Specific Gravity Formula: Why you need it

Why your formulation needs specific gravity

BPI Labs does all its manufacturing in mass measurements because it would be impractical to mix and compound your product without a mass measurement.  Measurements done in cups, liters, and gallons, which are volume measurements, are imperfect at best, but in a manufacturing environment it would be impossible to guarantee great production results.  With a simple calculation using your product formula’s specific gravity, we’re able to convert from volume measurements to mass measurements.     Click here to learn more about why BPI Labs manufactures in mass measurements.

Specific gravity is the key that allows us to convert from a purchase order written in volume, to manufacturing requirements in pounds or kilograms.  For example, if you were to order 4,200 bottles of conditioner and each bottle of conditioner contains 12 fluid ounces, then we could determine how many pounds of conditioner you are ordering with your formula’s specific gravity.  All we would need to do is find out how many gallons of conditioner would fill your 4,200 bottles, then use that number in a simple equation with your product formula’s specific gravity to see how many total pounds of conditioner is being ordered.

Download our sample formulation sheet to begin using specific gravity in your formulas.

What is Specific Gravity?

Specific gravity is a comparison of weights between two different substances using the same volume unit.  Usually we use water as the first substance since room temperature water has a well attested weight at sea level, and the other substance is your product formula.  To get a formula’s specific gravity, we need to know the weight of water and the weight of your formula at some volume, like a gallon, liter, or cup.  Then we take the two weights at the unit volume and divide them to get a specific gravity.  The result from a specific gravity calculation in the personal care industry tends to be a number that is pretty close to one (1.00 +/- .05).

Here’s an example where we derive a specific gravity ratio

Let’s find the specific gravity of a private label bath product called Fantasy Foam Bath (FFB). One gallon of this FFB weighs 8.08 pounds. We can derive the specific gravity of this product by dividing 8.08 pounds by the weight of a gallon of water, which is 8.33 pounds at sea level and at room temperature.  When we take 8.08 and divide by 8.33 we learn that our specific gravity ratio is (0.97).

[8.08 pounds of Fantasy Foam Bath per gallon  /  8.33 pounds of water per gallon = 0.97]

We now know that the weight of Fantasy Foam Bath (FFB) at one gallon is equivalent to 0.97 times the weight of water at one gallon.  Specific gravity does not need to use gallons; this is just the volume measurement we used in our example.  Any volumetric measurement will work as long as the weight of water and your formula is known for the chosen volumetric unit.  For example, we would have arrived at the same ratio (0.97) with the weights of a cup of water and a cup of FFB.

Any volumetric measurement will work as long as the weight of water and your formula is known for the chosen volumetric unit.

How we use specific gravity in a conversion

Let’s look at an example where we use specific gravity to make a conversion. Suppose a customer places an order for 4,000 bottles of FFB, each bottle holding 3.2 fl.oz. of product.  Filling this order requires [(4000 * 3.2) / 128] = 100 gallons of Fantasy Foam Bath.  Our formulation, written in percentages, says that for any amount of FFB there must be the following percentage of ingredients in the mixture:

• 95% Water
• 4.9% Surfactant X
• .1% Preservative Y

Our materials suppliers, who tend to sell in mass measurement, want to know how many pounds of Surfactant X we’ll be ordering, and the shipping company wants to know how much weight of Surfactant X will be shipped for pricing purposes. The production compounder, who will be mixing FFB, will also want to know how many pounds of Surfactant X to put into the mixing tank.  We need to use the specific gravity to find these weights.

To find Surfactant X’s weight, we first take 100 gallons of FFB and multiply it by a specific gravity of (0.97) to understand the volumetric equivalent of water.

[(100 gal. of FFB) x 0.97 = 97 gallons of water]

We can then multiply this result by 8.33 lbs., since this is the per gallon weight of water at room temperature and at sea level.

[(97 gal. of water) x 8.33 lbs./gal. = 808.01 lbs]

This is when we begin to appreciate a formulation’s specific gravity.  The total batch of product will weigh 808.01 pounds and 4.9% of the total formulation is Surfactant X.  We will multiply 808.01 lbs. by 4.9% to understand that we need to purchase and ship 39.59 lbs (~40 lbs.) of Surfactant X to our factory to complete the order of 4,000 bottles of Fantasy Foam Bath.

Conclusion

BPI Labs uses specific gravity daily because we’re using mass measurements to formulate and manufacture personal care product. Specific gravity removes error and assures production consistency by converting volume to mass for any unit of measurement. In our article entitled Turning Recipes into Formulas we explain why using mass measurements in production is better for consistent production in personal care manufacturing, and we encourage you learn more about this if you’re interested.

Download our sample formulation sheet to begin using specific gravity in your formulas.