Dear Valerie: I’m a little confused about how to calculate fill weight for a product. Our manufacturer says they need to do it for every new product, but isn’t it just the ounces, turned into grams?
—21 Grams

Dear Grams:

A pycnometer in action.I oversaw quality, regulatory and manufacturing in my previous role, so this is a topic I get excited to discuss because it’s often overlooked. It’s a misconception the US is unregulated when it comes to cosmetics, and things can certainly be made clearer for brands. There is one thing that is spelled out clearly and that is the Fair Packaging and Labeling Act, signed into law in 1966 by President Lyndon B. Johnson. One of the tenets of the law is consumer products must contain the net quantity of contents. Contents must include both US customary and metric units.

If a product is a solid or semi-solid, its measurement is denoted in ounce by weight or grams. This means you can simply tare the product’s packaging on a scale and add the required weight. Just use a calculator to convert ounces to grams or vice versa. Examples of cosmetic products labeled this way include hair pomades in jars, lipsticks and exfoliating scrubs. 

If the product is a liquid or flows, it cannot be measured by weight. It requires a measurement by volume. The units corresponding to this are fluid ounces and milliliters. Unfortunately, you cannot put a product on a scale and have it output fluid ounces or milliliters. Scales can’t read volume because products have different densities. Think of it this way—if you were to fill a one-liter container with sand, it would weigh considerably more than a one-liter container filled with feathers. However, they would take up the same volume or space.

So how is fill weight measured if we must denote net contents in fluid ounces, which is a volume? Your manufacturer is not pouring your product into a graduated cylinder before transferring it to your bottle to figure out if your product fits in there—at least, I hope they’re not…I guess you never know! Rather, they’re measuring the weight of the product, calculating the density or specific gravity, and converting the weight to volume.

Density, a term used interchangeably with specific gravity, is calculated by determining how much something weighs in a predetermined amount of space. In cosmetics, density is reported in grams per milliliter (g/mL). One milliliter of water weighs one gram. Therefore, it has a density of 1. If a product has a lower density than water (< 1.00), it must weigh less than water. If the product has a higher density than water (> 1.00), one milliliter of the product must weigh more than water. For example, as I write this column, I’m eating a delicious cake donut, which is denser than a yeast-raised confection of the same size. I know this because the cake donut is heavier. This is very practical!

More on Density

In the lab, we use a pycnometer or density cup that has a known capacity in milliliters. The cup is placed on a scale with at least two decimal places (more on this, later) and the scale is tared. The cup is filled with product and closed, ensuring the cup is filled. The post-weight is measured. If the density cup can hold 8.32ml of water, the cup should weigh 8.32g when filled. The density is 1.00 because 8.32g/8.32mL is 1.00 g/mL. If the product were a shampoo, the scale might read 8.65g. This would yield a density of 1.04 (8.65g/8.32mL = 1.04).

Now that you have the density, you can calculate how much product will fit a bottle. If your bottle was supposed to be filled with eight fluid ounces, this gets converted to milliliters (assuming a density of 1.00). Using Google, this is 236.58mL. If we rearrange the density equation to be Volume = Weight / Density, we multiply our volume of 236.58 by 1.04 to solve for the unknown, our fill weight. This is 246.04g.

As an aside, there are some other handy ways to gauge if the density will be higher or lower than water. A good rule of thumb is to look at the oil content of a product; since oil is less dense than water, a product with lots of oil will have a lower density than water. For example, a conditioner generally has a density of 0.96-1.00. If the product has surfactants or humectants like glycerin, the density will almost always be higher than 1.00. For example, a shampoo can have a density of up to 1.06 or more. If you ever measure a density of 0.95 and your product is a shampoo, you have a problem!

Earlier, I mentioned it’s important to use a scale with at least two decimal places. I do not recommend using a scale with no or one decimal place. We need a precise measurement and these scales just aren’t significant enough. There are significant weights and measures laws in the US—and around the world. If your product claims to have eight fluid ounces in it, it better have eight fluid ounces in it, and measuring specific gravity with a proper scale is of utmost importance to achieve that!

I’ve been in a situation where product manufactured was underfilled because of an erroneous specific gravity calculation. The product was aerated, and the manufacturer filled the product according to the aerated specific gravity. During shipment, the air collapsed out of the product, resulting in a higher density. If you look back at our density equation, where density = grams/mL we can see if the density increases and the grams stay the same, the volume decreases. We did not hit our volume claim and were fined.

Speaking of weights and measures, preventative maintenance is incredibly important. When was the last time you had your scale calibrated, either by an internal SOP or by a third party? I recommend conducting this at least every month in pre-weigh and quarterly in the lab and keeping a log. If you don’t have a protocol for this, develop one or implement routine scheduling. Consistent measurements are important for consistent batches.

Finding the Right Balance

On the topic of consistent batches, the right benchtop balance is critical. A scale is only as significant as its second-to-last reported number. If you often weigh out small amounts of ingredients, such as FD&C colorants (or hair dyes!) in the lab, a balance with two decimal places isn’t going to cut it. Consider five decimal places and know its significance for four. If you’re in manufacturing, you can either borrow the calibrated analytical balance in the lab or invest in a sturdy two-decimal place balance in pre-weigh that is suitable for lower-weight measurements. Not all scales are accurate or precise in the lower weight ranges as the higher weight ranges and vice versa. A floor scale best suited for 75kg weights might not be ideal for weighing under 5kg. You need more than one scale!

I didn’t want to be the one to tell you this, but since it’s MY column, I must! We’ve been talking a lot about weights, but most manufacturing equipment fills in volume. Before you close this magazine on me, hear me out! The specific gravity is still important because—as you now know—we need a way to verify that the correct volume went into the packaging. The way to do this is by knowing the density of the product and physically checking the weight. In using our hypothetical situation of an 8 fl oz shampoo bottle with a density of 1.04, filled with 246.04g of product; if a bottle comes off the filling line with a net weight of 260g, we are overfilling the bottle. If it only weighs 240g, we are underfilling and violating weights and measures laws. This is often checked by a scale near the filling area and having an SOP in place to check weights every so often, to ensure the filling machine hasn’t deviated depending on the pressure in the lines.

If you’re super lucky, your facility might have an in-line weight checker. These are neat because every filled component is checked for weight after filling. If it’s too under or overweight, the filling machine will kick it out. These can be pricey, but I always say, there is a price and a cost! The cost of having an under-filled product is worse than the price of the scale—throw one on your company’s Christmas wish list to Santa.

One last comment on scales; if you are a business engaged in selling bulk cosmetics or soaps based on weight, like a refill shop where you weigh product out and calculate the sell price based on the weight, you need a scale certified as “legal for trade.”

Looking for more insights on this weighty topic? Check out this eBook, sponsored by Minebea Intec.

Valerie George
[email protected]

Valerie George is a cosmetic chemist, science communicator, educator, leader, and avid proponent of transparency in the beauty industry. She works on the latest research in hair color and hair care at her company, Simply Formulas, and is the co-host of The Beauty Brains podcast. You can find her on Instagram at @cosmetic_chemist or showcasing her favorite ingredients to small brands and home formulators at simply-ingredients.com