Calculating the Net Weight of Soap (Part 1)

When you are labeling soap, one of the problems is figuring the net weight to put on the label. Since water will continue to evaporate out of the soap making over time it will gradually weigh less, even after the initial “cure time”. The net weight stated on the label may NOT be more than the actual weight of the product when the consumer is deciding whether to purchase.

There are a couple of ways to make sure that the net weight stated on the label is less than (or equal to) the actual weight of the bar.

Factors to Consider

When selecting a method to calculate your final net weight after time, there are several factors to consider.

How long will it be between the time your soap is packaged and the time it is sold?  The shorter the time between packaging and selling, the less change there will be in the weight.

Do you know the environment in which it will be stored between packaging and sale?  Will it be in a consistent temperature and humidity? When the environment is stable it is much easier to predict how it will affect the soap.

Second, what environment will the soap be in after it is packaged?  If you know that you packaged soap will always be in your shop with a controlled environment, then you can predict the rate of water evaporation (continued drying) over time.  If you sell wholesale and the soap will be in a store, or if the temperature and/or humidity changes in where you store the soap, then it is harder to predict what the drying rate will be.

Soap is sold immediately

If the soap is to be sold immediately (within a few days) of when it is packaged and labeled with the weight, the net weight is unlikely to change significantly.  To be on the safe side, you might want to round down a bit (just to cover possible changes during the day if you are outside or in the sun).

Stored in a stable environment

If you will be storing the finished soap  in a predictable and stable environment, you can calculate the drying rate over time. This method takes a little more time to set up because you have to measure the weights over time and determine the weight loss.
Once you’ve made the calculations, you can use them over and over to calculate the future weight of your soap.

How to Calculate the Weight Loss Percentage

Yes, it’s math, but it’s not too hard if you take it step by step.

First, the basic formula:

(1-{c-p}/o) * 100 = wl%
Where:
c is defined as the current weight
p is defined as the packaging weight
o is defined as the original weight
wl is defined as the weight loss

How to use the formula

Here’s an example of the step-by-step process to use the formula. Of course, you’ll need to use the actual numbers for your soap.

Step One
The first step is to plug in the numbers
c = current weight = 4.6 oz
p = packaging weight = .1 oz
0 = packaging weight = 5.0 oz
(1-{4.6-.1}/5.0) * 100 = wl%

Step Two
Then do the calculations, one at a time:

A. (1-4.5/5.0) * 100 = wl%

B. (1-.9) * 100 = wl%

C.  (.1) * 100 = wl%

D. .1 * 100 = 10%

Weight loss to that point in time is 10%.

Calculating Changes Over Time

In order to predict what the the weight of your soap will be in 30, 60, 90 or more days, you need to make the calculations over time (and keep records).

Step by Step

  1. Immediately after you unmold and trim a batch of soap, select several (2 – 4) bars to be your “test weight” bars.  Record the weight of each and then dry (cure) them as usual, with the rest of the bars in the batch.
  2. After 30 days (or one month), weigh each test bar again.  Record the weight and calculate the percentage of weight loss for each bar using the formula above.
  3. Calculate the average weight loss (add the weight loss of all the bars and then divide by the number of bars).
  4. When you are ready to package the soap, weigh each test bar again (before packaging).  Record the number of days, individual weight loss and  average weight loss.
  5. Package the test bars in the same way you package the other bars in the batch, and weigh them again.  Record the weight each time. The weight of the packaged soap less the weight of the unpackaged soap gives you the weight of the packaging. Keeping track of the time and weight of the packaging is important because it will probably affect the rate of weight loss.
  6. After 60 days (or 2 months) from the original weighing date, weigh the test bars again.  Record the weight LESS the weight of the packaging.  Calculate the percentage of weight lost for each bar, and the average between all bars.
  7. Record the weight and calculate the percentage of weight lost of the test bars every month until all the bars from the batch are used or sold.
  8. If it might be 90 or more days before your soap is sold, continue to weigh the test bars every 30 days (1 month) for as long as they might be held before being sold.

At the end, you should have a chart that looks something like this:

What to put on the label

Once you know the percentage of weight loss over time, you can figure out what to put on the label using this formula:

o-(o*wl)=label weight

If the soap will sell in not more than 60 days, then take the weight of the bar when first cut and trimmed (the original weight) and multiply it by the average weight loss in 60 days and deduct that from the total.  If it will be sold in 90 days, use the weight loss for 90 days.

Example:  Using the chart above, for a soap being sold in 60 days:

5.0-(5.0*12.52%)=label weight

5.0-.6886=4.8114

Round down, and put 4.8 oz net weight on the label.

Different Soaps, Different Calculations

If you make soaps with different formulations – particularly if they have different percentages of water or have bulk additives (such as oatmeal or herbs), they will have different drying rates.  You’ll need to do the calculations for each type of soap individually.

The good news is that, provided the formulation and environment don’t change much, you can continue to use the calculations over and over again.

In Part 2

In part 2 of this series, we’ll discuss how to calculate the final weight of the soap based on the soap formulation itself, without having to worry about taking measurements over time.

Comments

  1. ditto that! soap weight loss (and gain!) fascinates me and it’s reassuring, if not so fun, to know that you just have to collect the data.

  2. thanks marie….this was a very very helpful piece of info …thanks so much for sharing it with us 🙂

  3. Hi there. I see “80g when wet, 68g when dry” in a soap box. So what is the net weight of that soap?

    1. Author

      In the US there is a specific requirement that the net contents may not include any qualifying statement(s). For example, you can say “not less than XX oz” or “approximately XX oz”.

      If the example you gave was on a soap package in the US, it would be in violation of the labeling requirements. In the US, it would have to be NOT MORE than the weight of the product when it is looked at or purchased by the consumer. (How are they supposed to know if the soap is wet or dry …. or where in between?)

      Since the stated weight can’t be more than the actual weight, I recommend determining the lowest possible weight the soap might have, assuming evaporation over time, and then using that weight on the labeling, as explained in the article.

  4. Oh thank you!!!!!!! I’ve been killing my self over this. And the maths makes it even better. Thank you!!!! AWESOME POST

    1. Author

      The math is basically the same – just change from oz to grams.

  5. Can you source the labeling code you are referencing with regard to soap weight? It may have changed.

    When I search the FDA regarding this for cosmetic labeling I read, “Reasonable variations due to loss or gain of moisture, or deviations in good manufacturing practice, are acceptable.”

    This is found under accuracy at https://www.fda.gov/cosmetics/labeling/regulations/ucm126444.htm
    Last updated 12/23/2016

    1. Author

      Yes, that’s true. There are also statements elsewhere (can’t find on the FDA site at the moment) that the US is on a “minimum” net weight system, and the net weight, even though it can vary, cannot be LESS than is stated on the label. I am querying the FDA to get an “official” response.

  6. Hi Marie. Is it OK to leave the net weight amount blank and then fill it in by hand at the time of sale? Thank you.

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