Soap Recipe Calculator

Cold-process soap is a chemical reaction: oils plus a strong alkali (NaOH for bar soap, KOH for liquid soap) become soap and glycerine. Each oil has its own SAP (saponification) value that tells you exactly how much lye is needed to fully react with 1g of that oil. Coconut oil's SAP is 0.186 (NaOH); olive oil is 0.134; shea butter is 0.128; castor is 0.129. The calculator multiplies each oil's weight by its SAP value, sums the totals, then applies your superfat and lye-to-water ratio.

Skipping the calculator and using someone else's recipe with different oils is how soap-makers end up with bars that are either harsh and lye-heavy (causing skin burns) or soft and oil-heavy (causing rancidity). The numbers matter to within 1g for a 500g batch. Even the same oil from a different supplier can shift the SAP by 1-2% depending on origin and processing, but for hobby and small-batch commercial work, the standard SAP table the calculator uses is accurate enough. Always weigh, never measure by volume.

Superfat is the percentage of oils left unreacted in the finished bar. A 0% superfat means every gram of oil is converted to soap; a 5% superfat (the calculator's default) means 5% of the oils survive unreacted and remain in the bar as moisturising free oils. This is the buffer that makes the bar gentle on skin and accounts for the small variations in real-world SAP values. Below 3% superfat, the bar can feel drying or even slightly caustic on sensitive skin.

Standard ranges: 5% superfat for bar soap, 1-3% for liquid soap (KOH) because higher superfat in liquid soap leaves cloudy unreacted oil, 8-10% for facial bars where extra mildness matters, up to 15% for shaving soaps. The calculator applies the superfat as a discount on the total lye amount: at 5% superfat, you use 95% of the calculated lye, leaving 5% of the oils unreacted. Don't go above 8% for general body bars or the bar gets soft and rancid quickly.

Water dissolves the lye so it can react with the oils. Less water means a thicker, faster-reacting lye solution and faster trace; more water means a slower, gentler reaction. The calculator's default 2:1 water-to-lye ratio (e.g. 100g lye in 200g water) is the textbook standard. Master soapers use 1.5:1 for water discount (faster trace, less unmoulding wait), or 2.5:1 to 3:1 for cold-pour designs where you want extended workability.

The trade-off: less water makes a bar that unmoulds in 12-24 hours and cures in 4 weeks. More water makes a bar that needs 48-72 hours to unmould safely and cures in 6-8 weeks because the extra water has to evaporate. Beginners should stick with 2:1 until they've made 10+ batches. Advanced techniques like swirls, embeds and intricate designs benefit from a 2.5:1 ratio for the extra workable time. The calculator shows you the water amount in grams once you set the ratio.

NaOH (sodium hydroxide) makes solid bar soap; KOH (potassium hydroxide) makes liquid soap (or pastes that dilute into liquid soap). The chemistry is the same - oils react with the alkali to form soap - but the molecular weights are different (NaOH is 40.0, KOH is 56.1), so the SAP values are different and you'd never use the same recipe for both. The calculator switches all the maths automatically when you change the lye type dropdown.

Practical implications: solid bar work is more forgiving for beginners because the soap is easy to mould, unmould, slice and cure. Liquid soap requires a heated cook (60-80°C for 2-4 hours), pastes that look alarming during cooking, and a dilution stage where you add water to the cooled paste over hours or days. Liquid soap also needs a different storage approach - it ferments and cloudies if water content is too high. If you're starting out, do 5-10 NaOH bar batches before attempting your first KOH liquid batch. For pricing your finished bars to sell, see the [candle cost calculator](/candle-cost-calculator) - the costing approach transfers.