First identify what the original unit measures. Cups and milliliters measure volume; grams measure mass. Converting between them requires the ingredient's density, so a generic cup-to-gram number is not reliable for every food.
Use Unit Converter for like-for-like units, then note the original and converted quantities beside each ingredient. For baking, weigh ingredients where the recipe provides weights and avoid aggressive rounding of small amounts.
Which ingredient and units are you converting?