A very common question we get asked is whether you should formulate in weight or volume. Here we set out the differences and why one is better than the other. 
Volume vs. Weight
Volume is the space that a substance occupies, or that is enclosed in a container. That means it is a measure of the space that it takes up. Volume, when used in EU cosmetics, is measured in millilitres (‘ml’) or litres (‘l’) but is also measured in cm3, m3, pint. Most people will be familiar with the concept of weight, it is in effect the amount of force exerted downwards by an object. You can measure weight on a set of scales (see our section ‘Equipment for Production and formulating’ section for more information about scales for commercial use). The EU requires producers to describe weight in milligrams (mg), grams (g) or kilograms (kg).
If you are selling your product by weight these are the measurements you should use. Remember using the European Estimated sign also gives you some margin for error when using the average weights. More information is available on the Government website for Weights and Measures:
https://www.gov.uk/weights-measures-and-packaging-the-law/units-of-measurement
For production we recommend the use of weight, rather than volume. This is because it is the most accurate way to produce the same product each time and can be consistent with all your ingredients. For example it is not really possible to measure the volume of a wax pellet ingredient when in that form so the only way to be consistent is to use the weight of ingredients.
Remember: Even essential oils should be measured in weight, drops are not and accurate measurement for formulating or production.