Soap Calculator

Build a soap recipe from common oils and butters. The calculator works out the sodium hydroxide (or potassium hydroxide) and water you need, the fragrance amount, the total batch weight and the estimated qualities of the finished bar.

How the soap calculator works

Every oil and butter needs a specific amount of lye to turn completely into soap. That amount is its saponification value, or SAP value: the grams of sodium hydroxide needed for one gram of that oil. Coconut oil, for example, needs about 0.183 g of NaOH per gram, while olive oil needs about 0.135 g. The calculator multiplies each oil's weight by its SAP value, adds them up, and then reduces the total by your superfat percentage so a little oil stays unsaponified in the finished bar.

1. Lye for each oil = oil weight × SAP value (NaOH).
2. Total lye = sum of all oils, then × (1 − superfat).
3. For potassium hydroxide, multiply the NaOH amount by 1.403 (the ratio of their molecular weights); for 90% pure KOH, divide by 0.9.
4. Water is set from your chosen method: lye concentration, water-to-lye ratio or water as a percentage of oils.
5. Fragrance is a percentage of the oil weight.

Always weigh every ingredient, including water, on a digital scale. Volume measures are not accurate enough for soap.

Choosing a superfat

Superfat is the percentage of oils left over after the lye is used up. It protects against measuring errors and makes the bar milder. Five percent is the most common choice for body bars. Salt bars and pure coconut oil bars often use 15–20% because coconut oil is very cleansing. Laundry soap is made with 0–1% superfat so it cleans without leaving oil behind. Liquid soap made with KOH is usually made with a low superfat, around 1–3%, because excess oil can make the finished soap cloudy.

Water and lye concentration

Water dissolves the lye and carries it into the oils; most of it evaporates during cure. The same amount of lye can be dissolved in more or less water.

Beginners usually start around 33% concentration (roughly 2:1). A “water discount” — using less water — suits recipes with lots of soft oils or intricate swirls where you want more time before trace, only if you understand the trade-offs.

Understanding the bar quality numbers

The quality numbers are estimates built from each oil's fatty acids. They help compare recipes, not predict exactly how a bar will feel.

The ranges shown are the typical values many soapmakers aim for. A recipe outside a range is not wrong — castile soap made from 100% olive oil is famously low in hardness and cleansing and still makes a good, mild bar after a long cure.

Worked example

The starting recipe is 1,000 g of oils: 40% olive, 25% coconut, 25% palm, 5% castor and 5% shea. The oils need about 145 g of NaOH at full saponification; with a 5% superfat that becomes about 138 g. At 33% lye concentration the water is about 280 g. Three percent fragrance adds 30 g, for a batch of about 1,448 g before cure. The hardness comes out in the mid-40s and conditioning in the mid-50s, both inside the usual ranges — a balanced, beginner-friendly bar.

Safety with lye

• Wear splash-proof goggles and chemical-resistant gloves; keep sleeves long.
• Always add lye to water, never water to lye, in a heat-resistant plastic (#5 PP) or stainless steel container.
• Mix in a ventilated area and avoid breathing the fumes.
• Keep children and pets away and label lye containers clearly.
• Do not use aluminium tools — lye reacts with aluminium.
• Cure cold-process bars for about four to six weeks before use.

Why you should double-check any recipe

SAP values differ slightly between suppliers and references, and a typing error in a recipe can make a harsh bar. Recheck the oil names and weights before you mix, run the recipe through a second calculator if it is new to you, and make a small test batch before scaling up. This calculator gives estimates for hobby use and does not replace product safety testing or regulatory requirements if you sell soap.