Best oils for cold process soap
A good bar needs a hard fat, a soft fat and a lather booster. Everything past that is refinement — here is what each oil does and where to stop.
Four oils make a genuinely good bar: coconut for lather and hardness, olive as the conditioning base, palm or tallow for structure, and castor to hold the bubbles together. A standard beginner blend of 40% olive oil, 30% coconut oil (76°), 25% palm oil, 5% castor oil covers all four and needs 143 g of lye at the standard superfat.
Soap recipes get complicated faster than they get better. Beginners are routinely sold five-oil blends with avocado and sweet almond in them, and the bar that comes out is not meaningfully different from one made with four cheap oils in the right proportions.
What a bar actually needs is three jobs filled. Something hard and bubbly, which is nearly always coconut. Something soft and conditioning, which is nearly always olive. Something to give the bar structure and longevity — palm, or tallow if you are not making it vegan. Castor is the fourth because a small amount changes the lather out of proportion to its cost.
The percentages below are ceilings rather than targets, and they are the same ones the lye calculator warns against exceeding. Coconut past 30% strips skin; castor past 10% makes a sticky bar. Those two limits explain most disappointing first batches.
The oils worth buying first
Saponification values are grams of sodium hydroxide per gram of oil — the reason a blend cannot be averaged. The last column is what that oil alone would need at its own maximum share of a one-kilogram batch.
The three ceilings that matter
Hardness and big bubbles up to here. Past it the same oil starts stripping skin, which is what makes cheap coconut bars squeaky.
Five per cent makes the lather creamy and stable. Ten is the ceiling and the bar turns sticky above it — the cheapest upgrade in soap making, and easy to overdo.
Between all your butters, not each. More than this and the bar softens and the lather goes flat, having cost you a good deal to do it.
What each oil is for
The oil that makes soap feel like soap. It has the highest saponification value of the common oils at 0.183, so a coconut-heavy recipe needs noticeably more lye. Stop at 30%.
The only oil that works alone; 100% olive is Castile. Mild, slow to trace and slow to cure. Pomace olive has the same value of 0.1345 but traces much faster, which is useful until the day you want to swirl.
What makes a bar last in the shower rather than dissolving in a fortnight. Palm is the vegetable option and should be bought certified sustainable; tallow does the same job, is far cheaper, and is not vegan.
Not a base oil. At around 5% it turns thin bubbles into a stable creamy lather; past 10% it makes the bar tacky. The single highest-value small addition in a recipe.
Shea butter, cocoa and mango butter genuinely improve a bar at up to 20% between them. The 3 specialty oils and most of the 10 soft oils are refinements you will not notice until the basics are right.
Buying oils
Coconut, olive, palm or tallow, and castor. That covers every job a bar needs. Adding a fifth oil before you have made the four-oil version well tells you nothing about what the fifth did.
Supermarket olive oil makes excellent soap. Extra virgin is a waste of money here — the lye does not care about the flavour compounds you are paying for, and unrefined oils tint the bar green.
At 5% of a batch, a small bottle lasts an extremely long time. It is one of the few oils where the litre is genuinely more than you need.
Grapeseed, hemp and sunflower go rancid quickly, and free oil in a bar is the part that spoils first. Buy small, use fast, and keep them out of the sun.
Every oil has its own saponification value, so changing an oil changes the lye. Never carry a lye weight across from a recipe with a different blend.
Build your blend
Enter your oils and their percentages to get the lye, the water and every oil weighed out.
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Estimates, not advice. Every figure on this page is computed from published reference values and standard measures, so your own inputs, equipment and conditions will move it. Nothing here is professional advice — talk to a qualified professional before acting on it.