The basis of any soap recipe is the oil composition, expressed as percentages of the total oil weight (not the total soap weight).

You could have a 100% olive oil soap (Castile soap), but most soaps today are blends. For example, my Honey Lavender Soap is a blend of:

To scale this into my 3lb oil recipe, I can simple take each of these percentages x 3lbs, or 1361g. (I prefer measuring in grams for precision for soap making.)

This percentage-based structure makes resizing simple and accurate.

Once you have determined your oils, you need to calculate how much lye (sodium hydroxide) is needed to fully saponify them. Each oil requires a different amount of lye to fully saponify. These is really where a good soap calculator shines.

You can also calculate lye by hand, using SAP values. An oil’s SAP value is the measure of grams of lye needed to fully saponify one gram of oil.

Sticking with the Honey Lavender soap recipe example, if we add up the lye needed to fully saponify all the oils in that recipe, we get 197.89 grams, rounded to 198 g lye. Next, we will make a lye discount (explained in next section).

Superfat, or “Lye Discount” is an intentional reduction in the amount of lye so that a small portion of oil remains unsaponified in the final soap.
A common superfat amount is 5%.

This is default in almost all homemade soap recipes because it allows a small margin for errors in lye calculation or measurement (when you start looking into SAP values, you may find that different sources cite slightly different SAP values). Superfat also creates a more conditioning and luxurious-feeling bar.

High superfats are sometimes used to offset oils that can feel more cleansing or stripping. For example, 100% coconut oil soap is often superfatted at 20% to make it milder on the skin.

Continuing with our honey lavender soap example, let’s give it a 5% superfat:

lye weight × (100% – [superfat %]) = superfatted lye weight

198 g lye × 95% = 188.1 g lye (5% superfat)

Lye must be dissolved in liquid, usually distilled water, before being mixed with oils. This mixture is called your lye solution. There are three common ways to describe water amounts:

1. Water as a Percentage of Oil Weight
   Some soap calculators express water as a percentage of the total oil weight rather than as a lye concentration or water-to-lye ratio. In this method, the water amount is calculated based on how much oil is in the recipe. In my formulations, this number typically falls around 28%. While this notation is common, I prefer thinking in terms of lye concentration, since water’s primary role is to dissolve lye.
   
2. Lye Solution Percentage
   This simply expresses how much of your solution is lye. Most of my recipes are 33% lye concentration, or 2:1. The absolute minimum is 50%, but this is NOT recommended for beginners, as it will harden quickly.
   (188.1 g lye / 0.33 = 564.86 g solution) – 188.1 g lye = 376.8 g water
   
3. Water-to-Lye Ratio
   A good starting point is 2:1 water-to-lye. This is roughly 33% lye concentration, which provides a comfortable working time and reliability in most recipes. The absolute minimum is 1:1, but this is NOT recommended for beginners as it will harden quickly.
   188.1 g lye x [2 water : 1 lye] = 376.2 g water

Fragrance is calculated as a percentage of oil weight, not total batch weight. This is because most of the water eventually evaporates during cure, so oil weight more accurately reflects the final bar. A common fragrance range in cold process soap is 3% to 5% of oil weight. (When I say fragrance, I mean essential oils, I never use artificial fragrances.)

I tend to stay towards the lower end of that range to make sure that my soaps are suitable for facial use and sensitive skin. If you want to make a baby soap, I recommend either leaving essential oil out entirely or decreasing to 1%. Each fragrance has its own safe usage rate, consult the supplier’s IFRA documentation to confirm safe amounts.

For example, in my 1361g oil batch for the Honey Lavender Soap:

0.03 × 1361 g oil = 41 g lavender essential oil

Beyond oils and lye, small additions can enhance hardness, lather, color, and skin benefits. Here are a few common additives with guidelines per one pound of oils. I recommend limiting enhancers to a maximum of two, especially because you are probably using essential oils too.

• Natural Powders: Roots (alkanet, madder, turmeric, beet), herbs (nettle), clays (pink/green french clay, kaolin, bentonite), spices (cinnamon, cloves).
  • Used for coloring, scent, and skin benefits.
  • Use 1/2 to 1 tsp per pound of oils. Stir in a small amount of oil before mixing into the rest of the batch to mitigate clumping.
  • Be careful of spices as they can be irritating for sensitive skin.
• Honey and Beeswax:
  • Honey boosts lather and acts as a natural humectant.
  • Use 1/2 to 1 tsp per pound of oils. Stir into cooled lye solution before mixing with oils.
  • Beeswax increases bar hardness. This should be calculated with the base oils, and should generally stay below 3% of the oil weight.
• Sodium Lactate: This hardens bars and can be especially helpful for bars shaped in silicone molds. Mix in one tsp per pound of oil once the lye solution has cooled before it is stirred with the oils.

• Olive Oil – A gentle, conditioning oil that produces a mild bar with a creamy (rather than bubbly) lather. Ideal for sensitive or dry skin and often used at higher percentages. Contributes to hardness over a longer cure time.
• Coconut Oil – Adds hardness and abundant bubbly lather. Highly cleansing, so it is typically balanced with conditioning oils or superfatted to prevent dryness. Commonly used at 15–30%.
• Shea Butter – Contributes creaminess, conditioning properties, and a silky skin feel. Adds firmness and longevity to the bar. Often used at 5–15%.
• Cocoa Butter – A hard butter that produces a firm, long-lasting bar with a dense, creamy lather and a smooth, protective feel on the skin.
• Beeswax – Increases bar hardness and longevity. Used in small amounts (1–3%), as higher percentages can reduce lather and accelerate trace.
• Sweet Almond Oil – A lightweight, conditioning oil that adds mildness and softness to a recipe. Helps balance more cleansing oils.
• Castor Oil – A powerful lather booster that increases and stabilizes bubbles. Typically used at 3–8% to prevent softness in the final bar.
• Jojoba Oil – Technically a liquid wax, it adds a silky, conditioning feel and has excellent shelf stability. Often used at lower percentages due to cost.
• Argan Oil – A luxury oil that enhances conditioning and contributes a smooth, nourishing skin feel. Best used in smaller amounts.
• Tallow – Produces a hard, long-lasting bar with creamy, stable lather. Traditional, reliable, and excellent for balanced formulations.
• Lard – Creates a mild, creamy, conditioning bar with good hardness and longevity. Beginner-friendly and very stable.
• Avocado Oil – A rich, conditioning oil that supports a gentle, moisturizing bar and contributes to creamy lather.