Free Soap Calculator
Exact lye. Exact water. Exact superfat. No signup, no account, no guesswork.
This is a full soap calculator, not a lye calculator with the edges filed off. Enter your oils and butters, choose NaOH for bars or KOH for liquid, set your superfat, and get back the exact sodium hydroxide, water and yield figures your batch runs on. Cold process or hot process. Free, instant, and you will never be asked for an email to use it.
Ready to Calculate
Add your oils and weights on the left, then click Calculate Lye.
Before you pour.
Calculated to verified saponification data. You still run the batch — check your scale, your PPE, and your numbers before you pour. Lye is caustic. Goggles and gloves, every time. Always add lye to water, never water to lye. Work where the air moves.
How to Use This Soap Calculator
Choose your lye type
Select NaOH (sodium hydroxide) for bar soap or KOH (potassium hydroxide) for liquid soap. NaOH is used in cold process and hot process bar soap making. KOH at 90% purity is standard for liquid soap.
Select your soap making method
Cold process soap is made at room temperature and cured for 4–6 weeks. Hot process soap is cooked and can be used sooner. The lye amount is the same — the method changes how you handle the batter.
Add your oils and weights
Enter each oil or butter in your recipe with its weight in grams. Each oil has a unique saponification value (SAP value) — the calculator uses these to determine exactly how much lye is needed.
Set your superfat percentage
Superfat is the percentage of oils left unsaponified in your finished soap. A 5% superfat is standard for a mild, moisturising bar. Higher superfat means more free oils but a softer bar.
Check your yield against your mould
A recipe that calculates perfectly and overflows your mould is still a failed batch. Total your oil weight, then match it to the mould you are actually pouring into before you mix a thing.
Fragrance and Essential Oil Calculator for Soap
Fragrance is where good recipes quietly go wrong. Too little and the bar smells of nothing after cure. Too much and you have a batch you cannot legally or safely sell.
Usage rate is a percentage of your total oil weight, not a splash. For cold process soap, most fragrance oils run between 3% and 6% of oil weight, and most essential oils sit lower. A 1,000 g batch of oils at 5% is 50 g of fragrance. That is the whole calculation — and it is the one people do in their head and get wrong.
Essential oils and fragrance oils are not interchangeable at the same rate. Some essential oils accelerate trace, some seize, some fade to nothing over a four-week cure, and some carry usage limits well below what a fragrance oil would tolerate.
IFRA note: LatherForge is a planning aid, not a certification service. Every fragrance has its own IFRA category limit — check the current documentation for the specific oil in your hand, every time.
Melt and Pour Soap Calculator
Melt and pour needs no lye calculator. The base is already saponified — the lye work was done before it reached you. What you still need to calculate is fragrance load and mould yield, and both go wrong at exactly the same rate as they do in cold process.
Weigh your base, apply your fragrance percentage, check it against your mould volume. Melt and pour generally tolerates a lower fragrance rate than cold process — around 3% of base weight is a common ceiling. Check your specific base’s guidance.
Which Soap Calculator Is Best?
Most of them do the same arithmetic, and the good ones do it correctly. The long-established free calculators have been running for two decades and are accurate. Several supply retailers host free calculators, which is a reasonable thing for a supplier to do.
So the honest answer is that they are all fine at the calculation. The difference is what happens the moment it finishes.
Every one of those tools hands you a number and forgets you. You write it on a scrap of paper, or into a spreadsheet cell you will not find in March, and the recipe that made your best-selling bar exists only in your memory. Then a customer reorders and you cannot reproduce it exactly.
That is the actual problem. Not the arithmetic — the amnesia.
LatherForge calculates the same numbers, free and with no signup, and then keeps them. Versioned recipes, the batch you poured them into, its cure date, what the oils cost you that week, and what the bar has to sell for to be worth making.
The calculator above is free forever. The rest goes live 1 January 2027.
Soap Calculator FAQ
What is a lye calculator?
A lye calculator determines the exact amount of sodium hydroxide (NaOH) or potassium hydroxide (KOH) needed to saponify a specific blend of oils and butters. It uses saponification values (SAP values) for each oil to calculate the correct lye amount so no lye remains in your finished soap.
What is superfatting in soap making?
Superfatting is using slightly less lye than required to saponify all oils, leaving a small percentage of free oils in the finished soap. This results in a milder, more moisturising bar. A 5% superfat is the most common choice for handmade soap.
What is the difference between NaOH and KOH?
NaOH (sodium hydroxide) produces hard bar soap and is used for cold process and hot process soap. KOH (potassium hydroxide) produces soft or liquid soap. Most bar soap makers use NaOH. Liquid soap makers use KOH, typically at 90% purity.
How much water should I use?
The standard water to lye ratio for cold process soap is 2:1 — two parts water to one part lye by weight. This is the default used in most soap recipes. Some makers reduce water to speed cure time. The calculator recommends water based on your lye amount.
Is there a free soap calculator available?
Yes — the one on this page. No account, no email, no trial period. Enter your oils, get your lye, water, superfat and yield. It stays free after LatherForge launches.
How do you calculate a soap formula?
Every oil has a saponification value — the exact weight of lye needed to fully saponify one gram of it. A soap calculator multiplies each oil’s weight by its SAP value, totals the results, then reduces that total by your superfat percentage to leave some oil unsaponified in the finished bar. Water is calculated from the lye amount, commonly at a 2:1 ratio for cold process. Doing this by hand across six oils is where the errors come from.
What is a soap calculator?
A tool that converts a blend of oils and butters into the exact lye, water and superfat figures a batch needs. It is the one piece of equipment you cannot make cold process soap safely without, because getting lye wrong does not produce mediocre soap — it produces a caustic bar or a batch that never sets.
Can I use this soap calculator for beginners’ recipes?
It is built for exactly that. Choose NaOH, choose cold process, set superfat to 5%, and enter your oils. Those are the settings behind most beginner recipes you will find. If your recipe lists oils in percentages rather than grams, decide your total oil weight first, then convert.
Can I legally sell homemade soap?
In most markets, yes, but labelling and safety-assessment rules vary considerably by country, and cosmetic claims are regulated more tightly than soap itself. LatherForge is a planning aid, not a certification service — check the rules for the market you sell into before you list anything.
Work out what to charge with the Etsy pricing calculator →The Calculator Is Free. The Rest Is Coming.
You just calculated one batch. LatherForge holds the whole operation — every recipe versioned, every batch tracked from pour to cure date, every ingredient costed, every bar priced to actually make money.
Live 1 January 2027. Waitlist members get 14 days free on launch day.
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