Weight vs. Volume When Measuring Ingredients (Finished Products, too!)
Weight vs. Volume; What’s the Big Deal?
Which weighs more; a ton of feathers or a ton of bricks? You may have heard this riddle in middle school, and you may have even gotten the answer right. They both WEIGH a ton, however, your brain may have pictured a ton feathers and how you would need way more feathers than bricks, which may have lead you to guess that a ton of bricks weighs more. This tricky middle school riddle simply highlights the difference between weight and volume. We know that a ton of anything weighs a ton, however, to weigh a ton of feathers you would need an enormous container whereas if you were to weigh a ton of bricks, you would need a much smaller container. The amount of space that a ton of gold or a ton of feathers takes is the VOLUME. So, if the riddle were changed to ask what takes up more space, a ton of feathers, or a ton of bricks the answer would be a ton of feathers.
Specific Gravity – Volume and Weight
Simply defined, specific gravity is how density relates to weight. Specific Gravity is unique to every material and is the ratio of the density of a material to that of any standard substance, usually water at 4 degrees Celsius or 39.2 degrees Fahrenheit. By definition, water has a density of 1 kg per liter at this temperature. As the riddle above shows, they are two very different ways of measuring. Water is the starting point where 15 milliliters (volume) of water equals 15 grams (weight) of water. Generally speaking, Specific Gravity is the ratio of the density of a substance to the density of water. For example, Glycerin sits around 1.26 meaning that it would take 1.26 pounds of water to equal the same volume of 1 pound of Glycerin. In other words, you would need to put one pound of water in a container that is 1.26 times larger than you would need for one pound of Glycerin.
The one critical reason to understand Specific Gravity is to accurately calculate how much of an ingredient you need to order. Generally, formulations are written in weight and ingredients are sold in volume. So understanding how to convert your formula into a shopping list is important. Take Essential Oils and Carrier Oils, for example. Oils are lighter than water; you can see this by pouring a tablespoon of any oil into a clear glass and then pouring water on top of that oil. You will see the oil rise to the surface and float on water. Why? Because oil is lighter than water. Glycerin, on the other hand, sits at a specific gravity of about 1.26, so it sinks in water.
For example, many years ago when I learned this lesson, I was making a massage oil blend. I knew I needed a total of 20% Sunflower Oil in my blend. I was making 200 pounds of massage oil so I knew that I needed 40 lbs (weight) of Sunflower Oil. I grabbed a five-gallon pail of Sunflower Oil assuming that it would weigh exactly 40 pounds, just enough to complete my project. However, when I weighed out the Sunflower Oil, I only got 36.8 pounds. The pail was full, but I didn’t get the expected 40 pounds.
The reason the pail was full, but I didn’t get 40 pounds as expected, is because the Specific Gravity of Sunflower Oil is only .92 (it is not as dense, therefore not as heavy, as water) meaning that even though I had a full 5 gallons (volume) of Sunflower Oil, it still weighs less than water, so I only had 36.8lbs. The math looks like this – 0.92 (specific gravity of Sunflower Oil) x 40 ( weight of 5 gallons of water)= 36.8 lbs. I was able to adjust my formula to accommodate the lack of 3.2lbs, but that is not always an option, especially when you’re dealing with more sensitive formulas such as emulsions. A quick Google search can usually give you the Specific Gravity of an ingredient so you could conceivably do the math to order exactly the right amount of each ingredient for your projects.
The Same Goes for Your Finished Products
Have you ever been confounded about why your 4 oz jar doesn’t hold 4 oz of product? The volume (fl oz) of the jar may or may not be equal to the weight and so your 4 oz jar may hold only 3.8 oz (weight) when it holds 4 oz (volume) of a product. It’s the same principle at work. Take Calendula Butter Creme with Shea Butter and Avocado Oil. Both ingredients have a Specific Gravity less than 1. Compare that to Rose Water Toner, which has as its main ingredient Rosewater, with a Specific Gravity of about 1. These two products, at 4 oz in volume, will weigh different amounts and this goes back to figuring out how much raw ingredients you will need to produce, say 128 – 4 oz bottles of product. When you are doing your pricing and planning, we recommend getting a “Fill Weight” which is the weight of your product, or one similar to it, in your end packaging. With this Fill Weight, you can begin working backward to understand how much product you will need to make. Need 128 – 4 oz bottles of Rosewater Toner? The math looks like this: 1 (Specific Gravity of Rosewater) X 32 (Weight of 512 oz of Water) = 32 lbs of Rosewater Toner. For Calendula Butter, your math will look like this: 0.90 (Avg. Specific Gravity of Shea and Avocado) X 32 (Weight of 512 oz of Water) = 28.8 lbs of Calendula Butter.
Does it have to be so hard?
It’s can be confusing to get the hang of formulating in weight and then trying to purchase ingredients, especially when they are often sold in volume. You’ll see on our website that we sell some products and ingredients by weight, and others by volume. To make purchasing easier, we will be gradually changing our ingredients to sell by weight rather than volume. Your feedback has been instrumental in helping us decide how to best present your ingredient and product choices.
Cosmetic formulating requires all of us to dust off our chemistry books and refresh ourselves with some of the basic principles behind formulating. As you refamiliarize yourself with these fundamental concepts– and put them to use, you’ll soon see them as old hat and they’ll cease to confound you. At least that’s the dream, right?
As always, we look forward to your questions and your feedback. We’re here to help!