only an issue in heated dishes. Horseradish loses its spiciness when heated, which means that it is suitable only for cold items. Avoid grated horseradish as a mustard powder substitute as it does not retain its pungency and is therefore not similar enough to be a good stand-in. When using horseradish powder as a replacement for mustard powder, use half as much as your recipe specifies for mustard powder.
Turmeric can work as an alternative to mustard powder as long as its bright yellow color will not be an issue in your dish. It has a mild spiciness like that of mustard along with bitter notes that also bear some similarity to mustard. You can use exactly the same amount of turmeric that you would use of mustard powder.
Wasabi powder is yet another option. Wasabi has similarities to horseradish and wasabi powder has a similar consistency to that of mustard powder. It can provide the sharpness and kick that you want from mustard. Like horseradish, it is hotter than mustard powder so you should use it sparingly. Start by adding about half as much as your recipe requires for mustard powder and work your way up to the flavor you want.