Practical guide and verification
Volume needs density to become mass
Millilitres measure volume and grams measure mass. The relationship is mass = volume × density, so there is no universal mL-to-g conversion without a material or density assumption.
Ingredient variability
Powders can settle, liquids change density with temperature, and food products vary by formulation. A kitchen density is usually an approximation rather than a laboratory constant.
Common mistake
Do not assume the numeric value is unchanged just because water is close to 1 g/mL under ordinary conditions. Oils, syrups, flour and many other ingredients can differ substantially.