How to Make Soap From Scratch: Choose Oils and Build Your Own Recipe

To make soap from scratch with your own recipe, you choose a blend of oils for the bar you want, set a superfat and lye concentration, and let a lye calculator work out exactly how much lye and water those oils need. Then you make it like any cold process batch and cure it for four to six weeks. This guide is about the design step: how to pick oils, what the numbers mean, and how to check a recipe before you make it.

What “from scratch” means here

Every cold process or hot process soap is technically from scratch: oils plus lye, no pre-made base. This page is for the next step, when you stop following other people’s recipes and start writing your own.

That’s a good move once you’ve made at least one batch from a tested recipe, like our three-oil beginner soap, so you know what trace looks like and how a bar should feel after cure. If you haven’t, start with your first batch.

Pros and cons of writing your own recipes

Step 1: decide what the bar should do

Before you pick a single oil, decide what you’re after. A few questions:

• Hard and long-lasting, or soft and very mild? Hard oils and butters make firm bars; liquid oils make milder, softer ones.
• Big bubbles or creamy lather? Coconut and similar oils give big, quick bubbles; butters, tallow and castor give a denser, creamier lather.
• Any limits? Palm-free, vegan, nut-free, or built from what’s already in your pantry.
• Cold or hot process? The design is the same for both, but hot process needs more water.

Step 2: choose oils by role

Oils fall into rough roles. Most good bar recipes take one or two from each of the first three rows. Oils for soap making has detailed notes on each.

Role	Common choices	Typical share of the recipe	What too much does
Hardness and creaminess	Shea butter, cocoa butter, tallow, lard, palm (RSPO if used)	15–40% combined	Cocoa butter over about 15% can make a brittle bar
Lather and cleansing	Coconut oil, babassu, palm kernel	15–30%	Above about 30%, bars can feel drying
Mildness and conditioning	Olive, avocado, sweet almond, high-oleic sunflower, rice bran	30–60%	Very high amounts make a soft bar that needs a long cure
Lather stabilizer	Castor oil	About 5%, rarely above 10%	Sticky, soft bars

Oils to use sparingly

Oils high in linoleic and linolenic acids, such as regular sunflower, grapeseed, hemp and flaxseed, can go rancid and cause orange spots in the bar, known as DOS. Keep them low, roughly 10% or less, or choose high-oleic versions.

Step 3: set superfat and lye concentration

Two more settings complete the recipe:

Superfat is the percentage of oil you deliberately leave unsaponified. Around 5% is a sensible default for bar soap: enough margin that no lye is left over, not so much that the bar goes soft or rancid sooner. Some special bars use much more; salt bars and 100% coconut bars often use 15–20%.

Lye concentration sets how much water dissolves the lye. Around 33% suits most cold process recipes and is the calculator’s default. More water (a lower concentration) gives more working time; less water speeds unmolding and cure but traces faster. See water discount. For hot process, many makers drop to around 25–30% so the batter stays workable while it cooks.

Step 4: check it in the lye calculator

Now the numbers. Every oil needs a different amount of lye to turn fully into soap, its SAP value. The calculator multiplies each oil’s weight by its SAP value, adds them up and subtracts your superfat. How to use a lye calculator walks through the screen.

A worked example

Say you want a mild, hard-wearing bar with decent lather, palm-free, using common oils. A reasonable first draft for 1,000 g of oils:

Oil	%	Grams
Olive oil	45%	450 g
Coconut oil (76°)	25%	250 g
Shea butter	15%	150 g
Avocado oil	10%	100 g
Castor oil	5%	50 g

That’s the blend the lye calculator opens with. At 5% superfat and 33% lye concentration, it gives:

	Result
Sodium hydroxide	138 g
Distilled water	280 g
Hardness	36 (range 29–54)
Cleansing	17 (range 12–22)
Conditioning	59 (range 44–69)
Bubbly	21 (range 14–46)
Creamy	24 (range 16–48)

All five bar-quality numbers sit inside their typical ranges. That’s a balanced bar on paper.

