Soap Making Calculators

Cold process soap making has a dedicated following in the UK, with makers crafting artisan bars from kitchen workshops and selling at farmers markets, craft fairs and through online shops. The chemistry behind soap making requires precise calculations — too much sodium hydroxide (lye) and the soap will be harsh and potentially caustic; too little and the bars will be soft and greasy. Our lye calculator handles both NaOH for bar soap and KOH for liquid soap, working from your total oil weight in grams. The superfat calculator then adjusts the lye amount downward to leave a percentage of unsaponified oils in the finished bar, typically between 5 and 8 percent for a moisturising feel.

Building a balanced soap recipe means choosing oils that contribute different properties. Coconut oil adds hardness and lather, olive oil brings conditioning, and castor oil boosts bubbles. Our oil ratio calculator helps you balance these qualities, while the SAP value calculator provides the saponification value for each oil so your lye amount is accurate. The water discount calculator lets you reduce water below the standard amount (typically 38 percent of oil weight), which can speed up unmoulding time and reduce the chance of soda ash — a common cosmetic issue for UK makers working in cooler, humid conditions.

If you sell soap in the UK, your products must comply with UK Cosmetic Regulations (retained from the EU Cosmetics Regulation). This requires a Cosmetic Product Safety Report (CPSR) for each formulation, proper CLP labelling for any products classified as hazardous, and notification on the UK SCPN portal. Our fragrance load calculator ensures your essential oil or fragrance oil amounts stay within IFRA guidelines, and the soap cost calculator helps you work out the true cost per bar — including lye, oils, fragrance, colourants, and CPSR costs spread across your batch. The batch size calculator makes it easy to scale recipes up or down while keeping every ratio consistent, and the cure time calculator estimates the four to six week waiting period based on your recipe and curing environment.