You can assume ingredients are mostly water and use 1kg = 1L for conversions, but a lot of ingredients violate this density assumption. You don't have to do the hard work to figure out the densities, the USDA already did it in the FoodCentral database. If you want a service that allows you create define recipes with both mass and volume indications, check out CookTime! Here's a rather complex macaron recipe from one of my friends: https://letscooktime.com/Recipes/Details?id=8a67c391-ddcc-4d...