Substituting Diced Canned Tomatoes for Fresh
Most savory cooking simply isn't exact enough to worry about this in any level of detail.  Tomatoes are extremely high in water to start with, even whole, about 94%.

Savory recipes can be adjusted easily by adding liquid or reducing in most cases, if it does matter.

Simply use the entire weight of the can content in substitution for the weight of fresh tomatoes, and adjust the recipe as needed based on the outcome.



This is educated speculation, but note that while tomato packers may pack the diced tomatoes in juice, that juice almost certainly comes from the core of the very same tomatoes whose flesh was diced.  Otherwise it would be less economical to pack them that way, and they would pack them in water.