Every Formula Needs Specific Gravity

Introduction

What makes specific gravity so great?  With this simple calculation we’re able to manufacture using mass measurements and not volume measurements.  Why does that matter?  Because manufacturing personal care works best with mass measurements, but we fill bottles using volume measurements.  Translation between the two styles of measurement needs to be simple.

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

What is Specific Gravity?

The specific gravity equation is a comparison of weights between two different substances at a single known volume.  One of the substances is typically water since a single gallon of water has a well attested weight at sea level and room temperature.  The result from a specific gravity calculation in the personal care industry tends to be a number that is a little less or more than one (1.00).

What Does Specific Gravity look like?

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.  So, 8.08 pounds of FFB divided by 8.33 pounds of water creates a specific gravity of 0.97.

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

We now know that the weight Fantasy Foam Bath (FFB) at one gallon is equivalent to 0.97 times the weight of water  [Weight of 1 unit(x) FFB = .97 * unit(x) of water].

Specific gravity does not need to use gallons or lbs.  Any volumetric measurement will work as long as the weight of water is known for the chosen unit of volume, such as quart, liter, or cup.

Any volumetric measurement will work as long as the weight of water is known for the chosen unit of volume, such as quart, liter, or cup.

Why we need specific gravity

Let’s say a customer places an order for 4000 bottles of FFB, each bottle holding 3.2 fl.oz. of product.  Filling this order would require 100 gallons of Fantasy Foam Bath.  Our formulation says that for any amount of FFB there must be the following amounts of ingredients present 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.

But we don’t know the specific gravity of Surfactant X so there is no direct calculation we can make.  We’ll work backwards from the formulation’s specific gravity back to the weight of the ingredient to answer our question.

Getting Surfactant X’s weight

To find Surfactant X’s weight we first take 100 gallons of FFB and multiply it by .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.

[(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 must be comprised of 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.) to our factory.

Conclusion

BPI Labs finds itself using 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.

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