Why and how to use a soap calculator

Why would you use a soap calculator? A soap calculator is there to make your life a lot easier!

You need a specific quantity of lye to saponify one gram of oil or butter, this is called the saponification value (SAP). The Saponification Value number represents the number of milligrams of Sodium Hydroxide required to saponify 1g of fat under the conditions specified. Each oil and butter has a different saponification value which you need to know to calculate the quantity of lye needed for your formula. You will need to ensure your calculations are correct when scaling the formula up or down if you want to increase of decrease the quantity of soap you want to make. You will also need to recalculate the quantity of ingredients when changing a formula (when you add or remove oils or butters). To make things a lot easier you can use a soap calculator to calculate:

  • The quantity of oils and butters you need for your formula;
  • The quantity of lye necessary to saponify the oils and butters you chose. A soap calculator will make life much easier for you by calculating the quantities instantly;
  • How many grams of water you need to use for your formula if you use the standard 38% water of oils or you can adjust the water percentage for a water discount (you can use less than 38% but you should not use less then 25%);
  • How much fragrance or essential oil you need to use for your formula (note: you can also make non-fragranced soap for people with sensitive skin, skin issues such as eczema or if you just prefer non-fragranced soap);
  • The superfat you want to use. Standard is 5% but you can increase or decrease it. The higher the super fat, the more moisturising the soap will be. However it’s not recommended to go higher than 10-12% as it might make your soap unstable due to the high quantity of unsaponified oils and butters. Exception makes salt soap made with 100% coconut oil for which you can use even an 20% super-fat. The salt in the soap will make the bar of soap very hard and stable so using a 20% super-fat will not be an issue.

There are several soap calculators you can use, such as SoapCalc, Soap Queen or Brambleberry. To learn how to use SoapCalc please see the below demonstration.

2 thoughts on “Why and how to use a soap calculator”

  1. I dont understand how I should know if I need a water discount or if I stick to the standard 38%? What does a water discount do?

    • Hi Sophie, all the information you need is in the Module 7, Unit 4 How to apply a water discount. If you want your soap to reach trace and harden faster you can apply a water discount. This means you decrease the percentage of water as of oils used to suit you.

      You don’t need a water discount per se if you are happy with how the soap performs when you make it with 38% water, but if as mentioned above you want your soap to reach trace faster, and also have a slightly harder bar then you can apply a water discount. If you are a beginner, or want to make complicated swirls, you should keep the water as of oil percentage between 34 – 38 %. As you get more experienced with soap making you can decrease the percentage of water as of oils to help your soap reach trace faster and to also have a slightly harder bar. Using less water when making your soap (less than 38%) means there is less water to evaporate during the curing period and the soap will be slightly harder.

      When starting to experiment with water discounts start by dropping from 38% water as of oils to 35% or 30% and see how the soap looks and feels. Always re-calculate your formula in soap calc to see how much water you need to use for your formula!


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