Soap Calculator — Free Online Cold Process Lye Calculator

Superfat is the percentage of extra oil left unconverted in your soap. A 5% superfat means 5% of the oils won't react with lye, leaving a more moisturizing bar. It also provides a safety margin in case of measurement errors.

NaOH (sodium hydroxide) makes solid bar soap. KOH (potassium hydroxide) makes liquid soap. KOH requires about 40% more weight than NaOH for the same recipe because it's less potent.

Cold process soap should cure for 4-6 weeks. This allows water to evaporate, making a harder, longer-lasting bar. The lye continues to saponify during this time. Liquid soap (KOH) typically cures in 1-2 weeks.

The water:lye ratio determines how much water is used to dissolve the lye. A 2.5:1 ratio means 2.5g water per 1g of lye. Lower ratios (2:1) speed up trace and reduce cure time but are harder to work with. Higher ratios (3:1) give more working time but need longer to cure.

Trace is the point where the lye and oils have emulsified and the mixture thickens to a pudding-like consistency. "Light trace" is like thin custard — good for swirl designs. "Heavy trace" is like thick pudding — better for layering. After trace, the soap can be poured into molds.

Coconut oil makes hard, bubbly soap but can be drying above 30% of your recipe. At 15-30% it produces excellent lather without stripping skin. Some makers go up to 40% for laundry soap bars but this is too drying for most skin types.

INS is a number calculated from iodine value and SAP value that indicates overall soap quality. An INS of 136-170 is ideal. Below 136 may produce a soft bar. Above 170 may be brittle. It's a guideline, not a rule — many great soaps fall outside this range.

Yes. The lye and oil calculations are identical for hot process and cold process. The difference is in technique — hot process cooks the soap after trace using a slow cooker or oven. The lye amount doesn't change, only the method.

For hardness, use: coconut oil, palm oil, cocoa butter, babassu oil, tallow, or lard. A combination of coconut (15-25%) + palm or tallow (25-40%) + a soft oil like olive (30-50%) makes a well-balanced, long-lasting bar.

Saponification is the chemical reaction between lye (NaOH or KOH) and oils/fats that produces soap and glycerin. When all the lye has reacted with the oils, the soap is "fully saponified." The superfat ensures there's leftover oil after this reaction completes.

Castor oil is unique — it's the only common oil that boosts both bubbly and creamy lather. Even at 3-8% of your recipe, it dramatically improves lather stability and bubble size. It's also very conditioning. However, it can make soap sticky if used above 10%.

The iodine value measures how unsaturated an oil is. For soap, an iodine range of 41-70 is recommended. High iodine (above 70) can produce a soft bar that may go rancid quickly. Low iodine (below 41) creates a very hard bar. Hemp seed and sunflower oils are high-iodine — use them sparingly.

If your soap is lye-heavy (too much lye or too little oil), it will feel zappy on your tongue and may cause irritation. Unfortunately, there's no fix — the batch must be discarded safely. This is why superfat is important: it provides a safety buffer against small measurement errors.

You can substitute oils but you must recalculate the lye amount. Each oil has a different SAP value (the amount of lye needed to saponify it). Simply swapping oils without recalculating will result in too much or too little lye. Use this calculator to find the correct amount for your new recipe.