We often get asked "how many millilitres of oil should I add to my wax".
When making candles, it is best to use weight/mass. In Europe, this is measured in grams and kilograms, whereas in the US, it may be measured in pounds and ounces.
Litres and millilitres are measures of 'volume'. Volume does not always translate into mass. With water being the basis of the metric system, 1 litre of (liquid) water has a weight of 1 kilogram and 1 millilitre of water has a mass of 1 gram. This relationship is called the 'relative density' or 'specific gravity' and for water it equals 1.0.
This video from Khan Academy explains why this matters.
Candle wax and fragrance oils are lighter than water. So a litre of wax or oil will weigh somewhat less than a kilogram. Candle wax is typically around 10% lighter than water, so may have a relative density (relative to water) of 0.9
To make matters worse, volume changes with temperature and the relative density can be different for various waxes and oils, so 1 litre of one oil, may have a different weight to 1 litre of a different oil. It is for this reason that our perfume oils generally ship in larger bottles (i.e. 1kg of oil is shipped in a 1.25 litre bottle). This compensates for the extra volume. If we were shipping water, we could use 1 litre bottles.
So, we strongly recommend that you use weight and not volume when making your candles.