OpenERP supports several units of measure. Product can be expressed in several units of measure at once. Make sure that all the units of measure used for a product are in the same units of measure category.
Factor is used to convert from one unit of measure to another as long as they are in the same category.
Sales → Configuration → Products → Unit Of Measure → UoM Categories
Sales → Configuration → Products → Unit Of Measure → Unit of Measure
Here we are taking example of product: Milk.
We have created UoM Category named, ‘Volume’ and three units of measurement: Gram, Litre and Gallon.
As you can see, for Milk product, we configured ‘Litre’ unit as a default measurement unit and also for purchase unit.
Now we are going to sale 500 grams milk from our stock.
By this way, you can manage multiple UOM in OpenERP.