McCance and Widdowson's Composition of foods database

I am in need of some help and just need a point in the right direction.
I have McCance and Widdowson's Composition of foods database. This
gives nutrition values per 100g of the food item.
This is fine for solid foods such as bread, meat, cheese but what about
liquid products like milk, beer or olive oil. Liquid products are
normally measured out in ml or oz.
Is there another database or something that will tell me how many grams
are in a ml or oz?

This is for school menus. I am looking into developing an application
that will aid the school cooks in making healthy meals.

Thanks for your help

.