Steven,
The English system of measures is pretty difficult, but one really
convenient thing is that an ounce of water weighs an ounce. So when you
are using water, one type of ounce equals the other type of ounce. This is
true for any temperature of water, as long as it's liquid. So 8 fluid
ounces (i.e. one cup) of water weighs half a pound. One kiligram equals
2.2046 pounds, so that cup of water weighs about 0.227 kilograms. Maybe I
would round it to 250 grams to make the recipe easier.
I think that with its extra sugar, fat and protein, milk would weight a
little less per unit of volume, but not significantly enough to care.
This is not true with oils. Oil floats on top of water (or vinegar)
because it weighs less. I don't know how many ounces an ounce of oil
weighs, and I don't know how much the various oils vary in density (weight
per unit of volume).
A web search makes it easy to find conversion tables. For instance, I found
http://convert.french-property.co.uk/
which converts between metric and Imperial and also between metric and USA
measures.
Tom