Calculating the Net Weight of Soap (Part 2)

In Part 1 of this series, we discussed how to calculate the net weight of your soap over time by weighing actual bars and seeing what happened to them. That works if you know where your soap will be and can rely on the fact that the environment won’t change much.

If your soap may be in a retail shop somewhere, or if the environment in which you store it could change (due to weather or time of year, for example), then it won’t work to use what has happened  to the soap to predict what will happen to the soap.  In that case, you need a different way to calculate the net weight.

Weight Loss from Water Evaporation

Soaps get lighter and smaller over time primarily due to water evaporation.

While some volatiles (such as essential oils) can also evaporate, the bulk of the weight loss is from a reduction of the amount of water remaining in the soap.

In this method of calculating the net weight over time, we determine what remains in the soap when all, or nearly all, of the water is gone.

Water Percentage

The first thing to calculate is the percentage of water in your soap formulation – by weight. In order for this to work, it’s important that you know the actual weight of every ingredient that goes into the soap.

  1. Add up the weight of all the ingredients in the batch, including the water.
  2. Divide the weight of the water in the batch by the total weight of the batch.

That gives you the percentage of water in your batch (by weight).  (It will be an amount less than 0 by decimal point; multiply by 100 to get the percentage.)

Example:

Total Water Evaporation

That calculation tells you  how much lighter the soap would bethat if ALL the water evaporated out of it.  In the example above, each bar of soap from that batch would weigh 21% less than the original weight (when first made).

Using that number, you can determine the the lowest weight your soap could possibly reach.

Granted, it’s unlikely that every little bit of water will evaporate out of the soap before it’s sold, but it could.  I have a bar of coconut soap from the 40’s, and it is dry as a bone.  I doubt there is any water at all in it … but then, that soap is over 60 years old and made with coconut oil only, so it never went rancid.

Determining the Net Weight for the Label

  1. Take a bar of soap (or several, to get an average) and weigh each right after you unmold and trim (before any drying or curing time).
  2. Multiply that by the percentage of water to get  the amount of water in the bar. If your calculator doesn’t have a percentage function, convert the percentage to decimal for the calculation (i.e. 21% = .21)
  3. Subtract the water amount from the total weight.

That gives you the lowest possible weight for the soap.

Example:  

5 oz bar of soap with 21% water

5.0-(5.0*.21) = net

5.0-(1.05) = 3.95

Best Practice

The absolutely safest way to label your soap is to use that weight for the net weight on the label.

Making Adjustments

If you think that your soap will sell before all the water is evaporated out, you could adjust up a little in the net weight, but realize that you are making guess, albeit hopefully an educated guess, but don’t get carried away.

If you have also calculated the weight over time (as given in Part 1 of this series) you have some real numbers to work with, although they will change based on the environment the soap is in. For example, if those calculations showed that in 90 days the weight would be around 13% less, and the total water is 20%, you might adjust your net weight to 17% or 18% less than the original weight (rather than the full 20%).

Net Weight Rules

Keep in mind that the net weight stated on the product label can never be more than the actual net weight of the product, but it can be less.  In other words, the consumer can get more than they expect, but never less than they expect (based on what’s on the label).

The safest bet is to go with the lowest possible net weight, if you have any question about where the soap will be or how long it will be there before it is sold.

Comments

  1. I love these blog posts. Trying to guess what a bar of soap will be at the point of sale is so frustrating – but these calculations take the guess work out. It’s also a great reminder that, if anyone decided to weigh a bar of soap, they would be happy to realize it’s .5 oz heavier than it says on the label – but pretty upset if it was half an ounce lighter!

    Are there rules to how MUCH the difference can be? For example, you can’t mark a 1 pound bar of soap as being only 5 ounces, correct?

  2. Great post! Thank you! My husband and I were just discussing this last night as to how we were going to determine the weights accurately.

  3. Marie, I’m glad I read this post. I read part 1, and my head was swirling. The way I decided to do my weight calculations, and what made “common sense” to me was very similar to this–except I did not take into account the weight of the lye. So, I have a big THANK YOU to tell you–I just got a small percentage raise in my prices because I’ll now be calculating in the lye weight. So happy common sense did not fail me! Thank you for all your expert advise that we soapmakers can follow to ensure that we are in compliance of the rules, regulations, and laws that govern our industry and that, indubidably, crank our brains. You are a treasure to all of us!

    1. Author

      Glad it was of help!!

      (And thanks for the very nice compliments!)

  4. Thanks a lot!
    May you please help.
    What can be added to increase the net weight of the soap bar?
    Thanks

    1. Author

      Many different additives will change the net weight of the soap bar; anything that weighs more than soap will increase the weight of the soap bar.

      Materials that are used just to raise the weight cheaply (traditionally called “fillers”) are not normally used in handcrafted soap.

      1. Your post is incorrect. It doesn’t matter whether something weighs more than the soap or not, if you add it, then the weight of the product is going to change. I think what you meant was “density”, not “weight”. Of course, if you are talking about a fixed size bar that does not vary regardless of the ingredients that you are putting in your product, then you are talking about density whether you know it or not.

  5. Thank you for the information!
    I notice people include the weight of their packaging, is that standard practice?

    1. Author

      The weight on the package should be the NET weight – that is, the weight of the product only, not including any packaging.

  6. When creating a larger bar (say 7 oz) does the moisture in it evaporate more slowly than say a 4 oz bar of soap due to a thicker bar?

    1. Author

      I would expect so — but you should probably test it to see.

    2. Water evaporation should be based on surface area, so if your soap was a sphere instead of a bar, then the water would evaporate from it slower.

    3. The evaporation rate is going to be based on the ratio of the weight of the bar to the surface area. If the length and width are the same and only the thickness changes, then the water is going to evaporate slower for the thicker bar. Water would evaporate from a cube shaped bar slower than a traditional rectangular shaped bar and from a spherical bar even slower. Cylindrical shaped bars would be better than rectangular shaped bars for the most part, but that is still going to depend upon the thickness. One illustration of this that you might be familiar with is when you make dough for tortillas. This consists of just flour and water (and maybe a bit of salt for taste). After you have mixed the ingredients, if you place the ball of dough over to the side for awhile, you might notice a slight bit of a dry crust developing after an hour or so. On the other hand, if you flatten it out to tortilla thickness and let it sit for that same amount of time, it is going to be a lot more dried out.

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