How about starting off with an equation, that’s a great way to win friends and influence people isn’t it?

Specific Gravity formula from Wikipedia

Specific Gravity formula from Wikipedia

If that didn’t grab you then how about a little explanation:

“Specific gravity is a number given to represent how dense (mass per unit volume)  a substance is in relation to water.  That’s why the equation is written as it is – as a ratio.

Any wiser?

Having peaked as a mathematical genius in year 6  (or primary school) I sympathise with anyone who finds this sort of thing hard as even though I now ‘get’ this, every time I have to work out Specific Gravity I still need something a little more visual at hand to help me run a sanity check over the numbers.  I don’t know why I need to do that, maybe my math brain was sucked out by zombies dogs and replaced by a picture of a unicorn frolicking in the forest or maybe I have just always been a visual thinker.

So, with that in mind I’ll show you how I make sense of this and then I’ll explain why I need to.

Here is a picture to  help explain an important concept behind specific gravity, namely density. 

What 11 stone looks like from the Daily Mail, UK:

What 11 stone looks like from the Daily Mail, UK:

These women all weigh 11 stone according to a story ran by the UK’s Daily Mail.  As you can see while they all weigh the same they are not all the same size or shape.  When we weigh things we are only interested in that one dimension which is OK if space isn’t an issue but it often is.  If you were trying to squeeze these ladies into a container you would need to know something about their shape and how much space they take up to ensure the container you found was the right shape.

Density is where space and weight combine. This is really, really useful for the cosmetic chemist as it helps us ensure we buy the correct sized containers for our lotions and potions.  As a stand alone measurement the units used to describe density are Kg/m3

The density of a material can change with the temperature, especially when talking chemistry and as such that is often recorded also.

Demonstrating Density with Oil and Water – A cosmetic Scenario.

Specific Gravity Image


So back to our equation.

Specific Gravity is a number that shows us how dense our material or mixture is when compared to water.

As it is a ratio it has no units.

Numbers less than 1 mean that the material is lighter than water.  Practically speaking this means that our material will float if placed in water.

Numbers greater than 1 mean that the material is heavier than water. The material will sink.

I tried this out with an ointment I’ve made and tested, an SPF cream and a moisturiser.  As you can see my moisturiser floated whereas the other two materials sunk. This validated my laboratory test results where I worked out Specific Gravity.

Visualising Specific Gravity

Visualising Specific Gravity

I use this equation and visualisation in my daily life as a cosmetic chemist.  I watch out for changes in my specific gravity to see if a formula is changing over time. This may help me to detect instability early.  I use my knowledge of Specific Gravity  even before I make a recipe – to ensure that my oil and water phase stand a chance of being stable. Sometimes I seek to match densities to help build a more stable formula.  Finally, as mentioned above I use Specific Gravity to help me to work out what size of container I need and what I will put on the label.

I love the science that goes on behind the cosmetic brand and hope that after reading some of these blog posts that you do too!

Amanda x