Unfortunately you can't make the conversion simply, as it depends entirely on the density of what you are measuring.
Water is well known for having 1ml=1g. Oil on the other hand, is much less dense than water, and so 1ml weighs less than a gram.
Some common conversions are:
1 cup of water: 250g
1 cup plain flour: 128g
1 cup white sugar: 201g
It's worth using a converter like
this one as the density of each and every ingredient is different.
If your recipe is very finicky I highly recommend using weight and not volume because the volume will change with temperature. Again, water is a great example of this: think of how water increases in volume when frozen, yet its weight remains the same. Using weight for measurements is especially important in soap making and other chemical reactions.