I’m a big advocate for the transition of the bath bomb world from volumetric measurements to weight. C’mon people! It’s about time we get a little scientific with this stuff! If we want reliable, repeatable results with the elimination of as many variables as possible, then weighing ingredients is a MUST! 

There are already so many factors that can cause a batch of bath bombs to fail. Things like humidity, fragrance oils, and even the same ingredients from different suppliers can have a huge impact on how successful your bath bombs are. Sitting around and wondering if my idea of a cup is the same as your idea of a cup isn’t what I want to keep me up at night, and yet it totally would. Do you mean a level cup or a heaping cup? A sifted cup or a packed cup? Ack!

If you’ve found a recipe for bath bombs online it is likely that the ingredients are listed by volume, not weight. If you’ve tried the recipe and like it (or if others have suggested trying it) and you want to convert it to weight, here’s how to go about doing that! We’ll use Amanda’s Basic Bath Bomb Recipe as a reference and see how closely my numbers end up matching hers!

Her recipe is listed with measurements for both volume and weight, so we’ll ignore the weights for now and see what happens when I only use the volumes called for!

  • Baking Soda – 1 cup
  • Citric Acid – ½ cup
  • Cream of Tartar – 1 tablespoon
  • Kaolin Clay – 1 tablespoon
  • Cornstarch, Tapioca Starch or Arrowroot Powder – 1 tablespoon
  • Avocado Oil – 1 teaspoon
  • Polysorbate 80 – 1 teaspoon

To convert these I’ll first measure every ingredient by volume 3 times into an empty container on a scale tared to zero and come up with their average weight in grams. The more times you measure and weigh the more accurate your recipe will be, but 3 times seems pretty solid for this little experiment. 

Here is how my measurements looked when everything was said and done

  • Baking Soda – 248g, 245g, 240g
  • Citric Acid – 117g, 113g, 118g
  • Cream of Tartar – 14g, 15g, 16g
  • Kaolin Clay – 9g, 10g, 8g
  • Cornstarch – 9g 8g, 8g
  • Avocado Oil – 4g, 3g, 4g
  • Polysorbate 80 – 5g, 5g, 6g

(I averaged them out and rounded up or down as needed!)

Here’s why all of this is so important! In the blog post with Amanda’s Basic Bath Bomb Recipe she gives measurements by both volume and weight! While some of the measurements are close, there is still a ton of deviation between the two. Look how different our weights are from each other!

  • Baking Soda – 1 cup (250 grams)
  • Citric Acid – ½ cup (100 grams)
  • Cream of Tartar – 1 tablespoon (12 grams)
  • Kaolin Clay – 1 tablespoon (3 grams)
  • Cornstarch – 1 tablespoon (10 grams)
  • Avocado Oil – 1 teaspoon (5 grams)
  • Polysorbate 80 – 1 teaspoon (5 grams)
Baking Soda250 grams244 grams
Citric Acid100 grams116 grams
Cream of Tartar12 grams15 grams
Kaolin Clay3 grams9 grams
Cornstarch10 grams8 grams
Avocado Oil5 grams4 grams
Polysorbate 805 grams4 grams

While this could be viewed as our weights being different, the difference really lies in our volumetric measurements! Afterall, 100 grams is always 100 grams! It’s the variance in how we each measure our ingredients that is causing the discrepancies (in particular the kaolin clay! I must be pretty heavy handed with mine!). In a small recipe this might be negligible, but the larger this recipe is scaled up, the wider the gap between volume and weight would grow.  

This is why you can find a recipe online that promises perfect bath bombs–and they might in fact be perfect for the person giving the recipe away–and yet it still might not work for you. As I said before, there are so many factors that go into why a bath bomb might or might not work from one day to the next. Narrowing down and eliminating as many variables as possible is key! 

If you already have a recipe that you love and works for you, take a minute and convert it to weight! It makes scaling it up or down so much easier, and allows you to quickly calculate the percentages of ingredients being used. If you’ve found a volumetric recipe you want to test, try weighing it out! Once you’ve done that, it makes tweaking your recipe a much more pleasant task! 

Happy Making!!!