When Specific Gravity Matters

I'm sure you've seen the way oil always floats on top of water when the two are combined. Make (and shake) a simple oil and vinegar dressing and the 2 components will separate before they even make it out of the bottle.

This happens whether you are mixing oil and water for salad dressing or for a cosmetic product. Oil will be oil and water will be water.

Above: a little demonstration of 3 different essential oils in vinegar (or should I say on vinegar). One of the reasons oil and vinegar (or water) do not readily mix is because they do not have the same specific gravity. I made these vials up several months ago just to see if there was even the slightest possibility that the oils would decide to mix with the vinegar, but they are behaving exactly as expected. They are having nothing to do with the vinegar.

Today we're going to get a little bit nerdy, but I'll try and make it fun too (and promise to throw in a little surprise).

What Specific Gravity Means

Looking at our un-mixable oil-and-vinegar salad dressing is a pretty fair demonstration of what specific gravity (SG) means: the relative density of the vinegar (or water) is not the same as the relative density of the oil.

Read: a cup of water does not weigh the same as a cup of oil. Because oil is not as dense as water, it will float on top.

The tricky part is that word: relative. There is no absolute to measuring relative density because we are measuring one ingredient in relation to another ingredient. 


Because it has a specific gravity of 1.0, water is a common reference point. 
One gram of water is equal to one milliliter of water.
If we divide the mass of water by the volume of water we get: 1g/cm³

Specific Gravity of a Few Ingredients

If we look at a few additional ingredients, some things start to make sense, and a few surprises pop up.


Glycerine has a specific gravity of 1.21
Add glycerine to water and it will tend to sink to the bottom.

Admittedly, I poured this up carefully for demonstration purposes because glycerine will readily dissolve in water. The water in this photo is an infused vinegar so you could see where the water stops and the glycerine starts.

Even though these 2 ingredients do not have the same specific gravity, they will not behave like oil and water. Glycerine and water (or vinegar infusion) will mix and stay mixed with just a bit of agitation.


This one surprised me a bit so I'm sharing it here.

Cows milk has a specific gravity of 1.035

I would have expected the fat content of milk to make the specific gravity tend to go in the other direction, but apparently it doesn't. When I get time to get into details about this, I will see if I can find out why.

Note: this information is from a chart (link below) and the chart did not specify which kind of milk, so I am assuming they meant cows milk.


Ethanol has a SG of 0.79 (at about 20°C).
I mention the temperature because – just to make things really fun – the SG of ethanol varies slightly according to the temperature it has.

Add ethanol to water and it will behave like glycerine and mix with the water.


Oils vary in their specific gravity, so it is possible to 'float' one oil on top of the other simply because of the difference in their SG.

Pictured is an example of castor oil and apricot kernel oil.
SG of Castor oil:  0.96
SG of Apricot kernel oil: 0.90 - 0.92

Why Does all this Matter?

If you are making a small batch of something for personal use, knowing the SG of your ingredients may not matter very much. As a matter of fact, it may not matter at all.

Lots of people have been successfully making their own products for years without giving SG a single thought.

But the specific gravity of ingredients starts to matter a whole lot as soon as you want to increase the batch size.

If a face cream has been measured out with cups, teaspoons, and drops, the formula risks meeting all kinds of problems if it is scaled up using cups, teaspoons and drops.

When is a cupful full? To the line? The rim? How about a teaspoon? A cosmetic formula that calls for '1 cup' of an ingredient risks ending up with a generous percent more or less of that ingredient if it is measured out using different tools, by a different person, or even by the same person on 2 different days.

It is quite simply impossible to get an accurate result if ingredients are not all measured by weight. And even though it may seem like overkill to meticulously weigh out every ingredient for a small batch of lip balm 'just for personal use', it provides accuracy.  And accuracy is key to a continuously successful result.

Think you can weigh out drops and get the same result every time? Check this example of how much variation Lorraine Dallmeier of Formula Botanica experienced when she decided to measure out some essential oils by the drop.

More Information

A clever gent called Ganesh Subramaniam has a short and sweet description of the difference between density, specific gravity and volume on Quora right here.
Ethanol, density and weight, the Engineering Toolbox
Ethanol Water mixture at various temperatures
Specific gravity chart , Galloup
Formula Botanica Essential Oil weighing drops experiment
How to Dose Essential Oils in a Formula (this blog)