The term "dry mustard" refers to a powder made by crushing mustard seeds. "Ground mustard" and "mustard powder" are other terms for the same product. The seeds and powder can give your finished recipe a slightly different flavour, and the powder can make some final products cloudier than the seeds and act as an emulsifier in certain preparations. Despite these differences, you can substitute dry mustard for mustard seed in most recipes with no ill effects.
Determine the exact amount of mustard seed in the recipe you wish to make. Double this amount. For example, if your recipe calls for 15 ml (1 tbsp) of mustard seed, double this to 30 ml (2 tbsp).
Measure out an amount of dry mustard equal to the amount you calculated. Take care with the quantities as mustard powder is typically very strong.
Add the dry mustard to your recipe in the same manner as you would have added the mustard seed. Continue preparing the recipe as normal.
You can reverse this substitution by dividing the amount of dry mustard in half and adding that much mustard seed. This will not work in some recipes, such as those that rely on the texture of the powder to act as an emulsifier.