Now change one thing

Suppose you want bigger bubbles and move 10% from olive oil to coconut oil (olive 35%, coconut 35%). Run it again and the calculator shows:

• Lye rises to about 143 g, roughly 5 g more, because coconut oil needs more lye per gram than olive oil.
• Cleansing rises to about 23, just above the typical range, which usually means a bar that feels more stripping.
• Hardness rises to about 43 and conditioning drops to about 52.

That’s the whole lesson in one swap: changing the oils changes the lye. If you’d swapped the oils and kept 138 g of lye, the bar would have had less superfat than you planned. Always recalculate.

Sizing the batch to your mold

Recipes are designed as percentages, so you can make any size. To fit a particular mold:

1. Line the mold, set it on the scale, tare it, and fill it with water to where you want the soap to reach. Note the weight of water in grams.
2. Soap batter weighs roughly the same as the same volume of water, so that number is about your total batch weight.
3. In a typical bar recipe, oils make up roughly 70% of the total batch: in the example above, 1,000 g of oils in a batch of about 1,418 g. So multiply the water weight by about 0.7 to get your total oil weight.
4. Enter that oil weight into the calculator with your percentages, and it will give you the lye and water.

It’s an estimate, so make a little extra rather than too little. Leftover batter can go into a small single-cavity mold.

Step 5: make a small test batch

Make your new recipe at a small size first, around 500 g of oils, so a failed experiment costs little. Scale every ingredient by the same factor, or simply re-enter the smaller weights in the calculator.

Follow the cold process walkthrough, and take notes as you go: soaping temperatures, how long trace took, whether the batter thickened quickly, and whether it gelled. A soap batch log makes this easy.

Step 6: cure, then judge

Cure the bars for four to six weeks before deciding whether the recipe works. Fresh soap is softer and harsher than it will be. Curing soap explains why. Then use a bar for a week or two and ask:

• Does it last, or go soft and mushy in the dish?
• Is the lather the kind you wanted?
• Does it feel drying, or slippery?

Change one thing at a time between versions, and you’ll learn what each oil really does in your hands.

Adding liquids, scent and extras

Once the oil blend works, the rest of the recipe is easier to design:

• Other liquids. Milk, beer, coffee, tea or aloe can replace some or all of the water, at the same weight. Milks and sugary liquids heat up and scorch, so they’re usually frozen first and soaped cool. See goat milk soap.
• Fragrance. Plan it as a percentage of oil weight, within your supplier’s limits; for essential oils, check the essential oil usage chart. The fragrance calculator turns it into grams.
• Additives. Clays, oats, exfoliants and colorants don’t change the lye amount, but some change how the batter behaves. Soap additives covers typical amounts.

Add one new element per version. If a recipe change and a new fragrance arrive in the same batch, you won’t know which caused the problem.

Common mistakes

• Substituting oils without recalculating. Every swap changes the lye amount. See the example above.
• Too much coconut oil without extra superfat. Above about 30% the bar can feel harsh.
• Too many soft oils. A bar that’s mostly liquid oils can take a long time to firm up. See soft soap that won’t harden.
• Ignoring rancidity. High amounts of polyunsaturated oils lead to DOS.
• Using volume measurements. Oils must be weighed in grams.
• Skipping the test batch. Make a small one first, especially with an untried fragrance.

A starter recipe to build from

If you’d rather adapt than start from a blank page, take our three-oil beginner soap and change one oil at a time. Or start from castile soap and see how adding coconut and a butter changes it. Keep each version in the calculator, saved with its share link, so you can compare later.

Next steps

• Read SAP values and superfat to understand the numbers behind the calculator.
• Browse the SAP value chart for oils you’re curious about.
• Plan scent with the fragrance calculator and the essential oil usage chart.