there are so many factors involved in that it is impossible to say. A teaspoon is not always a teaspoon, if you tap it suddenly the product in it will compact and you will have less than a teaspoon. So you can never be totally sure.
What is the purpose behind adding it? Grams are fairly small units of measurement. You want to be careful with those epsoms, cos they are a laxative if you use too much! Have you checked your water quality before? You probably dont need calcium carb...or are you trying to duplicate a particular water from another region?
I think you are probably going to have to (gulp) buy a scale. You can get one at walmart pretty cheap.